Electronic Trend Setter

Dummy Alarm Project

2011-11-04T19:12:00.001+05:30

This Dummy Alarm project makes an LED flash briefly once every 5 seconds to imitate the indicator light of a real alarm. The circuit is designed to use very little current to prolong battery life so that it can be left on permanently. An on/off switch is not included, but could be added if you wish.

The 7555 timer IC used is a low power version of the standard 555 timer. A 'superbright' red LED is used because this provides a bright flash with a low current. The LED is off for most of the time so the average total current for the circuit is less than 0.2mA. With this very low current a set of 3 alkaline AA cells should last for several months, maybe as long as a year.

The circuit will work with a standard 555 timer IC (such as the popular NE555) but this will increase the average current to about 2mA and the battery life will be much shorter. You can use a greater supply voltage (15V maximum) for this circuit but the 1k resistor for the LED should be increased to keep the LED current low at about 3mA. For example to use a 9V PP3 battery change the 1k resistor to 3k3. Note that AA cells will last longer than a 9V PP3 battery.

This project uses a 555 astable circuit.

Parts Required

resistors: 1k, 10k, 680k
capacitor: 10µF radial
LED: red superbright, 5mm diameter
7555 low power timer IC
8-pin DIL socket for IC
battery clip
4.5V battery box for 3 AA cells
stripboard: 8 rows × 16 holes

Stripboard Layout

Circuit diagram

Printed circuit board

2009-06-27T13:26:00.000+05:30

Printed circuit board

A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, or traces, etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board (PWB) or etched wiring board. A PCB populated with electronic components is a printed circuit assembly (PCA), also known as a printed circuit board assembly (PCBA).

PCBs are rugged, inexpensive, and can be highly reliable. They require much more layout effort and higher initial cost than either wire-wrapped or point-to-point constructed circuits, but are much cheaper and faster for high-volume production. Much of the electronics industry's PCB design, assembly, and quality control needs are set by standards that are published by the IPC organization.

History

The inventor of the printed circuit was the Austrian engineer Paul Eisler (1907–1995) who, while working in England, made one circa 1936 as part of a radio set. Around 1943 the USA began to use the technology on a large scale to make rugged radios for use in World War II. After the war, in 1948, the USA released the invention for commercial use. Printed circuits did not become commonplace in consumer electronics until the mid-1950s, after the Auto-Sembly process was developed by the United States Army.

Before printed circuits (and for a while after their invention), point-to-point construction was used. For prototypes, or small production runs, wire wrap or turret board can be more efficient.

Originally, every electronic component had wire leads, and the PCB had holes drilled for each wire of each component. The components' leads were then passed through the holes and soldered to the PCB trace. This method of assembly is called through-hole construction. In 1949, Moe Abramson and Stanislaus F. Danko of the United States Army Signal Corps developed the Auto-Sembly process in which component leads were inserted into a copper foil interconnection pattern and dip soldered. With the development of board lamination and etching techniques, this concept evolved into the standard printed circuit board fabrication process in use today. Soldering could be done automatically by passing the board over a ripple, or wave, of molten solder in a wave-soldering machine. However, the wires and holes are wasteful since drilling holes is expensive and the protruding wires are merely cut off.

In recent years, the use of surface mount parts has gained popularity as the demand for smaller electronics packaging and greater functionality has grown.

Manufacturing

Materials

Conducting layers are typically made of thin copper foil. Insulating layers dielectric are typically laminated together with epoxy resin prepreg. The board is typically coated with a solder mask that is green in color. Other colors that are normally available are blue, and red. There are quite a few different dielectrics that can be chosen to provide different insulating values depending on the requirements of the circuit. Some of these dielectrics are polytetrafluoroethylene, FR-4, FR-1, CEM-1 or CEM-3. Well known prepreg materials used in the PCB industry are FR-2 (Phenolic cotton paper), FR-3 (Cotton paper and epoxy), FR-4 (Woven glass and epoxy), FR-5 (Woven glass and epoxy), FR-6 (Matte glass and polyester), G-10 (Woven glass and epoxy), CEM-1 (Cotton paper and epoxy), CEM-2 (Cotton paper and epoxy), CEM-3 (Woven glass and epoxy), CEM-4 (Woven glass and epoxy), CEM-5 (Woven glass and polyester).

Patterning (etching)

The vast majority of printed circuit boards are made by bonding a layer of copper over the entire substrate, sometimes on both sides, (creating a "blank PCB") then removing unwanted copper after applying a temporary mask (eg. by etching), leaving only the desired copper traces. A few PCBs are made by adding traces to the bare substrate (or a substrate with a very thin layer of copper) usually by a complex process of multiple electroplating steps.

There are three common "subtractive" methods (methods that remove copper) used for the production of printed circuit boards:

Silk screen printing uses etch-resistant inks to protect the copper foil. Subsequent etching removes the unwanted copper. Alternatively, the ink may be conductive, printed on a blank (non-conductive) board. The latter technique is also used in the manufacture of hybrid circuits.
Photoengraving uses a photomask and chemical etching to remove the copper foil from the substrate. The photomask is usually prepared with a photoplotter from data produced by a technician using CAM, or computer-aided manufacturing software. Laser-printed transparencies are typically employed for phototools; however, direct laser imaging techniques are being employed to replace phototools for high-resolution requirements.
PCB milling uses a two or three-axis mechanical milling system to mill away the copper foil from the substrate. A PCB milling machine (referred to as a 'PCB Prototyper') operates in a similar way to a plotter, receiving commands from the host software that control the position of the milling head in the x, y, and (if relevant) z axis. Data to drive the Prototyper is extracted from files generated in PCB design software and stored in HPGL or Gerber file format.

"Additive" processes also exist. The most common is the "semi-additive" process. In this version, the unpatterned board has a thin layer of copper already on it. A reverse mask is then applied. (Unlike a subtractive process mask, this mask exposes those parts of the substrate that will eventually become the traces.) Additional copper is then plated onto the board in the unmasked areas; copper may be plated to any desired weight. Tin-lead or other surface platings are then applied. The mask is stripped away and a brief etching step removes the now-exposed original copper laminate from the board, isolating the individual traces.

The additive process is commonly used for multi-layer boards as it facilitates the plating-through of the holes (to produce conductive vias) in the circuit board.

Lamination

Some PCBs have trace layers inside the PCB and are called multi-layer PCBs. These are formed by bonding together separately etched thin boards.

Drilling

Holes through a PCB are typically drilled with tiny drill bits made of solid tungsten carbide. The drilling is performed by automated drilling machines with placement controlled by a drill tape or drill file. These computer-generated files are also called numerically controlled drill (NCD) files or "Excellon files". The drill file describes the location and size of each drilled hole. These holes are often filled with annular rings to create vias. Vias allow the electrical and thermal connection of conductors on opposite sides of the PCB.

When very small vias are required, drilling with mechanical bits is costly because of high rates of wear and breakage. In this case, the vias may be evaporated by lasers. Laser-drilled vias typically have an inferior surface finish inside the hole. These holes are called micro vias.

It is also possible with controlled-depth drilling, laser drilling, or by pre-drilling the individual sheets of the PCB before lamination, to produce holes that connect only some of the copper layers, rather than passing through the entire board. These holes are called blind vias when they connect an internal copper layer to an outer layer, or buried vias when they connect two or more internal copper layers and no outer layers.

The walls of the holes, for boards with 2 or more layers, are plated with copper to form plated-through holes that electrically connect the conducting layers of the PCB. For multilayer boards, those with 4 layers or more, drilling typically produces a smear comprised of the bonding agent in the laminate system. Before the holes can be plated through, this smear must be removed by a chemical de-smear process, or by plasma-etch.

Exposed conductor plating and coating

The places to which components will be mounted are typically plated, because bare copper oxidizes quickly, and therefore is not readily solderable. Traditionally, any exposed copper was plated with solder by hot air solder levelling (HASL). This solder was a tin-lead alloy, however new solder compounds are now used to achieve compliance with the RoHS directive in the EU, which restricts the use of lead. One of these lead-free compounds is SN100CL, made up of 99.3% tin, 0.7% copper, 0.05% nickel, and a nominal of 60ppm germanium.

Other platings used are OSP (organic surface protectant), immersion silver (IAg), immersion tin, electroless nickel with immersion gold coating (ENIG), and direct gold. Edge connectors, placed along one edge of some boards, are often gold plated.

Electrochemical migration (ECM) is the growth of conductive metal filaments on or in a printed circuit board (PCB) under the influence of a DC voltage bias.

Solder resist

Areas that should not be soldered to may be covered with a polymer solder resist (solder mask) coating. The solder resist prevents solder from bridging between conductors and thereby creating short circuits. Solder resist also provides some protection from the environment. Solder resist is typically 20-30 microns thick.

Screen printing

Line art and text may be printed onto the outer surfaces of a PCB by screen printing. When space permits, the screen print text can indicate component designators, switch setting requirements, test points, and other features helpful in assembling, testing, and servicing the circuit board.

Screen print is also known as the silk screen, or, in one sided PCBs, the red print.

Lately some digital printing solutions have been developed to substitute the traditional screen printing process. This technology allows printing variable data onto the PCB, including serialization and barcode information for traceability purposes.

Test

Unpopulated boards may be subjected to a bare-board test where each circuit connection (as defined in a netlist) is verified as correct on the finished board. For high-volume production, a Bed of nails tester, a fixture or a Rigid needle adapter is used to make contact with copper lands or holes on one or both sides of the board to facilitate testing. A computer will instruct the electrical test unit to apply a small voltage to each contact point on the bed-of-nails as required, and verify that such voltage appears at other appropriate contact points. A "short" on a board would be a connection where there should not be one; an "open" is between two points that should be connected but are not. For small- or medium-volume boards, flying-probe and flying-grid testers use moving test heads to make contact with the copper/silver/gold/solder lands or holes to verify the electrical connectivity of the board under test.

Printed circuit assembly

After the printed circuit board (PCB) is completed, electronic components must be attached to form a functional printed circuit assembly^[3]^[4], or PCA (sometimes called a "printed circuit board assembly" PCBA). In through-hole construction, component leads are inserted in holes. In surface-mount construction, the components are placed on pads or lands on the outer surfaces of the PCB. In both kinds of construction, component leads are electrically and mechanically fixed to the board with a molten metal solder.

There are a variety of soldering techniques used to attach components to a PCB. High volume production is usually done with machine placement and bulk wave soldering or reflow ovens, but skilled technicians are able to solder very tiny parts (for instance 0201 packages which are 0.02" by 0.01") by hand under a microscope, using tweezers and a fine tip soldering iron for small volume prototypes. Some parts are impossible to solder by hand, such as ball grid array (BGA) packages.

Often, through-hole and surface-mount construction must be combined in a single PCA because some required components are available only in surface-mount packages, while others are available only in through-hole packages. Another reason to use both methods is that through-hole mounting can provide needed strength for components likely to endure physical stress, while components that are expected to go untouched will take up less space using surface-mount techniques.

After the board has been populated it may be tested in a variety of ways:

While the power is off, visual inspection, automated optical inspection. JEDEC guidelines for PCB component placement, soldering, and inspection are commonly used to maintain quality control in this stage of PCB manufacturing.

While the power is off, analog signature analysis, power-off testing.

While the power is on, in-circuit test, where physical measurements (i.e. voltage, frequency) can be done.

While the power is on, functional test, just checking if the PCB does what it had been designed for.

To facilitate these tests, PCBs may be designed with extra pads to make temporary connections. Sometimes these pads must be isolated with resistors. The in-circuit test may also exercise boundary scan test features of some components. In-circuit test systems may also be used to program nonvolatile memory components on the board.

In boundary scan testing, test circuits integrated into various ICs on the board form temporary connections between the PCB traces to test that the ICs are mounted correctly. Boundary scan testing requires that all the ICs to be tested use a standard test configuration procedure, the most common one being the Joint Test Action Group (JTAG) standard.

When boards fail the test, technicians may desolder and replace failed components, a task known as "rework".

The Processor

2009-06-27T13:05:00.003+05:30

So what is the processor? Well in the simplest of terms, it’s your computers brain. The processor tells your computer what to do and when to do it, it decides which tasks aremore important and prioritizes them to your computers needs.

There is and has been many processors on the market, running at many different speeds. The speed is measured in Megahertz or MHz. A single MHz is a calculation of 1 million cycles per second (or computer instructions), so if you have a processor running at 2000 MHz, then your computer is running at 2000,000,000 cycles per second, which in more basic terms is the amount of instructions your computer can carry out. Another important abbreviation is Gigahertz or GHz. A single GHz or 1 GHz is the same as 1000 MHz . Sounds a bit confusing, so here is a simple conversion :

1000 MHz (Megahertz) = 1GHz (Gigahertz) = 1000,000,000 Cycles per second (or computer instructions).

Now you can see why they abbreviate it, could you imagine going to a PC store and asking for a one thousand million cycle PC please. A bit of a mouth full isn’t it?So when buying a new computer always look for fastest you can afford. The fastest on the market at the time of writing this article is 3.8 GHz (3800 MHz). Remember though that it is not necessary to purchase such a fast processor, balance your needs, do you really need top of the range? Especially when the difference say between a 3.5 GHz (3500 MHz) and a 3.8 GHz (3800 MHz) processor will be barely noticed (if noticed at all) by you, while the price difference is around £100. With the money you save you could get a nice printer and scanner package.

Now that we have covered the speeds, there is one more important subject to cover. Which processor? There are 3 competitors at present, the AMD Athlon, Intel Pentium and the Intel Celeron. They come in many guises, but basically the more cores they have and the higher the speed means better and faster.

Processors now come as dual core, triple core and quad core. These processors are the equivalent of running two cpu's (Dual core), three CPU's ( Triple core) or four (Quad core).

In the past Intel Pentium the best and most expensive of them all, and remains today one of the most popular on the market. In layman’s terms it is/was the designer processor, although AMD have some superb if not better releases and equally highly priced and advanced products. It would be hard to say which is best as they are direct competitors.

Lastly there is the Intel Celeron; this processor is a budget version of the Intel Pentium 4, the processor you find in most budget computers. If the purse is tight, and you need a computer, then this is your port of call. You will find many sub £400 computers fitted with this processor.

Electronic Symbols : Logic Gates

2009-06-14T18:00:00.003+05:30

Logic Gates
Gate Type	Traditional Symbol	IEC Symbol	Function of Gate
NOT			A NOT gate can only have one input. The 'o' on the output means 'not'. The output of a NOT gate is the inverse (opposite) of its input, so the output is true when the input is false. A NOT gate is also called an inverter.
AND			An AND gate can have two or more inputs. The output of an AND gate is true when all its inputs are true.
NAND			A NAND gate can have two or more inputs. The 'o' on the output means 'not' showing that it is a Not AND gate. The output of a NAND gate is true unless all its inputs are true.
OR			An OR gate can have two or more inputs. The output of an OR gate is true when at least one of its inputs is true.
NOR			A NOR gate can have two or more inputs. The 'o' on the output means 'not' showing that it is a Not OR gate. The output of a NOR gate is true when none of its inputs are true.
EX-OR			An EX-OR gate can only have two inputs. The output of an EX-OR gate is true when its inputs are different (one true, one false).
EX-NOR			An EX-NOR gate can only have two inputs. The 'o' on the output means 'not' showing that it is a Not EX-OR gate. The output of an EX-NOR gate is true when its inputs are the same (both true or both false).

Electronic Symbols : Logic Gates

2009-06-14T18:00:00.000+05:30

Logic Gates
Gate Type	Traditional Symbol	IEC Symbol	Function of Gate
NOT			A NOT gate can only have one input. The 'o' on the output means 'not'. The output of a NOT gate is the inverse (opposite) of its input, so the output is true when the input is false. A NOT gate is also called an inverter.
AND			An AND gate can have two or more inputs. The output of an AND gate is true when all its inputs are true.
NAND			A NAND gate can have two or more inputs. The 'o' on the output means 'not' showing that it is a Not AND gate. The output of a NAND gate is true unless all its inputs are true.
OR			An OR gate can have two or more inputs. The output of an OR gate is true when at least one of its inputs is true.
NOR			A NOR gate can have two or more inputs. The 'o' on the output means 'not' showing that it is a Not OR gate. The output of a NOR gate is true when none of its inputs are true.
EX-OR			An EX-OR gate can only have two inputs. The output of an EX-OR gate is true when its inputs are different (one true, one false).
EX-NOR			An EX-NOR gate can only have two inputs. The 'o' on the output means 'not' showing that it is a Not EX-OR gate. The output of an EX-NOR gate is true when its inputs are the same (both true or both false).

Electronic Symbols : Sensors (input devices)

2009-06-14T17:58:00.000+05:30

Sensors (input devices)
Component	Circuit Symbol	Function of Component
LDR		A transducer which converts brightness (light) to resistance (an electrical property). LDR = Light Dependent Resistor
Thermistor		A transducer which converts temperature (heat) to resistance (an electrical property).

Electronic Symbols : Meters and Oscilloscope

2009-06-14T17:57:00.000+05:30

Meters and Oscilloscope
Component	Circuit Symbol	Function of Component
Voltmeter		A voltmeter is used to measure voltage. The proper name for voltage is 'potential difference', but most people prefer to say voltage!
Ammeter		An ammeter is used to measure current.
Galvanometer		A galvanometer is a very sensitive meter which is used to measure tiny currents, usually 1mA or less.
Ohmmeter		An ohmmeter is used to measure resistance. Most multimeters have an ohmmeter setting.
Oscilloscope		An oscilloscope is used to display the shape of electrical signals and it can be used to measure their voltage and time period.

Electronic Symbols :Audio and Radio Devices

2009-06-14T17:56:00.000+05:30

Audio and Radio Devices
Component	Circuit Symbol	Function of Component
Microphone		A transducer which converts sound to electrical energy.
Earphone		A transducer which converts electrical energy to sound.
Loudspeaker		A transducer which converts electrical energy to sound.
Piezo Transducer		A transducer which converts electrical energy to sound.
Amplifier (general symbol)		An amplifier circuit with one input. Really it is a block diagram symbol because it represents a circuit rather than just one component.
Aerial (Antenna)		A device which is designed to receive or transmit radio signals. It is also known as an antenna.

Electronic Symbols : Resistors , Capasitors , Diods and Transistors.

2009-06-14T17:53:00.001+05:30

Resistors
Component	Circuit Symbol	Function of Component
Resistor		A resistor restricts the flow of current, for example to limit the current passing through an LED. A resistor is used with a capacitor in a timing circuit. Some publications still use the old resistor symbol:
Variable Resistor (Rheostat)		This type of variable resistor with 2 contacts (a rheostat) is usually used to control current. Examples include: adjusting lamp brightness, adjusting motor speed, and adjusting the rate of flow of charge into a capacitor in a timing circuit.
Variable Resistor (Potentiometer)		This type of variable resistor with 3 contacts (a potentiometer) is usually used to control voltage. It can be used like this as a transducer converting position (angle of the control spindle) to an electrical signal.
Variable Resistor (Preset)		This type of variable resistor (a preset) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment. Presets are cheaper than normal variable resistors so they are often used in projects to reduce the cost.

Capacitors
Component	Circuit Symbol	Function of Component
Capacitor		A capacitor stores electric charge. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.
Capacitor, polarised		A capacitor stores electric charge. This type must be connected the correct way round. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.
Variable Capacitor		A variable capacitor is used in a radio tuner.
Trimmer Capacitor		This type of variable capacitor (a trimmer) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment.

Diodes
Component	Circuit Symbol	Function of Component
Diode		A device which only allows current to flow in one direction.
LED Light Emitting Diode		A transducer which converts electrical energy to light.
Zener Diode		A special diode which is used to maintain a fixed voltage across its terminals.
Photodiode		A light-sensitive diode.

Transistors
Component	Circuit Symbol	Function of Component
Transistor NPN		A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.
Transistor PNP		A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.
Phototransistor		A light-sensitive transistor.

Electronic Symbols : Switches

2009-06-14T17:52:00.000+05:30

Switches
Component	Circuit Symbol	Function of Component
Push Switch (push-to-make)		A push switch allows current to flow only when the button is pressed. This is the switch used to operate a doorbell.
Push-to-Break Switch		This type of push switch is normally closed (on), it is open (off) only when the button is pressed.
On-Off Switch (SPST)		SPST = Single Pole, Single Throw. An on-off switch allows current to flow only when it is in the closed (on) position.
2-way Switch (SPDT)		SPDT = Single Pole, Double Throw. A 2-way changeover switch directs the flow of current to one of two routes according to its position. Some SPDT switches have a central off position and are described as 'on-off-on'.
Dual On-Off Switch (DPST)		DPST = Double Pole, Single Throw. A dual on-off switch which is often used to switch mains electricity because it can isolate both the live and neutral connections.
Reversing Switch (DPDT)		DPDT = Double Pole, Double Throw. This switch can be wired up as a reversing switch for a motor. Some DPDT switches have a central off position.
Relay		An electrically operated switch, for example a 9V battery circuit connected to the coil can switch a 230V AC mains circuit. NO = Normally Open, COM = Common, NC = Normally Closed.

Electronic Symbols - Output Devices: Lamps, Heater, Motor, etc.

2009-06-14T17:51:00.000+05:30

Output Devices: Lamps, Heater, Motor, etc.
Component	Circuit Symbol	Function of Component
Lamp (lighting)		A transducer which converts electrical energy to light. This symbol is used for a lamp providing illumination, for example a car headlamp or torch bulb.
Lamp (indicator)		A transducer which converts electrical energy to light. This symbol is used for a lamp which is an indicator, for example a warning light on a car dashboard.
Heater		A transducer which converts electrical energy to heat.
Motor		A transducer which converts electrical energy to kinetic energy (motion).
Bell		A transducer which converts electrical energy to sound.
Buzzer		A transducer which converts electrical energy to sound.
Inductor (Coil, Solenoid)		A coil of wire which creates a magnetic field when current passes through it. It may have an iron core inside the coil. It can be used as a transducer converting electrical energy to mechanical energy by pulling on something.

Electronic Symbols : Power Supplies

2009-06-14T17:50:00.000+05:30

Power Supplies
Component	Circuit Symbol	Function of Component
Cell		Supplies electrical energy. The larger terminal (on the left) is positive (+). A single cell is often called a battery, but strictly a battery is two or more cells joined together.
Battery		Supplies electrical energy. A battery is more than one cell. The larger terminal (on the left) is positive (+).
DC supply		Supplies electrical energy. DC = Direct Current, always flowing in one direction.
AC supply		Supplies electrical energy. AC = Alternating Current, continually changing direction.
Fuse		A safety device which will 'blow' (melt) if the current flowing through it exceeds a specified value.
Transformer		Two coils of wire linked by an iron core. Transformers are used to step up (increase) and step down (decrease) AC voltages. Energy is transferred between the coils by the magnetic field in the core. There is no electrical connection between the coils.
Earth (Ground)		A connection to earth. For many electronic circuits this is the 0V (zero volts) of the power supply, but for mains electricity and some radio circuits it really means the earth. It is also known as ground.

Electronic Symbols : Wires and connections

2009-06-14T17:49:00.000+05:30

Wires and connections
Component	Circuit Symbol	Function of Component
Wire		To pass current very easily from one part of a circuit to another.
Wires joined		A 'blob' should be drawn where wires are connected (joined), but it is sometimes omitted. Wires connected at 'crossroads' should be staggered slightly to form two T-junctions, as shown on the right.
Wires not joined		In complex diagrams it is often necessary to draw wires crossing even though they are not connected. I prefer the 'bridge' symbol shown on the right because the simple crossing on the left may be misread as a join where you have forgotten to add a 'blob'!

Operating System (OS)

2009-06-14T17:28:00.002+05:30

How Operating Systems Work

When you turn on your computer, it's nice to think that you're in control. There's the trusty computer mouse, which you can move anywhere on the screen, summoning up your music library or Internet browser at the slightest whim. Although it's easy to feel like a director in front of your desktop or laptop, there's a lot going on inside, and the real man behind the curtain handling the necessary tasks is the operating system.

Most desktop or laptop PCs come pre-loaded with Microsoft Windows. Macintosh computers come pre-loaded with Mac OS X. Many corporate servers use the Linux or UNIX operating systems. The operating system (OS) is the first thing loaded onto the computer -- without the operating system, a computer is useless.

More recently, operating systems have started to pop up in smaller computers as well. If you like to tinker with electronic devices, you're probably pleased that operating systems can now be found on many of the devices we use every day, from cell phones to wireless access points. The computers used in these little devices have gotten so powerful that they can now actually run an operating system and applications. The computer in a typical modern cell phone is now more powerful than a desktop computer from 20 years ago, so this progression makes sense and is a natural development.

The purpose of an operating system is to organize and control hardware and software so that the device it lives in behaves in a flexible but predictable way. In this article, we'll tell you what a piece of software must do to be called an operating system, show you how the operating system in your desktop computer works and give you some examples of how to take control of the other operating systems around you.

What is an Operating System?

Not all computers have operating systems. The computer that controls the microwave oven in your kitchen, for example, doesn't need an operating system. It has one set of tasks to perform, very straightforward input to expect (a numbered keypad and a few pre-set buttons) and simple, never-changing hardware to control. For a computer like this, an operating system would be unnecessary baggage, driving up the development and manufacturing costs significantly and adding complexity where none is required. Instead, the computer in a microwave oven simply runs a single hard-wired program all the time.

A model displays Japanese mobile phone operator Willcom's smart phone, 'D4', which comes equipped with the Windows Vista operating system.

For other devices, an operating system creates the ability to:

serve a variety of purposes
interact with users in more complicated ways
keep up with needs that change over time

All desktop computers have operating systems. The most common are the Windows family of operating systems developed by Microsoft, the Macintosh operating systems developed by Apple and the UNIX family of operating systems (which have been developed by a whole history of individuals, corporations and collaborators). There are hundreds of other operating systems available for special-purpose applications, including specializations for mainframes, robotics, manufacturing, real-time control systems and so on.

In any device that has an operating system, there's usually a way to make changes to how the device works. This is far from a happy accident; one of the reasons operating systems are made out of portable code rather than permanent physical circuits is so that they can be changed or modified without having to scrap the whole device.

For a desktop computer user, this means you can add a new security update, system patch, new application or even an entirely new operating system rather than junk your computer and start again with a new one when you need to make a change. As long as you understand how an operating system works and how to get at it, in many cases you can change some of the ways it behaves. The same thing goes for your phone, too.

Operating System Functions

At the simplest level, an operating system does two things:

It manages the hardware and software resources of the system. In a desktop computer, these resources include such things as the processor, memory, disk space and more (On a cell phone, they include the keypad, the screen, the address book, the phone dialer, the battery and the network connection).
It provides a stable, consistent way for applications to deal with the hardware without having to know all the details of the hardware.

The first task, managing the hardware and software resources, is very important, as various programs and input methods compete for the attention of the central processing unit (CPU) and demand memory, storage and input/output (I/O) bandwidth for their own purposes. In this capacity, the operating system plays the role of the good parent, making sure that each application gets the necessary resources while playing nicely with all the other applications, as well as husbanding the limited capacity of the system to the greatest good of all the users and applications.

The second task, providing a consistent application interface, is especially important if there is to be more than one of a particular type of computer using the operating system, or if the hardware making up the computer is ever open to change. A consistent application program interface (API) allows a software developer to write an application on one computer and have a high level of confidence that it will run on another computer of the same type, even if the amount of memory or the quantity of storage is different on the two machines.

Even if a particular computer is unique, an operating system can ensure that applications continue to run when hardware upgrades and updates occur. This is because the operating system -- not the application -- is charged with managing the hardware and the distribution of its resources. One of the challenges facing developers is keeping their operating systems flexible enough to run hardware from the thousands of vendors manufacturing computer equipment. Today's systems can accommodate thousands of different printers, disk drives and special peripherals in any possible combination.

Types of Operating Systems

Within the broad family of operating systems, there are generally four types, categorized based on the types of computers they control and the sort of applications they support. The categories are:

Real-time operating system (RTOS) - Real-time operating systems are used to control machinery, scientific instruments and industrial systems. An RTOS typically has very little user-interface capability, and no end-user utilities, since the system will be a "sealed box" when delivered for use. A very important part of an RTOS is managing the resources of the computer so that a particular operation executes in precisely the same amount of time, every time it occurs. In a complex machine, having a part move more quickly just because system resources are available may be just as catastrophic as having it not move at all because the system is busy.
Single-user, single task - As the name implies, this operating system is designed to manage the computer so that one user can effectively do one thing at a time. The Palm OS for Palm handheld computers is a good example of a modern single-user, single-task operating system.
Single-user, multi-tasking - This is the type of operating system most people use on their desktop and laptop computers today. Microsoft's Windows and Apple's MacOS platforms are both examples of operating systems that will let a single user have several programs in operation at the same time. For example, it's entirely possible for a Windows user to be writing a note in a word processor while downloading a file from the Internet while printing the text of an e-mail message.
Multi-user - A multi-user operating system allows many different users to take advantage of the computer's resources simultaneously. The operating system must make sure that the requirements of the various users are balanced, and that each of the programs they are using has sufficient and separate resources so that a problem with one user doesn't affect the entire community of users. Unix, VMS and mainframe operating systems, such as MVS, are examples of multi-user operating systems.

It's important to differentiate between multi-user operating systems and single-user operating systems that support networking. Windows 2000 and Novell Netware can each support hundreds or thousands of networked users, but the operating systems themselves aren't true multi-user operating systems. The system administrator is the only "user" for Windows 2000 or Netware. The network support and all of the remote user logins the network enables are, in the overall plan of the operating system, a program being run by the administrative user.

With the different types of operating systems in mind, it's time to look at the basic functions provided by an operating system.

Computer Operating Systems

When you turn on the power to a computer, the first program that runs is usually a set of instructions kept in the computer's read-only memory (ROM). This code examines the system hardware to make sure everything is functioning properly. This power-on self test (POST) checks the CPU, memory, and basic input-output systems (BIOS) for errors and stores the result in a special memory location. Once the POST has successfully completed, the software loaded in ROM (sometimes called the BIOS or firmware) will begin to activate the computer's disk drives. In most modern computers, when the computer activates the hard disk drive, it finds the first piece of the operating system: the bootstrap loader.

The bootstrap loader is a small program that has a single function: It loads the operating system into memory and allows it to begin operation. In the most basic form, the bootstrap loader sets up the small driver programs that interface with and control the various hardware subsystems of the computer. It sets up the divisions of memory that hold the operating system, user information and applications. It establishes the data structures that will hold the myriad signals, flags and semaphores that are used to communicate within and between the subsystems and applications of the computer. Then it turns control of the computer over to the operating system.

The operating system's tasks, in the most general sense, fall into six categories:

Processor management
Memory management
Device management
Storage management
Application interface
User interface

While there are some who argue that an operating system should do more than these six tasks, and some operating-system vendors do build many more utility programs and auxiliary functions into their operating systems, these six tasks define the core of nearly all operating systems. Next, let's look at the tools the operating system uses to perform each of these functions.

Processor Management

The heart of managing the processor comes down to two related issues:

Ensuring that each process and application receives enough of the processor's time to function properly
Using as many processor cycles as possible for real work

The basic unit of software that the operating system deals with in scheduling the work done by the processor is either a process or a thread, depending on the operating system.

It's tempting to think of a process as an application, but that gives an incomplete picture of how processes relate to the operating system and hardware. The application you see (word processor, spreadsheet or game) is, indeed, a process, but that application may cause several other processes to begin, for tasks like communications with other devices or other computers. There are also numerous processes that run without giving you direct evidence that they ever exist. For example, Windows XP and UNIX can have dozens of background processes running to handle the network, memory management, disk management, virus checks and so on.

A process, then, is software that performs some action and can be controlled -- by a user, by other applications or by the operating system.

It is processes, rather than applications, that the operating system controls and schedules for execution by the CPU. In a single-tasking system, the schedule is straightforward. The operating system allows the application to begin running, suspending the execution only long enough to deal with interrupts and user input.

Interrupts are special signals sent by hardware or software to the CPU. It's as if some part of the computer suddenly raised its hand to ask for the CPU's attention in a lively meeting. Sometimes the operating system will schedule the priority of processes so that interrupts are masked -- that is, the operating system will ignore the interrupts from some sources so that a particular job can be finished as quickly as possible. There are some interrupts (such as those from error conditions or problems with memory) that are so important that they can't be ignored. These non-maskable interrupts (NMIs) must be dealt with immediately, regardless of the other tasks at hand.

While interrupts add some complication to the execution of processes in a single-tasking system, the job of the operating system becomes much more complicated in a multi-tasking system. Now, the operating system must arrange the execution of applications so that you believe that there are several things happening at once. This is complicated because the CPU can only do one thing at a time. Today's multi-core processors and multi-processor machines can handle more work, but each processor core is still capable of managing one task at a time.

In order to give the appearance of lots of things happening at the same time, the operating system has to switch between different processes thousands of times a second. Here's how it happens:

A process occupies a certain amount of RAM. It also makes use of registers, stacks and queues within the CPU and operating-system memory space.
When two processes are multi-tasking, the operating system allots a certain number of CPU execution cycles to one program.
After that number of cycles, the operating system makes copies of all the registers, stacks and queues used by the processes, and notes the point at which the process paused in its execution.
It then loads all the registers, stacks and queues used by the second process and allows it a certain number of CPU cycles.
When those are complete, it makes copies of all the registers, stacks and queues used by the second program, and loads the first program.

Random Acces Memory (RAM)

2009-06-14T17:18:00.001+05:30

How RAM Works

Random access memory (RAM) is the best known form of computer memory. RAM is considered "random access" because you can access any memory cell directly if you know the row and column that intersect at that cell.

The opposite of RAM is serial access memory (SAM). SAM stores data as a series of memory cells that can only be accessed sequentially (like a cassette tape). If the data is not in the current location, each memory cell is checked until the needed data is found. SAM works very well for memory buffers, where the data is normally stored in the order in which it will be used (a good example is the texture buffer memory on a video card). RAM data, on the other hand, can be accessed in any order.

Similar to a microprocessor, a memory chip is an integrated circuit (IC) made of millions of transistors and capacitors. In the most common form of computer memory, dynamic random access memory (DRAM), a transistor and a capacitor are paired to create a memory cell, which represents a single bit of data. The capacitor holds the bit of information -- a 0 or a 1 (see How Bits and Bytes Work for information on bits). The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state.

A capacitor is like a small bucket that is able to store electrons. To store a 1 in the memory cell, the bucket is filled with electrons. To store a 0, it is emptied. The problem with the capacitor's bucket is that it has a leak. In a matter of a few milliseconds a full bucket becomes empty. Therefore, for dynamic memory to work, either the CPU or the memory controller has to come along and recharge all of the capacitors holding a 1 before they discharge. To do this, the memory controller reads the memory and then writes it right back. This refresh operation happens automatically thousands of times per second.

This refresh operation is where dynamic RAM gets its name. Dynamic RAM has to be dynamically refreshed all of the time or it forgets what it is holding. The downside of all of this refreshing is that it takes time and slows down the memory.

Static RAM

Static RAM uses a completely different technology. In static RAM, a form of flip-flop holds each bit of memory (see How Boolean Logic Works for details on flip-flops). A flip-flop for a memory cell takes four or six transistors along with some wiring, but never has to be refreshed. This makes static RAM significantly faster than dynamic RAM. However, because it has more parts, a static memory cell takes up a lot more space on a chip than a dynamic memory cell. Therefore, you get less memory per chip, and that makes static RAM a lot more expensive.

Static RAM is fast and expensive, and dynamic RAM is less expensive and slower. So static RAM is used to create the CPU's speed-sensitive cache, while dynamic RAM forms the larger system RAM space.

Memory chips in desktop computers originally used a pin configuration called dual inline package (DIP). This pin configuration could be soldered into holes on the computer's motherboard or plugged into a socket that was soldered on the motherboard. This method worked fine when computers typically operated on a couple of megabytes or less of RAM, but as the need for memory grew, the number of chips needing space on the motherboard increased.

The solution was to place the memory chips, along with all of the support components, on a separate printed circuit board (PCB) that could then be plugged into a special connector (memory bank) on the motherboard. Most of these chips use a small outline J-lead (SOJ) pin configuration, but quite a few manufacturers use the thin small outline package (TSOP) configuration as well. The key difference between these newer pin types and the original DIP configuration is that SOJ and TSOP chips are surface-mounted to the PCB. In other words, the pins are soldered directly to the surface of the board, not inserted in holes or sockets.

Memory chips are normally only available as part of a card called a module. You've probably seen memory listed as 8x32 or 4x16. These numbers represent the number of the chips multiplied by the capacity of each individual chip, which is measured in megabits (Mb), or one million bits. Take the result and divide it by eight to get the number of megabytes on that module. For example, 4x32 means that the module has four 32-megabit chips. Multiply 4 by 32 and you get 128 megabits. Since we know that a byte has 8 bits, we need to divide our result of 128 by 8. Our result is 16 megabytes!

How Much RAM Do You Need?

It's been said that you can never have enough money, and the same holds true for RAM, especially if you do a lot of graphics-intensive work or gaming. Next to the CPU itself, RAM is the most important factor in computer performance. If you don't have enough, adding RAM can make more of a difference than getting a new CPU!

If your system responds slowly or accesses the hard drive constantly, then you need to add more RAM. If you are running Windows XP, Microsoft recommends 128MB as the minimum RAM requirement. At 64MB, you may experience frequent application problems. For optimal performance with standard desktop applications, 256MB is recommended. If you are running Windows 95/98, you need a bare minimum of 32 MB, and your computer will work much better with 64 MB. Windows NT/2000 needs at least 64 MB, and it will take everything you can throw at it, so you'll probably want 128 MB or more.

Linux works happily on a system with only 4 MB of RAM. If you plan to add X-Windows or do much serious work, however, you'll probably want 64 MB. Mac OS X systems should have a minimum of 128 MB, or for optimal performance, 512 MB.

The amount of RAM listed for each system above is estimated for normal usage -- accessing the Internet, word processing, standard home/office applications and light entertainment. If you do computer-aided design (CAD), 3-D modeling/animation or heavy data processing, or if you are a serious gamer, then you will most likely need more RAM. You may also need more RAM if your computer acts as a server of some sort (Web pages, database, application, FTP or network).

Another question is how much VRAM you want on your video card. Almost all cards that you can buy today have at least 16 MB of RAM. This is normally enough to operate in a typical office environment. You should probably invest in a 32-MB or better graphics card if you want to do any of the following:

Play realistic games
Capture and edit video
Create 3-D graphics
Work in a high-resolution, full-color environment
Design full-color illustrations

When shopping for video cards, remember that your monitor and computer must be capable of supporting the card you choose.

Power supply

2009-06-14T16:57:00.003+05:30

If there is any one component that is absolutely vital to the operation of a computer, it is the power supply. Without it, a computer is just an inert box full of plastic and metal. The power supply converts the alternating current (AC) line from your home to the direct current (DC) needed by the personal computer. In this article, we'll learn how PC power supplies work and what the wattage ratings mean.

In a personal computer (PC), the power supply is the metal box usually found in a corner of the case. The power supply is visible from the back of many systems because it contains the power-cord receptacle and the cooling fan.

Power supplies, often referred to as "switching power supplies", use switcher technology to convert the AC input to lower DC voltages. The typical voltages supplied are:

3.3 volts
5 volts
12 volts

The 3.3- and 5-volts are typically used by digital circuits, while the 12-volt is used to run motors in disk drives and fans. The main specification of a power supply is in watts. A watt is the product of the voltage in volts and the current in amperes or amps. If you have been around PCs for many years, you probably remember that the original PCs had large red toggle switches that had a good bit of heft to them. When you turned the PC on or off, you knew you were doing it. These switches actually controlled the flow of 120 volt power to the power supply.

Today you turn on the power with a little push button, and you turn off the machine with a menu option. These capabilities were added to standard power supplies several years ago. The operating system can send a signal to the power supply to tell it to turn off. The push button sends a 5-volt signal to the power supply to tell it when to turn on. The power supply also has a circuit that supplies 5 volts, called VSB for "standby voltage" even when it is officially "off", so that the button will work.

Switcher Technology

In this photo you can see three small transformers (yellow) in the center. To the left are two cylindrical capacitors. The large finned pieces of aluminum are heat sinks. The left heat sink has transistors attached to it. These are the transistors in charge of doing the switching -- they provide high-frequency power to the transformers. Attached to the right heat sink are diodes that rectify AC signals and turn them into DC signals.

Prior to 1980 or so, power supplies tended to be heavy and bulky. They used large, heavy transformers and huge capacitors (some as large as soda cans) to convert line voltage at 120 volts and 60 hertz into 5 volts and 12 volts DC.

The switching power supplies used today are much smaller and lighter. They convert the 60-Hertz (Hz, or cycles per second) current to a much higher frequency, meaning more cycles per second. This conversion enables a small, lightweight transformer in the power supply to do the actual voltage step-down from 110 volts (or 220 in certain countries) to the voltage needed by the particular computer component. The higher-frequency AC current provided by a switcher supply is also easier to rectify and filter compared to the original 60-Hz AC line voltage, reducing the variances in voltage for the sensitive electronic components in the computer.

A switcher power supply draws only the power it needs from the AC line. The typical voltages and current provided by a power supply are shown on the label on a power supply.

Switcher technology is also used to make AC from DC, as found in many of the automobile power inverters used to run AC appliances in an automobile and in uninterruptible power supplies. Switcher technology in automotive power inverters changes the direct current from the auto battery into alternating current. The transformer uses alternating current to make the transformer in the inverter step the voltage up to that of household appliances (120 VAC).

Power Supply Standardization

Over time, there have been at least six different standard power supplies for personal computers. Recently, the industry has settled on using ATX-based power supplies. ATX is an industry specification that means the power supply has the physical characteristics to fit a standard ATX case and the electrical characteristics to work with an ATX motherboard.

A PC power supply removed from its PC case. Cables and connectors at right supply DC voltages.

PC power-supply cables use standardized, keyed connectors that make it difficult to connect the wrong ones. Also, fan manufacturers often use the same connectors as the power cables for disk drives, allowing a fan to easily obtain the 12 volts it needs. Color-coded wires and industry standard connectors make it possible for the consumer to have many choices for a replacement power supply.

Advanced Power Management (APM) offers a set of five different states that your system can be in. It was developed by Microsoft and Intel for PC users who wish to conserve power. Each system component, including the operating system, basic input/output system (BIOS), motherboard and attached devices all need to be APM-compliant to be able to use this feature. Should you wish to disable APM because you suspect it is using up system resources or causing a conflict, the best way to do this is in the BIOS. That way, the operating system won't try to reinstall it, which could happen if it were disabled only in the software.

Power Supply Wattage

A 400-watt switching power supply will not necessarily use more power than a 250-watt supply. A larger supply may be needed if you use every available slot on the motherboard or every available drive bay in the personal computer case. It is not a good idea to have a 250-watt supply if you have 250 watts total in devices, since the supply should not be loaded to 100 percent of its capacity.

According to PC Power & Cooling, Inc., some power consumption values (in watts) for common items in a personal computer are:

PC Item	Watts
Accelerated Graphics Port (AGP) card	20 to 30W
Peripheral Component Interconnect (PCI) card	5W
small computer system interface (SCSI) PCI card	20 to 25W
floppy disk drive	5W
network interface card	4W
50X CD-ROM drive	10 to 25W
RAM	10W per 128M
5200 RPM Integrated Drive Electronics (IDE) hard disk drive	5 to 11W
7200 RPM IDE hard disk drive	5 to 15W
Motherboard (without CPU or RAM)	20 to 30W
550 MHz Pentium III	30W
733 MHz Pentium III	23.5W
300 MHz Celeron	18W
600 MHz Athlon	45W

Power supplies of the same form factor ("form factor" refers to the actual shape of the motherboard) are typically differentiated by the wattage they supply and the length of the warranty.

Power Supply Problems

The PC power supply is probably the most failure-prone item in a personal computer. It heats and cools each time it is used and receives the first in-rush of AC current when the PC is switched on. Typically, a stalled cooling fan is a predictor of a power supply failure due to subsequent overheated components. All devices in a PC receive their DC power via the power supply.

A typical failure of a PC power supply is often noticed as a burning smell just before the computer shuts down. Another problem could be the failure of the vital cooling fan, which allows components in the power supply to overheat. Failure symptoms include random rebooting or failure in Windows for no apparent reason.

For any problems you suspect to be the fault of the power supply, use the documentation that came with your computer. If you have ever removed the case from your personal computer to add an adapter card or memory, you can change a power supply. Make sure you remove the power cord first, since voltages are present even though your computer is off.

Power Supply Improvements

Recent motherboard and chipset improvements permit the user to monitor the revolutions per minute (RPM) of the power supply fan via BIOS and a Windows application supplied by the motherboard manufacturer. New designs offer fan control so that the fan only runs the speed needed, depending on cooling needs.

Recent designs in Web servers include power supplies that offer a spare supply that can be exchanged while the other power supply is in use. Some new computers, particularly those designed for use as servers, provide redundant power supplies. This means that there are two or more power supplies in the system, with one providing power and the other acting as a backup. The backup supply immediately takes over in the event of a failure by the primary supply. Then, the primary supply can be exchanged while the other power supply is in use.

Hard disk drive( HDD )

2009-06-14T14:36:00.006+05:30

Hard disk drive

hard disk which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.^[4] However, in more recent years, there has been a partial return to the use of removable hard disks, such as the Iomega Jaz and Rev drives and disks and the SyQuest SyJet and Sparq drives and disks, and the Castlewood Orb drive and disk, among other models.

History

HDDs (introduced in 1956 as data storage for an IBM accounting computer^[5]) were originally developed for use with general purpose computers. During the 1990s, the need for large-scale, reliable storage, independent of a particular device, led to the introduction of embedded systems such as RAIDs, network attached storage (NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data. In the 21st century, HDD usage expanded into consumer applications such as camcorders, cellphones (e.g. the Nokia N91), digital audio players, digital video players (e.g. the iPod Classic), digital video recorders, personal digital assistants and video game consoles.

Technology

HDDs record data by magnetizing ferromagnetic material directionally, to represent either a 0 or a 1 binary digit. They read the data back by detecting the magnetization of the material. A typical HDD design consists of a spindle which holds one or more flat circular disks called platters, onto which the data are recorded. The platters are made from a non-magnetic material, usually aluminum alloy or glass, and are coated with a thin layer of magnetic material. Older disks used iron(III) oxide as the magnetic material, but current disks use a cobalt-based alloy.^{[citation needed]}

A cross section of the magnetic surface in action. In this case the binary data is encoded using frequency modulation.

The platters are spun at very high speeds. Information is written to a platter as it rotates past devices called read-and-write heads that operate very close (tens of nanometers in new drives) over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. There is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator or in some older designs a stepper motor.

The magnetic recording media are CoCrPt-based magnetic thin films of about 10-20 nm in thickness. The thin films are normally deposited on glass/ceramic/metal substrate and covered by thin carbon layer for protection. The Co-based alloy thin films are polycrystalline and the size of grains has an order of 10 nm. Because the sizes of each grain are tiny, they are typical single domain magnets. The media are magnetically hard (coercivity is about 0.3T) so that a stable remnant magnetization can be achieved. The grain boundaries turn out to be very important. The reason is that, the grains are very small and close to each other, so the coupling between each grains are very strong. When one grain is magnetized, the adjacent grains tend to be aligned parallel to it or demagnetized. Then both the stability of the data and signal-to-noise ratio will be sabotaged. A clear grain boundary can weaken the coupling of the grains and subsequently increase the signal-to-noise ratio. During writing process, ideally one grain can store one bit (1/0). However, current technology can not reach that far yet. In practice, a group of grains (about 100) are magnetized as one bit. So, in order to increase the data density, smaller grains are required. From microstructure point of view, longitudinal and perpendicular recording are the same. Also, similar Co-based thin films are used in both longitudinal and perpendicular recording. However, the fabrication processes are different to gain different crystal structure and magnetic properties. In longitudinal recording, the single-domain grains have uniaxial anisotropy with easy axes lying in the film plane. The consequence of this arrangement is that adjacent magnets repel each other. Therefore the magnetostatic energy is so large that it is difficult to increase areal density. Perpendicular recording media, on the other hand, has the easy axis of the grains oriented perpendicular to the disk plane. Adjacent magnets attract to each other and magnetostatic energy are much lower. So, much higher areal density can be achieved in perpendicular recording. Another unique feature in perpendicular recording is that a soft magnetic underlayer are incorporated into the recording disk.This underlayer is used to conduct writing magnetic flux so that the writing is more efficient. This will be discussed in writing process. Therefore, a higher anisotropy medium film, such as L10-FePt and rare-earth magnets, can be used.

Older drives read the data on the platter by sensing the rate of change of the magnetism in the head; these heads had small coils, and worked (in principle) much like magnetic-tape playback heads, although not in contact with the recording surface. As data density increased, read heads using magnetoresistance (MR) came into use; the electrical resistance of the head changed according to the strength of the magnetism from the platter. Later development made use of spintronics; in these heads, the magnetoresistive effect was much greater than in earlier types, and was dubbed "giant" magnetoresistance (GMR). This refers to the degree of effect, not the physical size, of the head — the heads themselves are extremely tiny, and are too small to be seen without a microscope. GMR read heads are now commonplace.^{[citation needed]}

HD heads are kept from contacting the platter surface by the air that is extremely close to the platter; that air moves at, or close to, the platter speed.^{[citation needed]} The record and playback head are mounted on a block called a slider, and the surface next to the platter is shaped to keep it just barely out of contact. It's a type of air bearing.

The magnetic surface of each platter is conceptually divided into many small sub-micrometre-sized magnetic regions, each of which is used to encode a single binary unit of information. In today's HDDs, each of these magnetic regions is composed of a few hundred magnetic grains. Each magnetic region forms a magnetic dipole which generates a highly localized magnetic field nearby. The write head magnetizes a region by generating a strong local magnetic field. Early HDDs used an electromagnet both to generate this field and to read the data by using electromagnetic induction. Later versions of inductive heads included metal in Gap (MIG) heads and thin film heads. In today's heads, the read and write elements are separate, but in close proximity, on the head portion of an actuator arm. The read element is typically magneto-resistive while the write element is typically thin-film inductive.

In modern drives, the small size of the magnetic regions creates the danger that their magnetic state might be lost because of thermal effects. To counter this, the platters are coated with two parallel magnetic layers, separated by a 3-atom-thick layer of the non-magnetic element ruthenium, and the two layers are magnetized in opposite orientation, thus reinforcing each other.^[7] Another technology used to overcome thermal effects to allow greater recording densities is perpendicular recording, first shipped in 2005, as of 2007 the technology was used in many HDDs.

Modern drives also make extensive use of Error Correcting Codes (ECCs), particularly Reed–Solomon error correction. These techniques store extra bits for each block of data that are determined by mathematical formulas. The extra bits allow many errors to be fixed. While these extra bits take up space on the hard drive, they allow higher recording densities to be employed, resulting in much larger storage capacity for user data.

Architecture

A hard disk drive with the platters and spindle motor hub removed showing the copper colored stator coils surrounding a bearing at the center of the spindle motor. The orange stripe along the side of the arm is a thin printed-circuit cable. The spindle bearing is in the center.

A typical hard drive has two electric motors, one to spin the disks and one to position the read/write head assembly. The disk motor has an external rotor attached to the platters; the stator windings are fixed in place. The actuator has a read-write head under the tip of its very end (near center); a thin printed-circuit cable connects the read-write head to the hub of the actuator. A flexible, somewhat 'U'-shaped, ribbon cable, seen edge-on below and to the left of the actuator arm in the first image and more clearly in the second, continues the connection from the head to the controller board on the opposite side.

The head support arm is very light, but also rigid; in modern drives, acceleration at the head reaches 250 Gs.

Opened hard drive with top magnet removed, showing copper head actuator coil (top right).

The silver-colored structure at the upper left of the first image is the top plate of the permanent-magnet and moving coil motor that swings the heads to the desired position (it is shown removed in the second image). The plate supports a thin neodymium-iron-boron (NIB) high-flux magnet. Beneath this plate is the moving coil, often referred to as the voice coil by analogy to the coil in loudspeakers, which is attached to the actuator hub, and beneath that is a second NIB magnet, mounted on the bottom plate of the motor (some drives only have one magnet).

The voice coil, itself, is shaped rather like an arrowhead, and made of doubly-coated copper magnet wire. The inner layer is insulation, and the outer is thermoplastic, which bonds the coil together after it's wound on a form, making it self-supporting. The portions of the coil along the two sides of the arrowhead (which point to the actuator bearing center) interact with the magnetic field, developing a tangential force that rotates the actuator. Current flowing radially outward along one side of the arrowhead, and radially inward on the other produces the tangential force. (See magnetic field#Force on a charged particle.) If the magnetic field were uniform, each side would generate opposing forces that would cancel each other out. Therefore the surface of the magnet is half N pole, half S pole, with the radial dividing line in the middle, causing the two sides of the coil to see opposite magnetic fields and produce forces that add instead of canceling. Currents along the top and bottom of the coil produce radial forces that do not rotate the head.

Capacity and access speed

It has been suggested that Disk Overhead be merged into this article or section. (Discuss)

PC hard disk drive capacity (in GB). The vertical axis is logarithmic, so the fit line corresponds to exponential growth.

Using rigid disks and sealing the unit allows much tighter tolerances than in a floppy disk drive. Consequently, hard disk drives can store much more data than floppy disk drives and can access and transmit it faster.

As of April 2009^[update], the highest capacity HDDs are 2 TB.^[13]
A typical "desktop HDD" might store between 120 GB and 2 TB although rarely above 500GB of data (based on US market data^[14]) rotate at 5,400 to 10,000 rpm and have a media transfer rate of 1 Gbit/s or higher. Some newer have 3Gbit/s.^{[citation needed]} (1 GB = 10⁹ B; 1 Gbit/s = 10⁹ bit/s)
The fastest “enterprise” HDDs spin at 10,000 or 15,000 rpm, and can achieve sequential media transfer speeds above 1.6 Gbit/s.^[15] and a sustained transfer rate up to 125 MBytes/second.^[15] Drives running at 10,000 or 15,000 rpm use smaller platters to mitigate increased power requirements (due to air drag) and therefore generally have lower capacity than the highest capacity desktop drives.
"Mobile HDDs", i.e., laptop HDDs, which are physically smaller than their desktop and enterprise counterparts, tend to be slower and have lower capacity. A typical mobile HDD spins at 5,400 rpm, with 7,200 rpm models available for a slight price premium. Because of the smaller disks, mobile HDDs generally have lower capacity than the highest capacity desktop drives.

The exponential increases in disk space and data access speeds of HDDs have enabled the commercial viability of consumer products that require large storage capacities, such as digital video recorders and digital audio players.^[16] In addition, the availability of vast amounts of cheap storage has made viable a variety of web-based services with extraordinary capacity requirements, such as free-of-charge web search, web archiving and video sharing (Google, Internet Archive, YouTube, etc.).

The main way to decrease access time is to increase rotational speed, thus reducing rotational delay, while the main way to increase throughput and storage capacity is to increase areal density. Based on historic trends, analysts predict a future growth in HDD bit density (and therefore capacity) of about 40% per year.^[17] Access times have not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.

The first 3.5″ HDD marketed as able to store 1 TB was the Hitachi Deskstar 7K1000. It contains five platters at approximately 200 GB each, providing 935.5 GiB of usable space;^[18] note the discrepancy between its capacity in decimal units (1 TB = 10¹² bytes) and binary units (1 TiB = 1024 GiB = 2⁴⁰ bytes). Hitachi has since been joined by Samsung (Samsung SpinPoint F1, which has 3 × 334 GB platters), Seagate and Western Digital in the 1 TB drive market.^[19]^[20]

As of December 2008 a single 3.5" platter is able to hold 500GB worth of data.^[21]

Form factor	Width	Largest capacity	Platters (Max)
5.25″ FH	146 mm	47 GB^[22] (1998)	14
5.25″ HH	146 mm	19.3 GB^[23] (1998)	4^[24]
3.5″	102 mm	2 TB^[25] (2009)	5
2.5″	69.9 mm	500 GB^[26] (2008)	3
1.8″ (CE-ATA/ZIF)	54 mm	250 GB^[27] (2008)	3
1.3″	43 mm	40 GB^[28] (2007)	1
1″ (CFII/ZIF/IDE-Flex)	42 mm	20 GB (2006)	1
0.85″	24 mm	8 GB^[29] (2004)	1

Capacity measurements

A disassembled and labeled 1997 hard drive. All major components were placed on a mirror, which created the symmetrical reflections.

Raw unformatted capacity of a hard disk drive is usually quoted with SI prefixes (metric system prefixes), incrementing by powers of 1000; today that usually means Gigabytes (GB) and Terabytes(TB). This is conventional for data speeds and memory sizes which are not inherently manufactured in power of two sizes, as RAM and Flash memory are. Hard disks by contrast have no inherent binary size as capacity is determined by number of heads, tracks and sectors.

This can cause some confusion because some operating systems may report the formatted capacity of a hard drive using binary prefix units which increment by powers of 1024; today that would be Gibibytes (GiB) and Tebibytes (TiB). To make matters more confusing, these reports may be mislabeled as GB and TB rather than GiB and TiB.

A one Terabyte (1 TB) disk drive would be expected to hold around 1 trillion bytes (1,000,000,000,000) or 1000 GB; and indeed most 1 TB hard drives will contain slightly more than this number. However some Operating System utilities would report this correctly as around 931 GiB or 953,674 MiB, although the report might be incorrectly displayed as 931 GB or 953,674 MB, using the wrong labels. (The actual number for a formatted capacity will be somewhat smaller still, depending on the file system). The following are all correct, if possibly confusing, ways of reporting one Terabyte.

SI prefixs (Hard Drive)	equivalent	Binary prefixes (OS)	equivalent
1 TB (Terabytes)	1 * (1000 * 1000 * 1000 * 1000)	0.9095 TiB (Tebibytes)	0.9095 * (1024 * 1024 * 1024 * 1024)
1000 GB (Gigabytes)	1000 * (1000 * 1000 * 1000)	931.3 GiB (Gibibytes)	931.3 * (1024 * 1024 * 1024)
1,000,000 MB (Megabytes)	1,000,000 * (1000 * 1000)	953,674.3 MiB (Mebibytes)	953,674.2 * (1024 * 1024)
1,000,000,000 KB (Kilobytes)	1,000,000,000 * (1000)	976,562,500 KiB (Kibibytes)	976,562,500 * (1024)
1,000,000,000,000 B (bytes)	-	1,000,000,000,000 B (bytes)	-

Microsoft Windows reports disk capacity both in a decimal integer to 12 or more digits and in binary prefix units to three significant digits. The numbers will be correct, but the latter value will be mislabled with SI rather than binary prefixes (eg: shown as GB while calculated as GiB).

The capacity of an HDD can be calculated by multiplying the number of cylinders by the number of heads by the number of sectors by the number of bytes/sector (most commonly 512). Drives with the ATA interface and a capacity of eight gigabytes or more behave as if they were structured into 16383 cylinders, 16 heads, and 63 sectors, for compatibility with older operating systems. Unlike in the 1980s, the cylinder, head, sector (C/H/S) counts reported to the CPU by a modern ATA drive are no longer actual physical parameters since the reported numbers are constrained by historic operating-system interfaces and with zone bit recording the actual number of sectors varies by zone. Disks with SCSI interface address each sector with a unique integer number; the operating system remains ignorant of their head or cylinder count.

The old C/H/S scheme has been replaced by logical block addressing. In some cases, to try to "force-fit" the C/H/S scheme to large-capacity drives, the number of heads was given as 64, although no modern drive has anywhere near 32 platters.

Form factors

5¼″ full height 110 MB HDD,
2½″ (8.5 mm) 6495 MB HDD,
US/UK pennies for comparison.

Six hard drives with 8″, 5.25″, 3.5″, 2.5″, 1.8″, and 1″ disks, partially disassembled to show platters and read-write heads, with a ruler showing inches.

Before the era of PCs and small computers, hard disks were of widely varying dimensions, typically in free standing cabinets the size of washing machines (e.g. DEC RP06 Disk Drive) or designed so that dimensions enabled placement in a 19" rack (e.g. Diablo Model 31).

With increasing sales of small computers having built in floppy-disk drives (FDDs), HDDs that would fit to the FDD mountings became desirable, and this led to the evolution of the market towards drives with certain Form factors, initially derived from the sizes of 8", 5.25" and 3.5" floppy disk drives. Smaller sizes than 3.5" have emerged as popular in the marketplace and/or been decided by various industry groups.

8 inch: 9.5 in × 4.624 in × 14.25 in (241.3 mm × 117.5 mm × 362 mm)
In 1979, Shugart Associates' SA1000 was the first form factor compatible HDD, having the same dimensions and a compatible interface to the 8″ FDD.
5.25 inch: 5.75 in × 1.63 in × 8 in (146.1 mm × 41.4 mm × 203 mm)
This smaller form factor, first used in an HDD by Seagate in 1980, was the same size as full height 5¼-inch diameter FDD, i.e., 3.25 inches high. This is twice as high as "half height" commonly used today; i.e., 1.63 in (41.4 mm). Most desktop models of drives for optical 120 mm disks (DVD, CD) use the half height 5¼″ dimension, but it fell out of fashion for HDDs. The Quantum Bigfoot HDD was the last to use it in the late 1990s, with “low-profile” (≈25 mm) and “ultra-low-profile” (≈20 mm) high versions.
3.5 inch: 4 in × 1 in × 5.75 in (101.6 mm × 25.4 mm × 146 mm) = 376.77344 cm³
This smaller form factor, first used in an HDD by Rodime in 1984, was the same size as the "half height" 3½″ FDD, i.e., 1.63 inches high. Today has been largely superseded by 1-inch high “slimline” or “low-profile” versions of this form factor which is used by most desktop HDDs.
2.5 inch: 2.75 in × 0.374–0.59 in × 3.945 in (69.85 mm × 9.5–15 mm × 100 mm) = 66.3575 cm³-104.775 cm³
This smaller form factor was introduced by PrairieTek in 1988; there is no corresponding FDD. It is widely used today for hard-disk drives in mobile devices (laptops, music players, etc.) and as of 2008 replacing 3.5 inch enterprise-class drives. It is also used in the Playstation 3 video game console. Today, the dominant height of this form factor is 9.5 mm for laptop drives, but high capacity drives used to have a height of 12.5 mm. Enterprise-class drives can have a height up to 15 mm.^[30]
1.8 inch: 54 mm × 8 mm × 71 mm = 30.672 cm³
This form factor, originally introduced by Integral Peripherals in 1993, has evolved into the ATA-7 LIF with dimensions as stated. It is increasingly used in digital audio players and subnotebooks. An original variant exists for 2–5 GB sized HDDs that fit directly into a PC card expansion slot. These became popular for their use in iPods and other HDD based MP3 players.
1 inch: 42.8 mm × 5 mm × 36.4 mm
This form factor was introduced in 1999 as IBM's Microdrive to fit inside a CF Type II slot. Samsung calls the same form factor "1.3 inch" drive in its product literature.^[31]
0.85 inch: 24 mm × 5 mm × 32 mm
Toshiba announced this form factor in January 2004^[32] for use in mobile phones and similar applications, including SD/MMC slot compatible HDDs optimized for video storage on 4G handsets. Toshiba currently sells a 4 GB (MK4001MTD) and 8 GB (MK8003MTD) and holds the Guinness World Record for the smallest hard disk drive.

As of 2008, 3.5" and 2.5" hard disks dominate.

By 2009 all manufacturers had discontinued the development of new products for the 1.3-inch, 1-inch and 0.85-inch form factors due to falling prices of flash memory,^[34]^[35]

The inch-based nickname of all these form factors usually do not indicate any actual product dimension (which are specified in millimeters for more recent form factors), but just roughly indicate a size relative to disk diameters, in the interest of historic continuity.

Data transfer rate

As of 2008, a typical 7200rpm desktop hard drive has a sustained "disk-to-buffer" data transfer rate of about 70 megabytes per second. This rate depends on the track location, so it will be highest for data on the outer tracks (where there are more data sectors) and lower toward the inner tracks (where there are fewer data sectors); and is generally somewhat higher for 10,000rpm drives. A current widely-used standard for the "buffer-to-computer" interface is 3.0 Gbit/s SATA, which can send about 300 megabyte/s. from the buffer to the computer, and thus is still comfortably ahead of today's disk-to-buffer transfer rates. Data transfer rate (read/write) can be measured by writing a large file to disk using special file generator tools, then reading back the file. Transfer rate can be influenced by the fragmentation of the drive and the layout of the files. For example, a single file of 10GB will be read significantly faster than 1000 files of 10MB.

Power consumption

Power consumption has become considered increasingly important, not just in mobile devices such as laptops but also in server and desktop markets. Increasing data center machine density has led to problems delivering sufficient power to devices, and getting rid of the waste heat subsequently produced, as well as environmental and electrical cost concerns (see green computing). Similar issues exist for large companies with thousands of desktop PCs. Smaller form factor drives often use less power than larger drives. One interesting development in this area is actively controlling the seek speed so that the head arrives at its destination only just in time to read the sector, rather than arriving as quickly as possible and then having to wait for the sector to come around (i.e. the rotational latency).

Audible noise

Measured in dBA, audible noise is significant for certain applications, such as PVRs, digital audio recording and quiet computers. Low noise disks typically use fluid bearings, slower rotational speeds (usually 5,400 rpm) and reduce the seek speed under load (AAM) to reduce audible clicks and crunching sounds. Drives in smaller form factors (e.g. 2.5 inch) are often quieter than larger drives.

Shock resistance

Shock resistance is especially important for mobile devices. Some laptops now include a motion sensor that parks the disk heads if the machine is dropped, hopefully before impact, to offer the greatest possible chance of survival in such an event. Maximum shock tolerance to date is 350 Gs for operating and 900 Gs for non-operating.

Access and interfaces

This section does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed. (May 2008)

Hard disk drives are accessed over one of a number of bus types, including parallel ATA (P-ATA, also called IDE or EIDE), Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), and Fibre Channel. Bridge circuitry is sometimes used to connect hard disk drives to buses that they cannot communicate with natively, such as IEEE 1394, USB and SCSI.

Back in the days of the ST-506 interface, the data encoding scheme was also important. The first ST-506 disks used Modified Frequency Modulation (MFM) encoding, and transferred data at a rate of 5 megabits per second. Later on, controllers using 2,7 RLL (or just "RLL") encoding increased the transfer rate by 50%, to 7.5 megabits per second; this also increased disk capacity by fifty percent.

Many ST-506 interface disk drives were only specified by the manufacturer to run at the lower MFM data rate, while other models (usually more expensive versions of the same basic disk drive) were specified to run at the higher RLL data rate. In some cases, a disk drive had sufficient margin to allow the MFM specified model to run at the faster RLL data rate; however, this was often unreliable and was not recommended. An RLL-certified disk drive could run on a MFM controller, but with 1/3 less data capacity and speed.

Enhanced Small Disk Interface (ESDI) also supported multiple data rates (ESDI disks always used 2,7 RLL, but at 10, 15 or 20 megabits per second), but this was usually negotiated automatically by the disk drive and controller; most of the time, however, 15 or 20 megabit ESDI disk drives weren't downward compatible (i.e. a 15 or 20 megabit disk drive wouldn't run on a 10 megabit controller). ESDI disk drives typically also had jumpers to set the number of sectors per track and (in some cases) sector size.

Modern hard drives present a consistent interface to the rest of the computer, no matter what data encoding scheme is used internally. Typically a DSP in the electronics inside the hard drive takes the raw analog voltages from the read head and uses PRML and Reed–Solomon error to decode the sector boundaries and sector data, then sends that data out the standard interface. That DSP also watches the error rate detected by error detection and correction, and performs bad sector remapping, data collection for Self-Monitoring, Analysis, and Reporting Technology, and other internal tasks.

SCSI originally had just one signaling frequency of 5 MHz for a maximum data rate of 5 megabytes/second over 8 parallel conductors, but later this was increased dramatically. The SCSI bus speed had no bearing on the disk's internal speed because of buffering between the SCSI bus and the disk drive's internal data bus; however, many early disk drives had very small buffers, and thus had to be reformatted to a different interleave (just like ST-506 disks) when used on slow computers, such as early Commodore Amiga, IBM PC compatibles and Apple Macintoshes.

ATA disks have typically had no problems with interleave or data rate, due to their controller design, but many early models were incompatible with each other and couldn't run with two devices on the same physical cable in a master/slave setup. This was mostly remedied by the mid-1990s, when ATA's specification was standardised and the details began to be cleaned up, but still causes problems occasionally (especially with CD-ROM and DVD-ROM disks, and when mixing Ultra DMA and non-UDMA devices).

Serial ATA does away with master/slave setups entirely, placing each disk on its own channel (with its own set of I/O ports) instead.

FireWire/IEEE 1394 and USB(1.0/2.0) HDDs are external units containing generally ATA or SCSI disks with ports on the back allowing very simple and effective expansion and mobility. Most FireWire/IEEE 1394 models are able to daisy-chain in order to continue adding peripherals without requiring additional ports on the computer itself. USB however, is a point to point network and doesn't allow for daisy-chaining. USB hubs are used to increase the number of available ports and are used for devices that don't require charging since the current supplied by hubs is typically lower than what's available from the built-in USB ports.

Disk interface families used in personal computers

Notable families of disk interfaces include:

Historical bit serial interfaces — connect a hard disk drive (HDD) to a hard disk controller (HDC) with two cables, one for control and one for data. (Each drive also has an additional cable for power, usually connecting it directly to the power supply unit). The HDC provided significant functions such as serial/parallel conversion, data separation, and track formatting, and required matching to the drive (after formatting) in order to assure reliability. Each control cable could serve two or more drives, while a dedicated (and smaller) data cable served each drive.
- ST506 used MFM (Modified Frequency Modulation) for the data encoding method.
- ST412 was available in either MFM or RLL (Run Length Limited) encoding variants.
- Enhanced Small Disk Interface (ESDI) was an interface developed by Maxtor to allow faster communication between the processor and the disk than MFM or RLL.

Modern bit serial interfaces — connect a hard disk drive to a host bus interface adapter (today typically integrated into the "south bridge") with one data/control cable. (As for historical bit serial interfaces above, each drive also has an additional power cable, usually direct to the power supply unit.)
- Fibre Channel (FC), is a successor to parallel SCSI interface on enterprise market. It is a serial protocol. In disk drives usually the Fibre Channel Arbitrated Loop (FC-AL) connection topology is used. FC has much broader usage than mere disk interfaces, it is the cornerstone of storage area networks (SANs). Recently other protocols for this field, like iSCSI and ATA over Ethernet have been developed as well. Confusingly, drives usually use copper twisted-pair cables for Fibre Channel, not fibre optics. The latter are traditionally reserved for larger devices, such as servers or disk array controllers.
- Serial ATA (SATA). The SATA data cable has one data pair for differential transmission of data to the device, and one pair for differential receiving from the device, just like EIA-422. That requires that data be transmitted serially. Similar differential signaling system is used in RS485, LocalTalk, USB, Firewire, and differential SCSI.
- Serial Attached SCSI (SAS). The SAS is a new generation serial communication protocol for devices designed to allow for much higher speed data transfers and is compatible with SATA. SAS uses a mechanically identical data and power connector to standard 3.5" SATA1/SATA2 HDDs, and many server-oriented SAS RAID controllers are also capable of addressing SATA hard drives. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands.

Word serial interfaces — connect a hard disk drive to a host bus adapter (today typically integrated into the "south bridge") with one cable for combined data/control. (As for all bit serial interfaces above, each drive also has an additional power cable, usually direct to the power supply unit.) The earliest versions of these interfaces typically had a 8 bit parallel data transfer to/from the drive, but 16 bit versions became much more common, and there are 32 bit versions. Modern variants have serial data transfer. The word nature of data transfer makes the design of a host bus adapter significantly simpler than that of the precursor HDD controller.
- Integrated Drive Electronics (IDE), later renamed to ATA, with the alias P-ATA ("parallel ATA") retroactively added upon introduction of the new variant Serial ATA. The original name reflected the innovative integration of HDD controller with HDD itself, which was not found in earlier disks. Moving the HDD controller from the interface card to the disk drive helped to standardize interfaces, and to reduce the cost and complexity. The 40 pin IDE/ATA connection transfers 16 bits of data at a time on the data cable. The data cable was originally 40 conductor, but later higher speed requirements for data transfer to and from the hard drive led to an "ultra DMA" mode, known as UDMA. Progressively faster versions of this standard ultimately added the requirement for an 80 conductor variant of the same cable; where half of the conductors provides grounding necessary for enhanced high-speed signal quality by reducing cross talk. The interface for 80 conductor only has 39 pins, the missing pin acting as a key to prevent incorrect insertion of the connector to an incompatible socket, a common cause of disk and controller damage.
- EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement being the use of direct memory access (DMA) to transfer data between the disk and the computer without the involvement of the CPU, an improvement later adopted by the official ATA standards. By directly transferring data between memory and disk, DMA eliminates the need for the CPU to copy byte per byte, therefore allowing it to process other tasks while the data transfer occurs.
- Small Computer System Interface (SCSI), originally named SASI for Shugart Associates System Interface, was an early competitor of ESDI. SCSI disks were standard on servers, workstations, Commodore Amiga and Apple Macintosh computers through the mid-90s, by which time most models had been transitioned to IDE (and later, SATA) family disks. Only in 2005 did the capacity of SCSI disks fall behind IDE disk technology, though the highest-performance disks are still available in SCSI and Fibre Channel only. The length limitations of the data cable allows for external SCSI devices. Originally SCSI data cables used single ended (common mode) data transmission, but server class SCSI could use differential transmission, either low voltage differential (LVD) or high voltage differential (HVD). ("Low" and "High" voltages for differential SCSI are relative to SCSI standards and do not meet the meaning of low voltage and high voltage as used in general electrical engineering contexts, as apply e.g. to statutory electrical codes; both LVD and HVD use low voltage signals (3.3 V and 5 V respectively) in general terminology.)

Acronym or abbreviation	Meaning	Description
SASI	Shugart Associates System Interface	Historical predecessor to SCSI.
SCSI	Small Computer System Interface	Bus oriented that handles concurrent operations.
SAS	Serial Attached SCSI	Improvement of SCSI, uses serial communication instead of parallel.
ST-506	Seagate Technology	Historical Seagate interface.
ST-412	Seagate Technology	Historical Seagate interface (minor improvement over ST-506).
ESDI	Enhanced Small Disk Interface	Historical; backwards compatible with ST-412/506, but faster and more integrated.
ATA	Advanced Technology Attachment	Successor to ST-412/506/ESDI by integrating the disk controller completely onto the device. Incapable of concurrent operations.
SATA	Serial ATA	Modification of ATA, uses serial communication instead of parallel.

Integrity

Due to the extremely close spacing between the heads and the disk surface, any contamination of the read-write heads or platters can lead to a head crash — a failure of the disk in which the head scrapes across the platter surface, often grinding away the thin magnetic film and causing data loss. Head crashes can be caused by electronic failure, a sudden power failure, physical shock, wear and tear, corrosion, or poorly manufactured platters and heads.

The HDD's spindle system relies on air pressure inside the enclosure to support the heads at their proper flying height while the disk rotates. Hard disk drives require a certain range of air pressures in order to operate properly. The connection to the external environment and pressure occurs through a small hole in the enclosure (about 0.5 mm in diameter), usually with a carbon filter on the inside (the breather filter).^{[citation needed]} If the air pressure is too low, then there is not enough lift for the flying head, so the head gets too close to the disk, and there is a risk of head crashes and data loss. Specially manufactured sealed and pressurized disks are needed for reliable high-altitude operation, above about 3,000 m (10,000 feet). Note that modern commercial aircraft have a pressurized cabin, whose pressure altitude does not normally exceed 2,600 m(8,500 feet) - thus, ordinary hard drives can safely be used in flight.^{[citation needed]} Modern disks include temperature sensors and adjust their operation to the operating environment. Breather holes can be seen on all disk drives — they usually have a sticker next to them, warning the user not to cover the holes. The air inside the operating drive is constantly moving too, being swept in motion by friction with the spinning platters. This air passes through an internal recirculation (or "recirc") filter to remove any leftover contaminants from manufacture, any particles or chemicals that may have somehow entered the enclosure, and any particles or outgassing generated internally in normal operation. Very high humidity for extended periods can corrode the heads and platters.

For giant magnetoresistive (GMR) heads in particular, a minor head crash from contamination (that does not remove the magnetic surface of the disk) still results in the head temporarily overheating, due to friction with the disk surface, and can render the data unreadable for a short period until the head temperature stabilizes (so called "thermal asperity", a problem which can partially be dealt with by proper electronic filtering of the read signal).

Actuation of moving arm

The hard drive's electronics control the movement of the actuator and the rotation of the disk, and perform reads and writes on demand from the disk controller. Feedback of the drive electronics is accomplished by means of special segments of the disk dedicated to servo feedback. These are either complete concentric circles (in the case of dedicated servo technology), or segments interspersed with real data (in the case of embedded servo technology). The servo feedback optimizes the signal to noise ratio of the GMR sensors by adjusting the voice-coil of the actuated arm. The spinning of the disk also uses a servo motor. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media which have failed.

Landing zones and load/unload technology

Modern HDDs prevent power interruptions or other malfunctions from landing its heads in the data zone by parking the heads either in a landing zone or by unloading (i.e., load/unload) the heads. Some early PC HDDs did not park the heads and they could land on data.

A landing zone is an area of the platter usually near its inner diameter (ID), where no data is stored. This area is called the Contact Start/Stop (CSS) zone. Disks are designed such that either a spring or, more recently, rotational inertia in the platters is used to park the heads in the case of unexpected power loss. In this case, the spindle motor temporarily acts as a generator, providing power to the actuator.

Spring tension from the head mounting constantly pushes the heads towards the platter. While the disk is spinning, the heads are supported by an air bearing and experience no physical contact or wear. In CSS drives the sliders carrying the head sensors (often also just called heads) are designed to survive a number of landings and takeoffs from the media surface, though wear and tear on these microscopic components eventually takes its toll. Most manufacturers design the sliders to survive 50,000 contact cycles before the chance of damage on startup rises above 50%. However, the decay rate is not linear: when a disk is younger and has had fewer start-stop cycles, it has a better chance of surviving the next startup than an older, higher-mileage disk (as the head literally drags along the disk's surface until the air bearing is established). For example, the Seagate Barracuda 7200.10 series of desktop hard disks are rated to 50,000 start-stop cycles, in other words no failures attributed to the head-platter interface were seen before at least 50,000 start-stop cycles during testing.^[38]

Around 1995 IBM pioneered a technology where a landing zone on the disk is made by a precision laser process (Laser Zone Texture = LZT) producing an array of smooth nanometer-scale "bumps" in a landing zone,^[39] thus vastly improving stiction and wear performance. This technology is still largely in use today (2008), predominantly in desktop and enterprise (3.5 inch) drives. In general, CSS technology can be prone to increased stiction (the tendency for the heads to stick to the platter surface), e.g. as a consequence of increased humidity. Excessive stiction can cause physical damage to the platter and slider or spindle motor.

Load/Unload technology relies on the heads being lifted off the platters into a safe location, thus eliminating the risks of wear and stiction altogether. The first HDD RAMAC and most early disk drives used complex mechanisms to load and unload the heads. Modern HDDs use ramp loading, first introduced by Memorex in 1967,^[40] to load/unload onto plastic "ramps" near the outer disk edge.

All HDDs today still use one of these two technologies listed above. Each has a list of advantages and drawbacks in terms of loss of storage area on the disk, relative difficulty of mechanical tolerance control, non-operating shock robustness, cost of implementation, etc.

Addressing shock robustness, IBM also created a technology for their ThinkPad line of laptop computers called the Active Protection System. When a sudden, sharp movement is detected by the built-in accelerometer in the Thinkpad, internal hard disk heads automatically unload themselves to reduce the risk of any potential data loss or scratch defects. Apple later also utilized this technology in their PowerBook, iBook, MacBook Pro, and MacBook line, known as the Sudden Motion Sensor. Sony,^[41] HP with their HP 3D DriveGuard^[42] and Toshiba^[43] have released similar technology in their notebook computers.

This accelerometer based shock sensor has also been used for building cheap earthquake sensor networks.

Disk failures and their metrics

ost major hard disk and motherboard vendors now support S.M.A.R.T. (Self-Monitoring, Analysis, and Reporting Technology), which measures drive characteristics such as temperature, spin-up time, data error rates, etc. Certain trends and sudden changes in these parameters are thought to be associated with increased likelihood of drive failure and data loss.

However, not all failures are predictable. Normal use eventually can lead to a breakdown in the inherently fragile device, which makes it essential for the user to periodically back up the data onto a separate storage device. Failure to do so will lead to the loss of data. While it may sometimes be possible to recover lost information, it is normally an extremely costly procedure, and it is not possible to guarantee success. A 2007 study published by google suggested very little correlation between failure rates and either high temperature or activity level; however, the correlation between manufacturer/model and failure rate was relatively strong. Statistics in this matter is kept highly secret by most entities. Google did not publish the manufacturer's names along with their respective failure rates, though they have since revealed that they use Hitachi Deskstar drives in some of their servers. While several S.M.A.R.T. parameters have an impact on failure probability, a large fraction of failed drives do not produce predictive S.M.A.R.T. parameters. S.M.A.R.T. parameters alone may not be useful for predicting individual drive failures.

A common misconception is that a colder hard drive will last longer than a hotter hard drive. The Google study seems to imply the reverse -- "lower temperatures are associated with higher failure rates". Hard drives with S.M.A.R.T.-reported average temperatures below 27 °C had failure rates worse than hard drives with the highest reported average temperature of 50 °C, failure rates at least twice as high as the optimum S.M.A.R.T.-reported temperature range of 36 °C to 47 °C.

SCSI, SAS and FC drives are typically more expensive and are traditionally used in servers and disk arrays, whereas inexpensive ATA and SATA drives evolved in the home computer market and were perceived to be less reliable. This distinction is now becoming blurred.

The mean time between failures (MTBF) of SATA drives is usually about 600,000 hours (some drives such as Western Digital Raptor have rated 1.2 million hours MTBF), while SCSI drives are rated for upwards of 1.5 million hours.^{[citation needed]} However, independent research indicates that MTBF is not a reliable estimate of a drive's longevity.^[47] MTBF is conducted in laboratory environments in test chambers and is an important metric to determine the quality of a disk drive before it enters high volume production. Once the drive product is in production, the more valid metric is annualized failure rate (AFR).^{[citation needed]} AFR is the percentage of real-world drive failures after shipping.

SAS drives are comparable to SCSI drives, with high MTBF and high reliability.^{[citation needed]}

Enterprise S-ATA drives designed and produced for enterprise markets, unlike standard S-ATA drives, have reliability comparable to other enterprise class drives.^[48]^[49]

Typically enterprise drives (all enterprise drives, including SCSI, SAS, enterprise SATA and FC) experience between 0.70%-0.78% annual failure rates from the total installed drives.^{[citation needed]}

Eventually all mechanical hard disc drives fail. And thus the strategy to mitigate loss of data is to have redundancy in some form, like RAID and backup. RAID should never be relied on as backup, as RAID controllers also break down, making the disks inaccessible. Following a backup strategy, like daily differential and weekly full backups, is the only sure way to prevent data loss.

Manufacturers

The technological resources and know-how required for modern drive development and production mean that as of 2009, virtually all of the world's HDDs are manufactured by just five large companies: Seagate, Western Digital, Hitachi (which owns the former disk manufacturing division of IBM), Samsung, and Toshiba.

Dozens of former HDD manufacturers have gone out of business, merged, or closed their HDD divisions; as capacities and demand for products increased, profits became hard to find, and the market underwent significant consolidation in the late 1980s and late 1990s. The first notable casualty of the business in the PC era was Computer Memories Inc. or CMI; after an incident with faulty 20 MB AT disks in 1985,^[50] CMI's reputation never recovered, and they exited the HDD business in 1987. Another notable failure was MiniScribe, who went bankrupt in 1990 after it was found that they had engaged in accounting fraud and inflated sales numbers for several years. Many other smaller companies (like Kalok, Microscience, LaPine, Areal, Priam and PrairieTek) also did not survive the shakeout, and had disappeared by 1993; Micropolis was able to hold on until 1997, and JTS, a relative latecomer to the scene, lasted only a few years and was gone by 1999, after attempting to manufacture HDDs in India. Their claim to fame was creating a new 3″ form factor drive for use in laptops. Quantum and Integral also invested in the 3″ form factor; but eventually ceased support as this form factor failed to catch on. Rodime was also an important manufacturer during the 1980s, but stopped making disks in the early 1990s amid the shakeout and now concentrates on technology licensing; they hold a number of patents related to 3.5-inch form factor HDDs.

Motherboard Repairing

2009-06-14T14:32:00.000+05:30

Motherboard Repair

I had stopped repairing PC motherboard since i left my former computer distributor company back in 1994. Those days the famous motherboard board were the 486sx, 486dx and the Pentium. I was provided with every model of the motherboard schematic diagrams, Chipset (all range of IC’s), SMD desoldering tools, troubleshooting notes and even the POST diagnostic card. Repairing PC motherboards on those days were quite easy but i doubt nowadays it will be easy unless you are working in the motherboard manufacturers or you have the sources for all kind information related to motherboard.

Occasionally i do have motherboard sent for repair by my friend who deal with selling computer .Most of the time was the electrolytic capacitors bulged and quite simple to fix it. Nowadays PC motherboard is quite cheap and less people want to repair it and prefer to buy a new board unless if the motherboard is in the higher range (branded). As mentioned, if you can’t source for the parts it would be very difficult to repair the board. Those days the boards were only using normal SMD IC’s (spider IC’s) but most of the motherboard now are using the BGA IC’s which is very difficult to extract unless you have the special equipment to do the job.

It will be very time consuming to do this precision job unless the profit that you gonna make could cover your cost otherwise it would not be economical to repair the motherboard. If any of the readers in my blog repair motherboard i would like to listen to what you have to say about motherboard repairing business-thanks

Mother Board

2009-06-14T14:28:00.001+05:30

What is a Motherboard?

The motherboard is the main circuit board inside your PC. Every components at some point communicates through the motherboard, either by directly plugging into it or by communicating through one of the motherboards ports. The motherboard is one big communication highway. Its purpose inside your PC is to provide a platform for all the other components and peripherals to talk to each other.

Types of Motherboards

The type of motherboards depends on the CPU it was designed for. You can therefore categories motherboards by which socket type they have. e.g. Socket A, Socket 478 etc. The Type of motherboard you buy is very important, as it will need to house your CPU, and they are not interchangeable. When buying a motherboard, it will always tell you what socket type it has.

When Buying a Motherboard

As everything you have on the PC at some point needs the motherboard, you need to consider these components when buying a motherboard. Foe example, if you have a lot of devices with a PCI interface that you wish to use, there is little point buying a motherboard that only offers you 3 PCI slots. Like wise with memory, you have to make sure that there are enough slots for the amount of memory you have or wish to have.

The motherboard also needs the correct type of interface for your Memory, Graphics card, Hard disks and other items as well. You will find that most motherboards offer everything you need however it needs checking on when buying. Its especially important to pay detail to your motherboard if you want to use older components, which a new motherboard may or may not support.

The major difference between motherboards that support the same CPU is the model of the chipset (more on the chipset later). Different chipsets offer different performance and different features in terms of memory support, AGP port speed, Multiplier numbers, Bus speeds and much more.

Speed of a Motherboard

Motherboards have got to be one of the hardest components to measure the speed of. Performance can really only be measured by benchmarking using the same components in several motherboards of the same type. You often find that motherboards with the same chipset have roughly the same performance in real world tests. The minor differences that do occur are down to the quality of the materials used and the quality of the manufacturing.

The Motherboards speeds that are quoted on the box are maximum supported speeds for other components. For example motherboards will quote the maximum FSB (Front Side Bus) speed. However without a CPU that also supports this speed, it will never be reached. Likewise when it quotes the maximum memory speed. The memory of this speed has to be present.

What is a Motherboard Chipset?

A motherboard chipset controls all the data that flows through the data channels (buses) of the motherboard. The primary function of the motherboard chipset is to direct this data to the correct area's of the motherboard, and therefore the correct components.

Components of a Motherboard

The motherboard contains many connections for all type of components. Motherboards contain expansion slots such as the ISA, PCI, AGP and DIMM sockets. It also contains external connections for your onboard sound card, USB ports, Serial and Parallel ports, PS/2 ports for your keyboard and mouse as well as network and Firewire connections.

So the motherboard has a massive part to play in the workings of your PC. Components that you buy all rely on the motherboard to have the correct connections are available and working. Its best to buy a decent motherboard especially if you plan on buying extra's in the future.

How does a computer work ?

2009-06-14T13:55:00.001+05:30

Computer hardware. A computer consists of a few basic parts: the CPU, memory, disk
space, power supply, and IO (input/output) devices. The most important is the CPU
(Central Processing Unit), which is the “brains” of the computer. Computers also have
short-term memory (RAM) and long-term storage (disk drive). Short-term memory works
only when the computer is turned on while long-term storage (usually disk drives) saves
information even if powered off. Information flows from the CPU and memory via a set
of wires referred to as a bus. Most computers also possess some sort of monitor and a
variety of input and output ports (USB, network, etc) as well as a keyboard and mouse.
All the devices are connected to the motherboard. Power is supplied by the power supply
and usually cooling of the CPU and other parts is required.
The speed of a computer is controlled by the CPU which in turn depends on the
type of chips inside. Generally, speaking, the newer the CPU the faster the computer.
Most PC’s and Mac only have one CPU at present, but some larger computers may have
more (usually added as multiples of two). The amount of memory (not disk space) also
affects how fast a computer will work with some programs. Trying to run too many
programs at the same time will cause memory access to be inefficient and may even crash
the computer.
Binary numbers. Computers deal
with numbers. Binary numbers, to be
specific. Binary numbers are
composed of only two possible
digits: 0 or 1. This contrasts to the
usual way of counting (base 10, or
decimal) which uses the digits 0, 1,
2, 3, 4, 5, 6, 7, 8, and 9. The binary
number 0 corresponds to decimal
“0”, 1 is “1”, but decimal 2 is “10” in
binary (3 as “11”, 4 is “100” and so
on). It is possible to convert from
one system to the other and
computers do this frequently and
automatically. Computers use binary
numbers because it is easier to
devise electronics to handle “0” or
“1”’s than to deal with the decimal
system. Modern humans tend to use
base 10 because binary numbers get
very long very quickly and are hard
to remember (another possible
system is hexadecimal, which is base
16 and uses the numbers 0-9 and the
letters A through F).
So, the text message you sent
Figure 1. An example of how computers use binary
arithmetic is in disk drives, which are composed of an
extremely large number of small magnetic particles.
These magnetic particles can point “normal” or
“reverse” and can be switched from one to another.
(similar to the magnetic polarity fields recorded by
ocean floor basalt during seafloor spreading, but on a
smaller scale). In a disk drive, then, a “reverse” domain
would correspond to the binary number “1” and
“normal” would correspond to “0”. In this way binary
numbers can be recorded on a magnetic surface. Later,
this surface can be read by moving a small sensor over
the surface to measure the polarity of the magnetic field
over each particle. In disk drives, the read head(s) are
fixed while the disk spins rapidly underneath. Below,
the magnetic pattern produce by reversals on a seafloor
is analogous to a disk drive.
2
this morning, the image on the webpage, the music on the iPod, all were composed of just
1’s and 0’s. But why should we care? One reason is how computers store numbers is
essential to most programming and numerical analysis.
The first step in storing numbers on a computer is to define the basic building
block. The smallest block is a one or zero and is referred to as a “bit” (binary digit). All
the ones and zeros (“bits”) on the disk drive must be separated somehow, otherwise it
would all blur into one long number. Consequently, the bits are divided into groups of
eight, which are referred to as a “byte” – one byte equals eight bits. What can a byte
hold? Well, it can hold all possible combinations of eight bits, from [0,0,0,0,0,0,0,0] to
[1,1,1,1,1,1,1,1]. How many combinations are possible? 28, or 256. If we convert to
decimal, one byte can hold the integer numbers from 0 to 255. Often, this is all you need.
For example, an Internet address consists of four one-byte numbers (32 bits) and so the
number of possible Internet addresses is 256*256*256*256 which is a little over 4
billion. Storage and memory is usually measured in megabytes (~1 million bytes) or
gigabytes (~1 billion bytes) [confusingly, the exact number of bytes in a megabyte is
sometimes defined as 1,048,576].
Clearly, we often need to store large numbers than 255. If we use two bytes, we
now have16 bits, or 216 (65536) possible numbers. Attempting to place a number larger
than 65536 into a two-byte integer will not work and will result in an error. This way of
representing numbers is referred to as integers. If we use the first bit as a “sign” bit to
indicate positive or negative, we can accommodate negative integers.
Note that integers are “whole” numbers and cannot represent factions. But we
can’t always work with whole numbers – decimals and fractions are important. These are
usually represented by using a combination of four bytes to represent a number and a
multiplication factor, similar to scientific notation (0.25 is the same as (25)(10-2). This is
called a “floating point” number. Like integers, floating point numbers also have upper
limits, which differ on different machines. Note that it is possible to store a whole
number “integer” as a floating point number. The drawback is that they require more
memory to store and effort to use in computations.
One important problem emerges with floating point numbers – not all numbers
can be expressed exactly as decimals. For example, one third in decimal is the number
0.333333……where the threes go on forever. Since most computers has finite storage,
numbers like this have to be rounded off. In normal usage, rounding occurs after about 7
decimal digits. If extra precision is desired, programmers can use double precision, which
is accurate to about 16 decimal digits. In cases where a calculation is repeated many
times and the results put back into the calculation (perhaps in trying to model
groundwater flow over a long distance, or seismic waves from an earthquake), these
errors will get larger and larger and must be handled carefully.
Undetected errors can be catastrophic. The European Space Agency spent 10
years and $7 billion to produce their own space rocket, the Ariane 5. One minute after
launch it went off course and blew up. Why? The guidance software, which worked
flawlessly for Ariane 4, failed because Ariane 5 was faster. Ariane 5’s velocity exceeded
the two byte signed integer value allocated for it. This produced garbage numbers, which
confused the guidance system, which caused the rocket to go off course and explode. As
lead programmer Homer S. commented later, "Doh!." A more serious programming error
occurred in 1991, during the First Gulf War. A Patriot anti-missile unit relied on an
3
internal clock which kept count in seconds. Unfortunately, rounding errors lead to an
error of 0.34 seconds after about 1000 hours of operation. This error meant that this
particular Patriot unit failed to shoot down an incoming Scud missile, which then hit a
U.S. army barracks and killed 28 soldiers. A final, geology-related, example is the $700
million Sleipner oil platform in the North Sea, which sank on assembly in the North Sea
due to an error in design due to faulty software.
Review questions.
1. Write out the numbers 0-16 in decimal and binary.
2. The number of rows in an Excel spreadsheet (at least, the version I use) cannot exceed
65,536. Why this particular number?
3. How many different numbers can a one byte integer with a sign bit hold?
Digital and analog. Analog transmits or copies information by encoding it directly. An
old-timey phonograph record stores music by directly carving the vibrations onto a flat
vinyl disk. Digital means using numbers to represent information. Music is first
converted to numbers and then stored. Somewhat surprisingly, a perfect analog record is
in theory better than digital, as conversion to numbers almost always creates some error.
The primary advantage of digital is that can be manipulated by a computer and
allows error checking. Error checking means that when we receive a digital signal in the
raw state, we know that the values are either 1 or 0. A simple, grossly simplified example
might help. Suppose we install a seismometer on a remote, sheep-infested mountaintop in
Central Asia and that the waves recorded by the seismometers are converted to binary
numbers and sent out via a small 5 watt radio powered by solar cells. At the beginning of
an earthquake, the numbers might be something like [0.0, 0.0, 1.0, 1.0, 1.0].
Unfortunately, interference from a nearby military 50,000 watt radar unit is interfering
with our radio signals and at our central receiving site instead of [0.0, 0.0, 1.0, 1.0, 1.0]
we get [0.3, 0.2, 0.9, 1.1, 1.3]. Since we know the individual numbers must be 0 or 1, we
can correct our numbers at the central receive site to get the correct answer [0.0, 0.0, 1.0,
1.0, 1.0] (it should be pointed out that actual digital radio is more complicated than this).
Errors cannot be detected or fixed with analog transmission. If noise corrupts an analog
signal, too bad. While we have become accustomed to everything being digital, the
Apollo mission to the moon were done with almost all analog instruments, computers at
that time were heavy and bulky. Currently, only radios, TV’s and some telephones are
analog but are rapidly changing to digital.
4
However, most signals start out as analog (voice, music, etc) and must be
converted to digital somehow. The usual method is to measure the signal at regularly
spaced intervals. The length of the interval depends on the frequency of the signal. For
example, if we wanted to make a digital record of sea level, we could go down to the
beach and measure the height of the water every day at midnight. Each measurement is
commonly called a sample even though the water is only measured. However, if we later
plotted the sea level using those numbers, the results would be misleading (Figure 2).
Note that the sampling completely missed the daily variation due to tides. Instead, it
appears that sea level is gradually rising over a few days. This effect, which is caused by
an inadequate sample rate, is known as aliasing because it makes one signal look like
another. To accurately capture the variation, we need to sample faster than the signal is
changing. How fast? To ensure an adequate representation, the signal must be sampled
twice as fast as the highest frequency. This is known as the Nyquist theorem. Because the
tide changes twice pre day, the Nyquist theorem says that it must be sampled four times
per day to capture the variation. The Nyquist theorem is important in capturing audio as
well. Normal (young) human ears can hear frequencies up to about 20,000 Hz. The
Nyquist theorem says that this signal would have to be sampled at twice that to accurately
reproduce it. Therefore, in theory music needs to be digitized at 40,000 Hz (or 40,000
Figure 2. Example of insufficient sampling. Sea level is measured every 24 hours
and yields an inaccurate portrayal of true sea level because of tides. If we assume
two high tides per day, we would need to sample at least 4 times per day to record
the sea level based on the Nyquist theorem.
5
samples per second) to reproduce exactly what is heard live. If we assume that each
sample requires a two-byte integer, then each second of music requires (2)(40,000) bytes
of storage and a minute would require (60)(2)(40,000) bytes (or about 4.8 megabytes). If
each sample was stored using floating point representation (4 bytes per sample), then one
minute would require about 9.6 mb. Music files do not typically record these high
frequencies and therefore are much smaller (and may be compressed as well).
Sampling theory applies to spatial variations as well as time series. If a
geochemist is trying to find a 1 km wide diamond pipe, it is necessary to take samples
with a grid spacing less than 1 km to guarantee success.
Characters and ASCII. The preceding section discussed numbers on computers and
sidetracked a little into sample theory. The claim was made that computers only can
handle numbers. But words are used all the time on computers and on digital telephones.
How are letters represented? The answer is to assign a number to each letter, and then
save the text as a sequence of numbers. The most standard conversion is American
Standard Code for Information Interchange, or ASCII (other, older standards do exist but
are incompatible with most computers). Standard ASCII encodes only 127 characters –
i.e. one character per byte. These include the letters A-Z and a-z (uppercase and
lowercase), as well as punctuation and numerals (0-9). Special ASCII characters that are
not printable exist for control of devices such as printers and modems. More recently,
encoding forms such as Unicode expand traditional ASCII to include other symbols (such
as those used in other languages).
The advantage of ASCII is that is understood by almost any computer. An ASCII
file can be read on a PC, Mac, or UNIX computer without a special program. Email is
commonly in ASCII. A difficulty with ASCII is that does not contain much formatting
information other than line breaks and tabs. To save text with other formats usually
requires special programs such as Microsoft Word or other text-processing software.
Unfortunately, a Microsoft Word file requires Microsoft Word to read it (or another
program designed to read word formats).
Review questions.
1) Write out the ASCII equivalent of your name.
2) List three different ways the number “1” can be stored on a computer.
Files. We now know how computers store numbers and letters. Usually, groups
(sometimes large groups) of numbers and letters are stored as a file. A file is defined by a
sequence of bytes that ends in a special character named “EOF” (End Of File). Files are
the building block for most computer systems but the exact format may vary depending
on the type (Unix, Mac, PC) of computer system.
A special type of file that tells the computer how to do something is called a
program. At the very lowest level, a program is a series of bits that the computer
interprets as instructions (“machine code”). Creating a program at this level is very
difficult and consequently, a program is usually created by writing text in a higher-level
language (such as Fortran or C) and using a program called a compiler to convert it into a
format that the computer can understand (the intermediate step between a higher-level
language and machine code is called assembly language). Programs are very dependent
6
on the type of computer – a program that works on a Mac will not work on a PC, for
example.
Other types of files are used to represent pictures or text. Different types of files
are often distinguished by the suffix on the file name. A file named fred.doc is likely a
Microsoft word document and fred.jpg is likely a picture. However, changing the file
name does not change the underlying file format – if I simply rename fred.doc to fred.jpg
it will not change the word document to a picture but may confuse the computer.
Images. Again, a picture must be converted into numbers in some way and a wide
variety of ways are available. The method chosen often depends on the type of image.
One method is to break up the image into many tiny squares called pixels. Each pixel is
then assigned a number or number(s) based on the color and brightness of the pixel. For
example, a black and white image might use one byte per pixel, with the number 0
representing pure black and 255 pure white and varying shades of gray. Color images
require three bytes per pixel with one byte for shades of red, one for shades of green, and
one for shades of blue (or RGB color scheme). This is typically used for monitors and
web-pages. Printers tend to use a CYMK scheme. Representing an image by specifying
the color of exact pixel is called a “bitmap” or raster image (scanner and digital cameras
create raster images). Raster images work well for photographs and similar images. A
disadvantage is that high resolution requires large files, even if most of the image is the
same color. Scanned in text often looks rough and pixilated.
An alternate method is to represent a drawing by telling the computer to draw
lines between points using a coordinate system. For example, [100 50 moveto; 200 75
lineto; stroke] would tell the computer to go to the point (100,50) and then draw a line
from there to the point (200, 75) using the postscript language. Text is not represented as
pixels but rather as defined letters (fonts). This type of representing images is known as
vector graphics.
The best type of representation depends on the type of image. Digital photographs
are best if saved as raster images; simple black and white line drawings and text are saved
most efficiently as vector images. Modern graphics languages such as PDF allow for both
representations. Some confusion exists because many raster file formats exists including
tiff, jpg, gif, and png. Each format saves a raster image slightly differently. Note that, as
before, changing the suffix does not necessarily change the format.
Compression is a way of reducing the size of a file by looking for repeated
patterns and then substituting for each occurrence with a defined but smaller pattern.
Some compression techniques are lossless, i.e. compressing and uncompressing a file
results in the exact same file. Other compression techniques are referred to as lossy
because some information is lost forever. "jpg" files are a form of lossy compression but
is used because is very good at reducing file size and most of the time the difference is
difficult to tell. But black and white text converted to jpg will look smeared out, for
example.
Operating systems. Over the years, various ways of handling files on computers have
evolved (and some operating systems or OS, to the regret of those who spent hours, days,
weeks, and months learning them, have gone extinct). Currently, the main varieties are
Windows (proprietary, owned by Microsoft, and cost money, at least in the US), Unix
7
(free to anyone), and MacIntosh (proprietary). Strictly speaking, Mac is now based on
Unix but is essentially a different OS now. Each type of OS has several slightly different
versions. Windows is mostly XP now, but a few computers still run Windows 2000 or
Windows 98, and a new version, Vista, is just coming out. Unix, is the oldest and
simplest. The most common type of Unix is Linux, which refers to Unix that runs on a
PC. Various flavors of Linux, (Redhat, Suse, etc) exist but are all similar.
The operating system controls how files are handled. Two basic types exist:
command or graphical. A command driven system requires the user to type in
instructions while a graphical system uses a device such as a mouse. The commanddriven
version of Window is DOS (and can still be accessed at the start-run-command).
Mouse driven systems were invented by Xerox, adopted by Apple in the 1980’s and then
took over everywhere. Mouse driven systems are easier to learn but more inefficient than
command line systems because good typists can move single fingers faster than their
entire hand.
Computer security. Perhaps the most important topic is computer security. Computers
hold information. Information can be useful. And profitable, especially when the
information is bank account information or private documents (such as grades or answers
to a final exam). The advent of the worldwide Internet means that in principle, anyone
anywhere can gain access to any other information. Unfortunately, when the first
computers networks and operating systems were developed, security was not a primary
concern. This has lead to a serious and ongoing security problem.
One of the first “malware” (or malicious software) events was the CREEPER
virus which infected the ARPANET, a forerunner of the Internet, in the early 1970’s. The
first major virus (a program capable of self-replication) occurred in 1987 when the
“Jerusalem virus” would delete all programs from a PC on Friday the 13th. These early
viruses were usually spread by transfer from floppy disks. The first major Internet virus
was a Unix worm in 1988 written by Robert Morris, son of the chief scientist of the NSA
and now professor at MIT. But malware did not become a major problem until the early
2000’s, when several viruses (such as “loveletter”,”mydoom”, “sobig”) swamped the
Internet and caused billions of dollars in damage in lost time. At one point, it was
estimated that 1 in 5 emails were generated by the mydoom virus. This generation of
viruses was written by professional programmers on purpose, and not just for “fun” as
had most previous malware. Most spread via email after people clicked on attachments.
Currently, popular methods of attack included spreading via Internet into
unguarded ports, download from websites, or transfer from portable media such as flash
drives. In most cases but not all, it is necessary to click on button for a website to cause
damage. The button is disguised in some fashion and usually does not say “click here to
download virus”. Avoid disreputable sites (free music, free movies, video codecs, people
with scanty clothing, etc) although even apparently reputable ones can get hacked. For
example, visitors to Paris Hilton’s website saw a pop-up that asked to install display
software. If the answer was yes, the site downloaded a Trojan capable of stealing bank
account information. The goal of the software is to obtain information or send out emails
and spam. The best defense is to use anti-virus software and to update the operating
system frequently. Unfortunately, a single anti-virus program will not detect all threats,
as most viruses automatically change themselves to avoid detection and some will
8
attempt to block anti-virus websites. A recent trend is the creation of “botnets”, remotely
controlled networks of computers to send spam and perform other tasks, such as target
computer security researchers. Typically, most users will be completely unaware that
their computer is infected.

What is a computer ?

2009-06-14T13:49:00.002+05:30

What is a computer ?
A programmable machine. The two principal characteristics of a computer are:

It responds to a specific set of instructions in a well-defined manner.

It can execute a prerecorded list of instructions (a program).

Modern computers are electronic and digital. The actual machinery -- wires, transistors, and circuits -- is called hardware; the instructions and data are called software.

All general-purpose computers require the following hardware components:

memory : Enables a computer to store, at least temporarily, data and programs.

mass storage device : Allows a computer to permanently retain large amounts of data. Common mass storage devices include disk drives and tape drives.

input device : Usually a keyboard and mouse, the input device is the conduit through which data and instructions enter a computer.

output device : A display screen, printer, or other device that lets you see what the computer has accomplished.

central processing unit (CPU): The heart of the computer, this is the component that actually executes instructions.

In addition to these components, many others make it possible for the basic components to work together efficiently. For example, every computer requires a bus that transmits data from one part of the computer to another.

Computers can be generally classified by size and power as follows, though there is considerable overlap:

personal computer : A small, single-user computer based on a microprocessor. In addition to the microprocessor, a personal computer has a keyboard for entering data, a monitor for displaying information, and a storage device for saving data.

workstation : A powerful, single-user computer. A workstation is like a personal computer, but it has a more powerful microprocessor and a higher-quality monitor.

minicomputer : A multi-user computer capable of supporting from 10 to hundreds of users simultaneously.

mainframe : A powerful multi-user computer capable of supporting many hundreds or thousands of users simultaneously.

supercomputer : An extremely fast computer that can perform hundreds of millions of instructions per second.

Inductor (coil)

2009-06-11T13:42:00.001+05:30

Inductor (miniature)

circuit symbol

Inductor (coil)

An inductor is a coil of wire which may have a core of air, iron or ferrite (a brittle material made from iron). Its electrical property is called inductance and the unit for this is the henry, symbol H. 1H is very large so mH and µH are used, 1000µH = 1mH and 1000mH = 1H. Iron and ferrite cores increase the inductance. Inductors are mainly used in tuned circuits and to block high frequency AC signals (they are sometimes called chokes). They pass DC easily, but block AC signals, this is the opposite of capacitors.

Inductors are rarely found in simple projects, but one exception is the tuning coil of a radio receiver. This is an inductor which you may have to make yourself by neatly winding enamelled copper wire around a ferrite rod. Enamelled copper wire has very thin insulation, allowing the turns of the coil to be close together, but this makes it impossible to strip in the usual way - the best method is to gently pull the ends of the wire through folded emery paper.

Warning: a ferrite rod is brittle so treat it like glass, not iron!

An inductor may be connected either way round and no special precautions are required when soldering.

Buzzer and Bleeper

2009-06-11T13:40:00.000+05:30


Buzzer (about 400Hz)	Bleeper (about 3kHz)
circuit symbol

Buzzer and Bleeper

These devices are output transducers converting electrical energy to sound. They contain an internal oscillator to produce the sound which is set at about 400Hz for buzzers and about 3kHz for bleepers.

Buzzers have a voltage rating but it is only approximate, for example 6V and 12V buzzers can be used with a 9V supply. Their typical current is about 25mA.

Bleepers have wide voltage ranges, such as 3-30V, and they pass a low current of about 10mA.

Buzzers and bleepers must be connected the right way round, their red lead is positive (+).

Loudspeaker

2009-06-11T13:39:00.000+05:30

capacitor in series to block DC

circuit symbol

Loudspeaker

Loudspeakers are output transducers which convert an electrical signal to sound. Usually they are called 'speakers'. They require a driver circuit, such as a 555 astable or an audio amplifier, to provide a signal. There is a wide range available, but for many electronics projects a 300mW miniature loudspeaker is ideal. This type is about 70mm diameter and it is usually available with resistances of 8 and 64. If a project specifies a 64 speaker you must use this higher resistance to prevent damage to the driving circuit.

Most circuits used to drive loudspeakers produce an audio (AC) signal which is combined with a constant DC signal. The DC will make a large current flow through the speaker due to its low resistance, possibly damaging both the speaker and the driving circuit. To prevent this happening a large value electrolytic capacitor is connected in series with the speaker, this blocks DC but passes audio (AC) signals. See capacitor coupling.

Loudspeakers may be connected either way round except in stereo circuits when the + and - markings on their terminals must be observed to ensure the two speakers are in phase.

Correct polarity must always be observed for large speakers in cabinets because the cabinet may contain a small circuit (a 'crossover network') which diverts the high frequency signals to a small speaker (a 'tweeter') because the large main speaker is poor at reproducing them.

Miniature loudspeakers can also be used as a microphone and they work surprisingly well, certainly good enough for speech in an intercom system for example.

Speaker

2009-06-11T13:38:00.000+05:30

Piezo transducer

circuit symbol

Piezo transducers are output transducers which convert an electrical signal to sound. They require a driver circuit (such as a 555 astable) to provide a signal and if this is near their natural (resonant) frequency of about 3kHz they will produce a particularly loud sound.

Piezo transducers require a small current, usually less than 10mA, so they can be connected directly to the outputs of most ICs. They are ideal for buzzes and beeps, but are not suitable for speech or music because they distort the sound. They are sometimes supplied with red and black leads, but they may be connected either way round. PCB-mounting versions are also available.

Piezo transducers can also be used as input transducers for detecting sudden loud noises or impacts, effectively behaving as a crude microphone.

Thermistor

2009-06-11T13:37:00.001+05:30

Thermistor



circuit symbol

A thermistor is an input transducer (sensor) which converts temperature (heat) to resistance. Almost all thermistors have a negative temperature coefficient (NTC) which means their resistance decreases as their temperature increases. It is possible to make thermistors with a positive temperature coefficient (resistance increases as temperature increases) but these are rarely used. Always assume NTC if no information is given.

A multimeter can be used to find the resistance at various temperatures, these are some typical readings for example:

Icy water 0°C: high resistance, about 12k.
Room temperature 25°C: medium resistance, about 5k.
Boiling water 100°C: low resistance, about 400.

Suppliers usually specify thermistors by their resistance at 25°C (room temperature). Thermistors take several seconds to respond to a sudden temperature change, small thermistors respond more rapidly. A thermistor may be connected either way round and no special precautions are required when soldering. If it is going to be immersed in water the thermistor and its connections should be insulated because water is a weak conductor; for example they could be coated with polyurethane varnish.

Light Dependent Resistor (LDR)

2009-06-11T13:34:00.000+05:30

circuit symbol

Light Dependent Resistor (LDR)

An LDR is an input transducer (sensor) which converts brightness (light) to resistance. It is made from cadmium sulphide (CdS) and the resistance decreases as the brightness of light falling on the LDR increases.

A multimeter can be used to find the resistance in darkness and bright light, these are the typical results for a standard LDR:

Darkness: maximum resistance, about 1M.
Very bright light: minimum resistance, about 100.

For many years the standard LDR has been the ORP12, now the NORPS12, which is about 13mm diameter. Miniature LDRs are also available and their diameter is about 5mm. An LDR may be connected either way round and no special precautions are required when soldering.

Variable Resistors

2009-06-11T13:31:00.000+05:30

Variable Resistors

Construction

Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along the track as you turn the spindle. The track may be made from carbon, cermet (ceramic and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight track versions, usually called sliders, are also available.

Variable resistors may be used as a rheostat with two connections (the wiper and just one end of the track) or as a potentiometer with all three connections in use. Miniature versions called presets are made for setting up circuits which will not require further adjustment.

Variable resistors are often called potentiometers in books and catalogues. They are specified by their maximum resistance, linear or logarithmic track, and their physical size. The standard spindle diameter is 6mm.

The resistance and type of track are marked on the body:
4K7 LIN means 4.7 k linear track.
1M LOG means 1 M logarithmic track.

Some variable resistors are designed to be mounted directly on the circuit board, but most are for mounting through a hole drilled in the case containing the circuit with stranded wire connecting their terminals to the circuit board.

Linear (LIN) and Logarithmic (LOG) tracks

Linear (LIN) track means that the resistance changes at a constant rate as you move the wiper. This is the standard arrangement and you should assume this type is required if a project does not specify the type of track. Presets always have linear tracks.

Logarithmic (LOG) track means that the resistance changes slowly at one end of the track and rapidly at the other end, so halfway along the track is not half the total resistance! This arrangement is used for volume (loudness) controls because the human ear has a logarithmic response to loudness so fine control (slow change) is required at low volumes and coarser control (rapid change) at high volumes. It is important to connect the ends of the track the correct way round, if you find that turning the spindle increases the volume rapidly followed by little further change you should swap the connections to the ends of the track.

Rheostat

Rheostat Symbol

This is the simplest way of using a variable resistor. Two terminals are used: one connected to an end of the track, the other to the moveable wiper. Turning the spindle changes the resistance between the two terminals from zero up to the maximum resistance.

Rheostats are often used to vary current, for example to control the brightness of a lamp or the rate at which a capacitor charges.

If the rheostat is mounted on a printed circuit board you may find that all three terminals are connected! However, one of them will be linked to the wiper terminal. This improves the mechanical strength of the mounting but it serves no function electrically.

Potentiometer

Potentiometer Symbol

Variable resistors used as potentiometers have all three terminals connected.

This arrangement is normally used to vary voltage, for example to set the switching point of a circuit with a sensor, or control the volume (loudness) in an amplifier circuit. If the terminals at the ends of the track are connected across the power supply then the wiper terminal will provide a voltage which can be varied from zero up to the maximum of the supply.

Presets

Preset Symbol

These are miniature versions of the standard variable resistor. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built. For example to set the frequency of an alarm tone or the sensitivity of a light-sensitive circuit. A small screwdriver or similar tool is required to adjust presets.

Presets are much cheaper than standard variable resistors so they are sometimes used in projects where a standard variable resistor would normally be used.

Multiturn presets are used where very precise adjustments must be made. The screw must be turned many times (10+) to move the slider from one end of the track to the other, giving very fine control.


Preset (open style)	Presets (closed style)	Multiturn preset

Transistors

2009-06-11T13:26:00.002+05:30

Transistors

This page covers practical matters such as precautions when soldering and identifying leads. The operation and use of transistors is covered by the Transistor Circuits page.

Function

Transistors amplify current, for example they can be used to amplify the small output current from a logic IC so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage.

A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on).

The amount of current amplification is called the current gain, symbol h_FE.
For further information please see the Transistor Circuits page.

Types of transistor


Transistor circuit symbols

There are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. If you are new to electronics it is best to start by learning how to use NPN transistors.

The leads are labelled base (B), collector (C) and emitter (E).
These terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labels!

A Darlington pair is two transistors connected together to give a very high current gain.

In addition to standard (bipolar junction) transistors, there are field-effect transistors which are usually referred to as FETs. They have different circuit symbols and properties and they are not (yet) covered by this page.

Transistor leads for some common case styles.

Connecting

Transistors have three leads which must be connected the correct way round. Please take care with this because a wrongly connected transistor may be damaged instantly when you switch on.

If you are lucky the orientation of the transistor will be clear from the PCB or stripboard layout diagram, otherwise you will need to refer to a supplier's catalogue to identify the leads.

The drawings on the right show the leads for some of the most common case styles.

Please note that transistor lead diagrams show the view from below with the leads towards you. This is the opposite of IC (chip) pin diagrams which show the view from above.

Please see below for a table showing the case styles of some common transistors.

Crocodile clip

Soldering

Transistors can be damaged by heat when soldering so if you are not an expert it is wise to use a heat sink clipped to the lead between the joint and the transistor body. A standard crocodile clip can be used as a heat sink.

Do not confuse this temporary heat sink with the permanent heat sink (described below) which may be required for a power transistor to prevent it overheating during operation.

Heat sink

Heat sinks

Waste heat is produced in transistors due to the current flowing through them. Heat sinks are needed for power transistors because they pass large currents. If you find that a transistor is becoming too hot to touch it certainly needs a heat sink! The heat sink helps to dissipate (remove) the heat by transferring it to the surrounding air.

Testing a transistor

Transistors can be damaged by heat when soldering or by misuse in a circuit. If you suspect that a transistor may be damaged there are two easy ways to test it:

Testing an NPN transistor

1. Testing with a multimeter

Set a digital multimeter to diode test and an analogue multimeter to a low resistance range.

Test each pair of leads both ways (six tests in total):

The base-emitter (BE) junction should behave like a diode and conduct one way only.
The base-collector (BC) junction should behave like a diode and conduct one way only.
The collector-emitter (CE) should not conduct either way.

The diagram shows how the junctions behave in an NPN transistor. The diodes are reversed in a PNP transistor but the same test procedure can be used.

A simple switching circuit
to test an NPN transistor

2. Testing in a simple switching circuit

Connect the transistor into the circuit shown on the right which uses the transistor as a switch. The supply voltage is not critical, anything between 5 and 12V is suitable. This circuit can be quickly built on Bread Board for example. Take care to include the 10k resistor in the base connection or you will destroy the transistor as you test it!

If the transistor is OK the LED should light when the switch is pressed and not light when the switch is released.

To test a PNP transistor use the same circuit but reverse the LED and the supply voltage.

Transistor codes

Codes beginning with B (or A), for example BC108, BC478
The first letter B is for silicon, A is for germanium (rarely used now). The second letter indicates the type; for example C means low power audio frequency; D means high power audio frequency; F means low power high frequency. The rest of the code identifies the particular transistor. There is no obvious logic to the numbering system. Sometimes a letter is added to the end (eg BC108C) to identify a special version of the main type, for example a higher current gain or a different case style. If a project specifies a higher gain version (BC108C) it must be used, but if the general code is given (BC108) any transistor with that code is suitable.
Codes beginning with TIP, for example TIP31A
TIP refers to the manufacturer: Texas Instruments Power transistor. The letter at the end identifies versions with different voltage ratings.
Codes beginning with 2N, for example 2N3053
The initial '2N' identifies the part as a transistor and the rest of the code identifies the particular transistor. There is no obvious logic to the numbering system.

Choosing a transistor

Most projects will specify a particular transistor, but if necessary you can usually substitute an equivalent transistor from the wide range available. The most important properties to look for are the maximum collector current I_C and the current gain h_FE. To make selection easier most suppliers group their transistors in categories determined either by their typical use or maximum power rating.

To make a final choice you will need to consult the tables of technical data which are normally provided in catalogues. They contain a great deal of useful information but they can be difficult to understand if you are not familiar with the abbreviations used. The table below shows the most important technical data for some popular transistors, tables in catalogues and reference books will usually show additional information but this is unlikely to be useful unless you are experienced.

NPN transistors
Code	Structure	Case style	I_C max.	V_CE max.	h_FE min.	P_tot max.	Category (typical use)	Possible substitutes
BC107	NPN	TO18	100mA	45V	110	300mW	Audio, low power	BC182 BC547
BC108	NPN	TO18	100mA	20V	110	300mW	General purpose, low power	BC108C BC183 BC548
BC108C	NPN	TO18	100mA	20V	420	600mW	General purpose, low power
BC109	NPN	TO18	200mA	20V	200	300mW	Audio (low noise), low power	BC184 BC549
BC182	NPN	TO92C	100mA	50V	100	350mW	General purpose, low power	BC107 BC182L
BC182L	NPN	TO92A	100mA	50V	100	350mW	General purpose, low power	BC107 BC182
BC547B	NPN	TO92C	100mA	45V	200	500mW	Audio, low power	BC107B
BC548B	NPN	TO92C	100mA	30V	220	500mW	General purpose, low power	BC108B
BC549B	NPN	TO92C	100mA	30V	240	625mW	Audio (low noise), low power	BC109
2N3053	NPN	TO39	700mA	40V	50	500mW	General purpose, low power	BFY51
BFY51	NPN	TO39	1A	30V	40	800mW	General purpose, medium power	BC639
BC639	NPN	TO92A	1A	80V	40	800mW	General purpose, medium power	BFY51
TIP29A	NPN	TO220	1A	60V	40	30W	General purpose, high power
TIP31A	NPN	TO220	3A	60V	10	40W	General purpose, high power	TIP31C TIP41A
TIP31C	NPN	TO220	3A	100V	10	40W	General purpose, high power	TIP31A TIP41A
TIP41A	NPN	TO220	6A	60V	15	65W	General purpose, high power
2N3055	NPN	TO3	15A	60V	20	117W	General purpose, high power
PNP transistors
Code	Structure	Case style	I_C max.	V_CE max.	h_FE min.	P_tot max.	Category (typical use)	Possible substitutes
BC177	PNP	TO18	100mA	45V	125	300mW	Audio, low power	BC477
BC178	PNP	TO18	200mA	25V	120	600mW	General purpose, low power	BC478
BC179	PNP	TO18	200mA	20V	180	600mW	Audio (low noise), low power
BC477	PNP	TO18	150mA	80V	125	360mW	Audio, low power	BC177
BC478	PNP	TO18	150mA	40V	125	360mW	General purpose, low power	BC178
TIP32A	PNP	TO220	3A	60V	25	40W	General purpose, high power	TIP32C
TIP32C	PNP	TO220	3A	100V	10	40W	General purpose, high power	TIP32A

Structure	This shows the type of transistor, NPN or PNP. The polarities of the two types are different, so if you are looking for a substitute it must be the same type.
I_C max.	Maximum collector current.
V_CE max.	Maximum voltage across the collector-emitter junction. You can ignore this rating in low voltage circuits.
h_FE	This is the current gain (strictly the DC current gain). The guaranteed minimum value is given because the actual value varies from transistor to transistor - even for those of the same type! Note that current gain is just a number so it has no units. The gain is often quoted at a particular collector current I_C which is usually in the middle of the transistor's range, for example '100@20mA' means the gain is at least 100 at 20mA. Sometimes minimum and maximum values are given. Since the gain is roughly constant for various currents but it varies from transistor to transistor this detail is only really of interest to experts. Why h_FE? It is one of a whole series of parameters for transistors, each with their own symbol. There are too many to explain here.
P_tot max.	Maximum total power which can be developed in the transistor, note that a heat sink will be required to achieve the maximum rating. This rating is important for transistors operating as amplifiers, the power is roughly I_C × V_CE. For transistors operating as switches the maximum collector current (I_C max.) is more important.
Category	This shows the typical use for the transistor, it is a good starting point when looking for a substitute. Catalogues may have separate tables for different categories.
Possible substitutes	These are transistors with similar electrical properties which will be suitable substitutes in most circuits. However, they may have a different case style so you will need to take care when placing them on the circuit board.

Darlington pair

This is two transistors connected together so that the amplified current from the first is amplified further by the second transistor. This gives the Darlington pair a very high current gain such as 10000. Darlington pairs are sold as complete packages containing the two transistors. They have three leads (B, C and E) which are equivalent to the leads of a standard individual transistor.

You can make up your own Darlington pair from two transistors.
For example:

For TR1 use BC548B with h_FE1 = 220.
For TR2 use BC639 with h_FE2 = 40.

The overall gain of this pair is h_FE1 × h_FE2 = 220 × 40 = 8800.
The pair's maximum collector current I_C(max) is the same as TR2.

Lamps

2009-06-11T13:20:00.003+05:30

Lamps

Function and Construction

Lamps emit light when an electric current passes through them. All of the lamps shown on this page have a thin wire filament which becomes very hot and glows brightly when a current passes through it. The filament is made from a metal with a high melting point such as tungsten and it is usually wound into a small coil. Filament lamps have a shorter lifetime than most electronic components because eventually the filament 'blows' (melts) at a weak point.

Circuit symbols

There are two circuit symbols for a lamp, one for a lamp used to provide illumination and another for a lamp used as an indicator. Small lamps such as torch bulbs can be used for both purposes so either circuit symbol may used in simple educational circuits.


Lamp used for lighting (for example a car headlamp or torch bulb)	Lamp used as an indicator (for example a warning light on a car dashboard)

Selecting a Lamp

There are three important features to consider when selecting a lamp:

Voltage rating - the supply voltage for normal brightness.
Power or current rating - small lamps are usually rated by current.
Lamp type - please see the table below.

The voltage and power (or current) ratings are usually printed or embossed on the body of a lamp.

Voltage rating

This is the supply voltage required for normal brightness. If a slightly higher voltage is used the lamp will be brighter but its lifetime will be shorter. With a lower supply voltage the lamp will be dimmer and its lifetime will be longer. The light from dim lamps has a yellow-orange colour.

Torch lamps pass a relatively large current and this significantly reduces the output voltage of the battery. Some voltage is used up inside the battery driving the large current through the small resistance of the battery itself (its 'internal resistance'). As a result the correct voltage rating for a torch lamp is lower than the normal voltage of the battery which lights it!

For example: a lamp rated 3.5V 0.3A is correct for a 4.5V battery (three 1.5V cells) because when the lamp is connected the voltage across the battery falls to about 3.5V.

Power or current rating

This is the power or current for the lamp when connected to its rated voltage. Low power lamps are usually rated by their current and high power lamps by their power. It is easy to convert between the two ratings:

P = I × V
or
I = P / V

where:

P = power in watts (W)
I = current in amps (A)
V = voltage in volts (V)

Examples:

A lamp rated 3.5V 0.3A has a power rating P = I × V = 0.3 × 3.5 = 1.05W
A lamp rated 6V 0.06A has a power rating P = I × V = 0.06 × 6 = 0.36W
A lamp rated 12W 2.4W has a current rating I = P / V = 2.4 / 12 = 0.2A

Lamp Type

Type of Lamp	Example
MES Miniature Edison Screw These are the standard small lamps. The bulb diameter is usually about 10mm, but tubular bulbs are also available. MES lamps have one contact on the base and the body forms the other contact. They are available with a good range of voltage and power (or current) ratings. Lens ended versions are available to produce a focused beam of light. LES Lilliput Edison Screw Smaller than MES, these have a bulb diameter of about 5mm.
MCC Miniature Centre Contact These have a bayonet style fitting, like a standard mains lamp in the UK. They have one contact on the base and the body forms the other contact. The bulb diameter is about 10mm.
SBC Small Bayonet Cap These have a bayonet style fitting, like a standard mains lamp in the UK. They have two contacts on the base so the metal body is not connected in the circuit. SBC lamps have high power ratings (24W for example) and their bulbs are large with a diameter of up to about 40mm. Note the two filament arrangements in the lamps shown, horizontal on the left, vertical on the right.
Pre-focus This type of lamp is used in torches and lanterns. The flange at the top of the metal body is used to hold the lamp in place. Lampholders are not readily available so this type is unsuitable for most projects.
Wire ended These are very small lamps with a bulb about 3mm diameter and 6mm long. Take care to avoid snapping the wires where they enter the glass bulb.
Grain of Wheat These are similar to the wire ended lamps above but they have stranded wire leads usually about 150mm long. The bulb is about 3mm diameter and 6mm long - the size of a grain of wheat!


screw terminals	solder tags

Connecting and soldering

Lamps may be connected either way round in a circuit and the supply may beAC or DC.

Most lamps are designed to be used in a lampholder but the small 'wire ended' and 'grain of wheat' lamps have wires which may be soldered directly onto a circuit board.

Lampholders usually have screw terminals or solder tags to attach wires. Some small holders have contacts which may be soldered directly to a circuit board.

Lamps in Series

Several lamps can be successfully connected in series provided they all have identical voltage and power (or current) ratings. The supply voltage is divided equally between identical lamps so their voltage rating must be suitable for this. For example Christmas tree lights may have 20 lamps connected in series to a 240V supply, so each lamp will have 240V ÷ 20 = 12V across it.

A disadvantage of connecting lamps in series is that if one lamp blows all of them will go out because the circuit is broken. Christmas tree lamps have a special feature to overcome this problem; they are designed to short circuit (conduct like a wire link) when they blow, so the circuit is not broken and the other lamps remain lit, making it easier to locate the faulty lamp. Sets also include one 'fuse' lamp which blows normally.

WARNING! The Christmas tree lamps may seem safe because they use only 12V but they are connected to the mains supply which can be lethal. Always unplug from the mains before changing lamps. The voltage across the holder of a missing lamp is the full 240V of the mains supply! (Yes, it really is!)

Integrated Circuits (Chips)

2009-06-11T13:12:00.002+05:30

Integrated Circuits (Chips)

Integrated Circuits are usually called ICs or chips. They are complex circuits which have been etched onto tiny chips of semiconductor (silicon). The chip is packaged in a plastic holder with pins spaced on a 0.1" (2.54mm) grid which will fit the holes on stripboard and breadboards. Very fine wires inside the package link the chip to the pins.

Pin numbers

The pins are numbered anti-clockwise around the IC (chip) starting near the notch or dot. The diagram shows the numbering for 8-pin and 14-pin ICs, but the principle is the same for all sizes.

IC holders (DIL sockets)

ICs (chips) are easily damaged by heat when soldering and their short pins cannot be protected with a heat sink. Instead we use an IC holder, strictly called a DIL socket (DIL = Dual In-Line), which can be safely soldered onto the circuit board. The IC is pushed into the holder when all soldering is complete.

IC holders are only needed when soldering so they are not used on breadboards.

Commercially produced circuit boards often have ICs soldered directly to the board without an IC holder, usually this is done by a machine which is able to work very quickly. Please don't attempt to do this yourself because you are likely to destroy the IC and it will be difficult to remove without damage by de-soldering.

Removing an IC from its holder

If you need to remove an IC it can be gently prised out of the holder with a small flat-blade screwdriver. Carefully lever up each end by inserting the screwdriver blade between the IC and its holder and gently twisting the screwdriver. Take care to start lifting at both ends before you attempt to remove the IC, otherwise you will bend and possibly break the pins.

Static precautions

Many ICs are static sensitive and can be damaged when you touch them because your body may have become charged with static electricity, from your clothes for example. Static sensitive ICs will be supplied in antistatic packaging with a warning label and they should be left in this packaging until you are ready to use them.

It is usually adequate to earth your hands by touching a metal water pipe or window frame before handling the IC but for the more sensitive (and expensive!) ICs special equipment is available, including earthed wrist straps and earthed work surfaces. You can make an earthed work surface with a sheet of aluminium kitchen foil and using a crocodile clip to connect the foil to a metal water pipe or window frame with a 10k resistor in series.

Sinking and sourcing current

IC outputs are often said to 'sink' or 'source' current. The terms refer to the direction of the current at the IC's output.

If the IC is sinking current it is flowing into the output. This means that a device connected between the positive supply (+Vs) and the IC output will be switched on when the output is low (0V).

If the IC is sourcing current it is flowing out of the output. This means that a device connected between the IC output and the negative supply (0V) will be switched on when the output is high (+Vs).

It is possible to connect two devices to an IC output so that one is on when the output is low and the other is on when the output is high. This arrangement is used in the Level crossing to make the red LEDs flash alternately.

Using diodes to combine outputs

The outputs of ICs must never be directly connected together. However, diodes can be used to combine two or more digital (high/low) outputs from an IC such as a counter. This can be a useful way of producing simple logic functions without using logic gates!

The diagram shows two ways of combining outputs using diodes. The diodes must be capable of passing the output current. 1N4148 signal diodes are suitable for low current devices such as LEDs.

For example the outputs Q0 - Q9 of a 4017 of-10 counter go high in turn. Using diodes to combine the 2nd (Q1) and 4th (Q3) outputs as shown in the bottom diagram will make the LED flash twice followed by a longer gap. The diodes are performing the function of an OR Gate

The 555 and 556 Timers

The 8-pin 555 timer IC is used in many projects, a popular version is the NE555. Most circuits will just specify '555 timer IC' and the NE555 is suitable for these. The 555 output (pin 3) can source up to 200mA. This is more than most ICs and it is sufficient to supply LEDs, relay coils and low current lamps. To switch larger currents you canconnect a transistor.

The 556 is a dual version of the 555 housed in a 14-pin package. The two timers (A and B) share the same power supply pins.

Low power versions of the 555 are made, such as the ICM7555, but these should only be used when specified (to increase battery life) because their maximum output current of about 20mA (with 9V supply) is too low for many standard 555 circuits. The ICM7555 has the same pin arrangement as a standard 555.

Logic ICs (chips)

Logic ICs process Digital signals there are many devices, including Logic gates , flip-flops, shift registers, counters and display drivers. They can be split into two groups according to their pin arrangements: the 4000 series the 74 Series which consists of various families such as the 74HC, 74HCT and 74LS.

For most new projects the 74HC family is the best choice. The older 4000 series is the only family which works with a supply voltage of more than 6V. The 74LS and 74HCT families require a 5V supply so they are not convenient for battery operation.

The table below summarises the important properties of the most popular logic families:

Property	4000 Series	74 Series 74HC	74 Series 74HCT	74 Series 74LS
Technology	CMOS	High-speed CMOS	High-speed CMOS TTL compatible	TTL Low-power Schottky
Power Supply	3 to 15V	2 to 6V	5V ±0.5V	5V ±0.25V
Inputs	Very high impedance. Unused inputs must be connected to +Vs or 0V. Inputs cannot be reliably driven by 74LS outputs unless a 'pull-up' resistor is used (see below).		Very high impedance. Unused inputs must be connected to +Vs or 0V. Compatible with 74LS (TTL) outputs.	'Float' high to logic 1 if unconnected. 1mA must be drawn out to hold them at logic 0.
Fan-out	One output can drive up to 50 CMOS, 74HC or 74HCT inputs, but only one 74LS input.	One output can drive up to 50 CMOS, 74HC or 74HCT inputs, but only 10 74LS inputs.		One output can drive up to 10 74LS inputs or 50 74HCT inputs.
Maximum Frequency	about 1MHz	about 25MHz	about 25MHz	about 35MHz
Power consumption of the IC itself	A few µW.	A few µW.	A few µW.	A few mW.

Driving 4000 or 74HC inputs from a
74LS output using a pull-up resistor.

Mixing Logic Families

It is best to build a circuit using just one logic family, but if necessary the different families may be mixed providing the power supply is suitable for all of them. For example mixing 4000 and 74HC requires the power supply to be in the range 3 to 6V. A circuit which includes 74LS or 74HCT ICs must have a 5V supply.

A 74LS output cannot reliably drive a 4000 or 74HC input unless a 'pull-up' resistor of 2.2k is connected between the +5V supply and the input to correct the slightly different logic voltage ranges used.

Note that a 4000 series output can drive only one 74LS input.

4000 Series CMOS

This family of logic ICs is numbered from 4000 onwards, and from 4500 onwards. They have a B at the end of the number (e.g. 4001B) which refers to an improved design introduced some years ago. Most of them are in 14-pin or 16-pin packages. They use CMOS circuitry which means they use very little power and can tolerate a wide range of power supply voltages (3 to 15V) making them ideal for battery powered projects. CMOS is pronounced 'see-moss' and stands for Complementary Metal Oxide Semiconductor.

However the CMOS circuitry also means that they are Static Sensitive a pin while charged with static electricity (from your clothes for example) may damage the IC. In fact most ICs in regular use are quite tolerant and earthing your hands by touching a metal water pipe or window frame before handling them will be adequate. ICs should be left in their protective packaging until you are ready to use them. For the more sensitive (and expensive!) ICs special equipment is available, including earthed wrist straps and earthed work surfaces.

74 Series: 74LS, 74HC and 74HCT

There are several families of logic ICs numbered from 74xx00 onwards with letters (xx) in the middle of the number to indicate the type of circuitry, eg 74LS00 and 74HC00. The original family (now obsolete) had no letters, eg 7400.

The 74LS (Low-power Schottky) family (like the original) uses TTL (Transistor-Transistor Logic) circuitry which is fast but requires more power than later families.

The 74HC family has High-speed CMOS circuitry, combining the speed of TTL with the very low power consumption of the 4000 series. They are CMOS ICs with the same pin arrangements as the older 74LS family. Note that 74HC inputs cannot be reliably driven by 74LS outputs because the voltage ranges used for logic 0 are not quite compatible, use 74HCT instead.

The 74HCT family is a special version of 74HC with 74LS TTL-compatible inputs so 74HCT can be safely mixed with 74LS in the same system. In fact 74HCT can be used as low-power direct replacements for the older 74LS ICs in most circuits. The minor disadvantage of 74HCT is a lower immunity to noise, but this is unlikely to be a problem in most situations.

Beware that the 74 series is often still called the 'TTL series' even though the latest ICs do not use TTL!

The CMOS circuitry used in the 74HC and 74HCT series.

PIC microcontrollers

A PIC is a Programmable Integrated Circuit microcontroller, a 'computer-on-a-chip'. They have a processor and memory to run a program responding to inputs and controlling outputs, so they can easily achieve complex functions which would require several conventional ICs.

Programming a PIC microcontroller may seem daunting to a beginner but there are a number of systems designed to make this easy. The PICAXE system is an excellent example because it uses a standard computer to program (and re-program) the PICs; no specialist equipment is required other than a low-cost download lead. Programs can be written in a simple version of BASIC or using a flowchart. The PICAXE programming software and extensive documentation is available to download free of charge, making the system ideal for education and users at home.

If you think PICs are not for you because you have never written a computer program, please look at the PICAXE system! It is very easy to get started using a few simple BASIC commands and there are a number of projects available as kits which are ideal for beginners. The system is

Switches

2009-06-11T13:05:00.003+05:30

Switches

Selecting a Switch

There are three important features to consider when selecting a switch:

Contacts (e.g. single pole, double throw)
Ratings (maximum voltage and current)
Method of Operation (toggle, slide, key etc.)

Switch Contacts

Several terms are used to describe switch contacts:

Pole - number of switch contact sets.
Throw - number of conducting positions, single or double.
Way - number of conducting positions, three or more.
Momentary - switch returns to its normal position when released.
Open - off position, contacts not conducting.
Closed - on position, contacts conducting, there may be several on positions.

For example: the simplest on-off switch has one set of contacts (single pole) and one switching position which conducts (single throw). The switch mechanism has two positions: open (off) and closed (on), but it is called 'single throw' because only one position conducts.

Switch Contact Ratings

Switch contacts are rated with a maximum voltage and current, and there may be different ratings for AC and DC. The AC values are higher because the current falls to zero many times each second and an arc is less likely to form across the switch contacts.

For low voltage electronics projects the voltage rating will not matter, but you may need to check the current rating. The maximum current is less for inductive loads (coils and motors) because they cause more sparking at the contacts when switched off.

Standard Switches

Type of Switch	Circuit Symbol	Example
ON-OFF Single Pole, Single Throw = SPST A simple on-off switch. This type can be used to switch the power supply to a circuit. When used with mains electricity this type of switch must be in the live wire, but it is better to use a DPST switch to isolate both live and neutral.		SPST toggle switch
(ON)-OFF Push-to-make = SPST Momentary A push-to-make switch returns to its normally open (off) position when you release the button, this is shown by the brackets around ON. This is the standard doorbell switch.		Push-to-make switch
ON-(OFF) Push-to-break = SPST Momentary A push-to-break switch returns to its normally closed (on) position when you release the button.		Push-to-break switch
ON-ON Single Pole, Double Throw = SPDT This switch can be on in both positions, switching on a separate device in each case. It is often called a changeover switch. For example, a SPDT switch can be used to switch on a red lamp in one position and a green lamp in the other position. A SPDT toggle switch may be used as a simple on-off switch by connecting to COM and one of the A or B terminals shown in the diagram. A and B are interchangeable so switches are usually not labelled. ON-OFF-ON SPDT Centre Off A special version of the standard SPDT switch. It has a third switching position in the centre which is off. Momentary (ON)-OFF-(ON) versions are also available where the switch returns to the central off position when released.		SPDT toggle switch SPDT slide switch (PCB mounting) SPDT rocker switch
Dual ON-OFF Double Pole, Single Throw = DPST A pair of on-off switches which operate together (shown by the dotted line in the circuit symbol). A DPST switch is often used to switch mains electricity because it can isolate both the live and neutral connections.		DPST rocker switch
Dual ON-ON Double Pole, Double Throw = DPDT A pair of on-on switches which operate together (shown by the dotted line in the circuit symbol). A DPDT switch can be wired up as a reversing switch for a motor as shown in the diagram. ON-OFF-ON DPDT Centre Off A special version of the standard SPDT switch. It has a third switching position in the centre which is off. This can be very useful for motor control because you have forward, off and reverse positions. Momentary (ON)-OFF-(ON) versions are also available where the switch returns to the central off position when released.		DPDT slide switch Wiring for Reversing Switch

Special Switches

Type of Switch	Example
Push-Push Switch (e.g. SPST = ON-OFF) This looks like a momentary action push switch but it is a standard on-off switch: push once to switch on, push again to switch off. This is called a latching action.
Microswitch (usually SPDT = ON-ON) Microswitches are designed to switch fully open or closed in response to small movements. They are available with levers and rollers attached.
Keyswitch A key operated switch. The example shown is SPST.
Tilt Switch (SPST) Tilt switches contain a conductive liquid and when tilted this bridges the contacts inside, closing the switch. They can be used as a sensor to detect the position of an object. Some tilt switches contain mercury which is poisonous.
Reed Switch (usually SPST) The contacts of a reed switch are closed by bringing a small magnet near the switch. They are used in security circuits, for example to check that doors are closed. Standard reed switches are SPST (simple on-off) but SPDT (changeover) versions are also available.
DIP Switch (DIP = Dual In-line Parallel) This is a set of miniature SPST on-off switches, the example shown has 8 switches. The package is the same size as a standard DIL (Dual In-Line) integrated circuit. This type of switch is used to set up circuits, e.g. setting the code of a remote control.
Multi-pole Switch The picture shows a 6-pole double throw switch, also known as a 6-pole changeover switch. It can be set to have momentary or latching action. Latching action means it behaves as a push-push switch, push once for the first position, push again for the second position etc.
Multi-way Switch Multi-way switches have 3 or more conducting positions. They may have several poles (contact sets). A popular type has a rotary action and it is available with a range of contact arrangements from 1-pole 12-way to 4-pole 3 way. The number of ways (switch positions) may be reduced by adjusting a stop under the fixing nut. For example if you need a 2-pole 5-way switch you can buy the 2-pole 6-way version and adjust the stop. Contrast this multi-way switch (many switch positions) with the multi-pole switch (many contact sets) described above.	Multi-way rotary switch 1-pole 4-way switch symbol

Light Emitting Diodes (LEDs)

2009-06-11T12:58:00.002+05:30

Light Emitting Diodes (LEDs)

Example: Circuit symbol:

Function

LEDs emit light when an electric current passes through them.

Connecting and soldering

LEDs must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method).

LEDs can be damaged by heat when soldering, but the risk is small unless you are very slow. No special precautions are needed for soldering most LEDs.

Testing an LED

Never connect an LED directly to a battery or power supply!
It will be destroyed almost instantly because too much current will pass through and burn it out.

LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1k resistor is suitable for most LEDs if your supply voltage is 12V or less. Remember to connect the LED the correct way round!

Colours of LEDs

LEDs are available in red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colours.

The colour of an LED is determined by the semiconductor material, not by the colouring of the 'package' (the plastic body). LEDs of all colours are available in uncoloured packages which may be diffused (milky) or clear (often described as 'water clear'). The coloured packages are also available as diffused (the standard type) or transparent.

Tri-colour LEDs

The most popular type of tri-colour LED has a red and a green LED combined in one package with three leads. They are called tri-colour because mixed red and green light appears to be yellow and this is produced when both the red and green LEDs are on.

The diagram shows the construction of a tri-colour LED. Note the different lengths of the three leads. The centre lead (k) is the common cathode for both LEDs, the outer leads (a1 and a2) are the anodes to the LEDs allowing each one to be lit separately, or both together to give the third colour.

Bi-colour LEDs

A bi-colour LED has two LEDs wired in 'inverse parallel' (one forwards, one backwards) combined in one package with two leads. Only one of the LEDs can be lit at one time and they are less useful than the tri-colour LEDs described above.

Sizes, Shapes and Viewing angles of LEDs

LEDs are available in a wide variety of sizes and shapes. The 'standard' LED has a round cross-section of 5mm diameter and this is probably the best type for general use, but 3mm round LEDs are also popular.

Round cross-section LEDs are frequently used and they are very easy to install on boxes by drilling a hole of the LED diameter, adding a spot of glue will help to hold the LED if necessary. LED clips are also available to secure LEDs in holes. Other cross-section shapes include square, rectangular and triangular.

As well as a variety of colours, sizes and shapes, LEDs also vary in their viewing angle. This tells you how much the beam of light spreads out. Standard LEDs have a viewing angle of 60° but others have a narrow beam of 30° or less.

Calculating an LED resistor value

An LED must have a resistor connected in series to limit the current through the LED, otherwise it will burn out almost instantly.

The resistor value, R is given by:

R = (V_S - V_L) / I

V_S = supply voltage
V_L = LED voltage (usually 2V, but 4V for blue and white LEDs)
I = LED current (e.g. 20mA), this must be less than the maximum permitted

If the calculated value is not available choose the nearest standard resistor value which is greater, so that the current will be a little less than you chose. In fact you may wish to choose a greater resistor value to reduce the current (to increase battery life for example) but this will make the LED less bright.

For example

If the supply voltage V_S = 9V, and you have a red LED (V_L = 2V), requiring a current I = 20mA = 0.020A,
R = (9V - 2V) / 0.02A = 350, so choose 390 (the nearest standard value which is greater).

Working out the LED resistor formula using Ohm's law

Ohm's law says that the resistance of the resistor, R = V/I, where:
V = voltage across the resistor (= V_S - V_L in this case)
I = the current through the resistor

So R = (V_S - V_L) / I

Connecting LEDs in series

If you wish to have several LEDs on at the same time it may be possible to connect them in series. This prolongs battery life by lighting several LEDs with the same current as just one LED.

All the LEDs connected in series pass the same current so it is best if they are all the same type. The power supply must have sufficient voltage to provide about 2V for each LED (4V for blue and white) plus at least another 2V for the resistor. To work out a value for the resistor you must add up all the LED voltages and use this for V_L.

Example calculations:
A red, a yellow and a green LED in series need a supply voltage of at least 3 × 2V + 2V = 8V, so a 9V battery would be ideal.
V_L = 2V + 2V + 2V = 6V (the three LED voltages added up).
If the supply voltage V_S is 9V and the current I must be 15mA = 0.015A,
Resistor R = (V_S - V_L) / I = (9 - 6) / 0.015 = 3 / 0.015 = 200,
so choose R = 220 (the nearest standard value which is greater).

Avoid connecting LEDs in parallel!

Connecting several LEDs in parallel with just one resistor shared between them is generally not a good idea.

If the LEDs require slightly different voltages only the lowest voltage LED will light and it may be destroyed by the larger current flowing through it. Although identical LEDs can be successfully connected in parallel with one resistor this rarely offers any useful benefit because resistors are very cheap and the current used is the same as connecting the LEDs individually. If LEDs are in parallel each one should have its own resistor.

Reading a table of technical data for LED's

Suppliers' catalogues usually include tables of technical data for components such as LEDs. These tables contain a good deal of useful information in a compact form but they can be difficult to understand if you are not familiar with the abbreviations used.

The table below shows typical technical data for some 5mm diameter round LEDs with diffused packages (plastic bodies). Only three columns are important and these are shown in bold. Please see below for explanations of the quantities.

Type	Colour	I_F max.	V_F typ.	V_F max.	V_R max.	Luminous intensity	Viewing angle	Wavelength
Standard	Red	30mA	1.7V	2.1V	5V	5mcd @ 10mA	60°	660nm
Standard	Bright red	30mA	2.0V	2.5V	5V	80mcd @ 10mA	60°	625nm
Standard	Yellow	30mA	2.1V	2.5V	5V	32mcd @ 10mA	60°	590nm
Standard	Green	25mA	2.2V	2.5V	5V	32mcd @ 10mA	60°	565nm
High intensity	Blue	30mA	4.5V	5.5V	5V	60mcd @ 20mA	50°	430nm
Super bright	Red	30mA	1.85V	2.5V	5V	500mcd @ 20mA	60°	660nm
Low current	Red	30mA	1.7V	2.0V	5V	5mcd @ 2mA	60°	625nm

I_F max.	Maximum forward current, forward just means with the LED connected correctly.
V_F typ.	Typical forward voltage, V_L in the LED resistor calculation. This is about 2V, except for blue and white LEDs for which it is about 4V.
V_F max.	Maximum forward voltage.
V_R max.	Maximum reverse voltage You can ignore this for LEDs connected the correct way round.
Luminous intensity	Brightness of the LED at the given current, mcd = millicandela.
Viewing angle	Standard LEDs have a viewing angle of 60°, others emit a narrower beam of about 30°.
Wavelength	The peak wavelength of the light emitted, this determines the colour of the LED. nm = nanometre.

Flashing LEDs

Flashing LEDs look like ordinary LEDs but they contain an integrated circuit (IC) as well as the LED itself. The IC flashes the LED at a low frequency, typically 3Hz (3 flashes per second). They are designed to be connected directly to a supply, usually 9 - 12V, and no series resistor is required. Their flash frequency is fixed so their use is limited and you may prefer to build your own circuit to flash an ordinary LED, for example our Flashing LED project which uses a 555 astable circuit.

LED Displays

LED displays are packages of many LEDs arranged in a pattern, the most familiar pattern being the 7-segment displays for showing numbers (digits 0-9). The pictures below illustrate some of the popular designs:


Bargraph	7-segment	Starburst	Dot matrix

Pin connections of LED displays

There are many types of LED display and a supplier's catalogue should be consulted for the pin connections. The diagram on the right shows an example from the Rapid electronics . Like many 7-segment displays, this example is available in two versions: Common Anode (SA) with all the LED anodes connected together and Common Cathode (SC) with all the cathodes connected together. Letters a-g refer to the 7 segments, A/C is the common anode or cathode as appropriate (on 2 pins). Note that some pins are not present (NP) but their position is still numbered.

Diodes

2009-06-11T12:55:00.001+05:30

Diodes

Example: Circuit symbol:

Function

Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves.

Forward Voltage Drop

Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph).

Reverse Voltage

When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.

Connecting and soldering

Diodes must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is marked by a line painted on the body. Diodes are labelled with their code in small print, you may need a magnifying glass to read this on small signal diodes!

Small signal diodes can be damaged by heat when soldering, but the risk is small unless you are using a germanium diode (codes beginning OA...) in which case you should use a heat sink clipped to the lead between the joint and the diode body. A standard crocodile clip can be used as a heat sink.

Rectifier diodes are quite robust and no special precautions are needed for soldering them.

Signal diodes (small current)

Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA.

General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V.

Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V and this makes them suitable to use in radio circuits as detectors which extract the audio signal from the weak radio signal.

For general use, where the size of the forward voltage drop is less important, silicon diodes are better because they are less easily damaged by heat when soldering, they have a lower resistance when conducting, and they have very low leakage currents when a reverse voltage is applied.

Protection diodes for relays

Signal diodes are also used to protect transistors and ICs from the brief high voltage produced when a relay coil is switched off. The diagram shows how a protection diode is connected 'backwards' across the relay coil.

Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs.

Diode	Maximum Current	Maximum Reverse Voltage
1N4001	1A	50V
1N4002	1A	100V
1N4007	1A	1000V
1N5401	3A	100V
1N5408	3A	1000V

Rectifier diodes (large current)

Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification. They are also used elsewhere in circuits where a large current must pass through the diode.

All rectifier diodes are made from silicon and therefore have a forward voltage drop of 0.7V. The table shows maximum current and maximum reverse voltage for some popular rectifier diodes. The 1N4001 is suitable for most low voltage circuits with a current of less than 1A.

Bridge rectifiers

There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required. Bridge rectifiers are rated by their maximum current and maximum reverse voltage. They have four leads or terminals: the two DC outputs are labelled + and -, the two AC inputs are labelled .

The diagram shows the operation of a bridge rectifier as it converts AC to DC. Notice how alternate pairs of diodes conduct.

Connectors and Cables

2009-06-11T12:45:00.002+05:30

Connectors and Cables

Battery clips and holders

The standard battery clip fits a 9V PP3 battery and many battery holders such as the 6 × AA cell holder shown. Battery holders are also available with wires attached, with pins for PCB mounting, or as a complete box with lid, switch and wires.

Many small electronic projects use a 9V PP3 battery but if you wish to use the project for long periods a better choice is a battery holder with 6 AA cells. This has the same voltage but a much longer battery life and it will work out cheaper in the long run.

Larger battery clips fit 9V PP9 batteries but these are rarely used now.


PCB terminal block	Terminal block

Terminal blocks and PCB terminals

Terminal blocks are usually supplied in 12-way lengths but they can be cut into smaller blocks with a sharp knife, large wire cutters or a junior hacksaw. They are sometimes called 'chocolate blocks' because of the way they can be easily cut to size.

PCB mounting terminal blocks provide an easy way of making semi-permanent connections to PCBs. Many are designed to interlock to provide more connections.

Crocodile clips


Crocodile clip

The 'standard' crocodile clip has no cover and a screw contact. However, miniature insulated crocodile clips are more suitable for many purposes including test leads. They have a solder contact and lugs which fold down to grip the cable's insulation, increasing the strength of the joint. Remember to feed the cable through the plastic cover before soldering! Add and remove the cover by fully opening the clip, a piece of wood can be used to hold the jaws open.



4mm terminal and solder tag

4mm plugs, sockets and terminals

These are the standard single pole connectors used on meters and other electronic equipment. They are capable of passing high currents (typically 10A) and most designs are very robust. Shrouded plugs and sockets are available for use with high voltages where there is a risk of electric shock. A wide variety of colours is available from most suppliers.

Plugs
Plugs may have a screw or solder terminal to hold the cable. Check if you need to thread the cable through the cover before connecting it. Some plugs, such as those illustrated, are 'stackable' which means that they include a socket to accept another plug, allowing several plugs to be connected to the same point - a very useful feature for test leads.

Sockets
These are usually described as 'panel mounting' because they are designed to be fitted to a case. Most sockets have a solder contact but the picture shows other options. Fit the socket in the case before attaching the wire otherwise you will be unable to add the mounting nut.

Terminals
In addition to a socket these have provision for attaching a wire by threading it through a hole (or wrapping it around the post) and tightening the top nut by hand. They usually have a threaded stud to fit a solder tag inside the case.

2mm plugs and sockets

These are smaller versions of the 4mm plugs and sockets described above, but terminals are not readily available. The plugs illustrated are stackable. Despite their small size these connectors can pass large currents and some are rated at 10A.

DC power plugs and sockets

These 2-pole plugs and sockets ensure that the polarity of a DC supply cannot be accidentally reversed. The standard sizes are 2.1 and 2.5mm plug diameter. Standard plugs have a 10mm shaft, 'long' plugs have a 14mm shaft. Sockets are available for PCB or chassis mounting and most include a switch on the outer contact which is normally used to disconnect an internal battery when a plug is inserted.

Miniature versions with a 1.3mm diameter plug are used where small size is essential, such as for personal cassette players.


¼" (6.3mm) jack plug and socket

3.5mm jack plug and socket

3.5mm jack line socket (for fitting to a cable)

Jack plugs and sockets

These are intended for audio signals so mono and stereo versions are available. The sizes are determined by the plug diameter: ¼" (6.3mm), 3.5mm and 2.5mm. The 2.5mm size is only available for mono.

Screened plugs have metal bodies connected to the COM contact. Most connections are soldered, remember to thread cables through plug covers before soldering! Sockets are designed for PCB or chassis mounting.

¼" plug connections are similar to those for 3.5mm plugs shown below. ¼" socket connections are COM, R and L in that order from the mounting nut, ignore R for mono use. Most ¼" sockets have switches on all contacts which open as the plug is inserted so they can be used to isolate internal speakers for example.

The connections for 3.5mm plugs and sockets are shown below. Plugs have a lug which should be folded down to grip the cable's insulation and increase the strength of the joint. 3.5mm mono sockets have a switch contact which can be used to switch off an internal speaker as the plug is inserted. Ignore this contact if you do not require the switching action.

3.5mm jack plug and socket connections
(the R connection is not present on mono plugs)

L = left channel signal
R = right channel signal
COM = common (0V, screen)

Phono plugs and sockets

These are used for screened cables carrying audio and video signals. Stereo connections are made using a pair of phono plugs and sockets. The centre contact is for the signal and the outer contact for the screen (0V, common). Screened plugs have metal bodies connected to the outer contact to give the signal additional protection from electrical noise. Sockets are available for PCB or chassis mounting, singly for mono, or in pairs for stereo. Line sockets are available for making extension leads.

Construction of a screened cable

Coax plugs and sockets

These are similar to the phono plugs and sockets described above but they are designed for use with screened cables carrying much higher frequency signals, such as TV aerial leads. They provide better screening because at high frequencies this is essential to reduce electrical noise.

BNC plugs and sockets

These are designed for screened cables carrying high frequency signals where an undistorted and noise free signal is essential, for example ociloscope . BNC plugs are connected with a push and twist action, to disconnect you need to twist and pull.

Plugs and sockets are rated by their impedance (50 or 75) which must be the same as the cable's impedance. If the connector and cable impedances are not matched the signal will be distorted because it will be partly reflected at the connection, this is the electrical equivalent of the weak reflection which occurs when light passes through a glass window.

DIN plug

5 way 180° DIN socket
(chassis mounting)

DIN plugs and sockets

These are intended for audio signals but they can be used for other low-current purposes where a multi-way connector is required. They are available from 3 way to 8 way. 5 way is used for stereo audio connections. The contacts are numbered on the connector, but they are not in numerical order! For audio use the 'common' (0V) wire is connected to contact 2. 5 way plugs and sockets are available in two versions: 180° and 270° (the angle refers to the arc formed by the contacts).

Plastic covers of DIN plugs (and line sockets) are removed by depressing the retaining lug with a small screwdriver. You may also need small pliers to extract the body from the cover but do not pull on the pins themselves to avoid damage. Remember to thread the cable through the cover before starting to solder the connections!

Soldering DIN plugs is easier if you clamp the insert with the pins. Wires should be pushed into the hollow pins - first 'tin' the wires (coat them with a thin layer of solder) then melt a little solder into the hollow pin and insert the wire while keeping the solder molten. Take care to avoid melting the plastic base, stop and allow the pin to cool if necessary.

Mini-DIN connectors are used for computer equipment such as keyboards and mice but they are not a good choice for general use unless small size is essential.

D connectors

These are multi-pole connectors with provision for screw fittings to make semi-permanent connections, for example on computer equipment. The D shape prevents incorrect connection. Standard D-connectors have 2 rows of contacts (top picture); 9, 15 and 25-way versions are the most popular. High Density D-connectors have 3 rows of contacts (bottom picture); a 15-way version is used to connect computer monitors for example.

Note that covers (middle picture) are usually sold separately because both plugs and sockets can be fitted to cables by fitting a cover to a chassis mounted connector. PCB mounting versions of plugs and sockets are also available. The contacts are usually numbered on the body of the connector, although you may need a magnifying glass to see the very small markings. Soldering D-connectors requires a steady hand due to the closeness of the contacts, it is easy to accidently unsolder a contact you have just completed while attempting to solder the next one!

IDC communication connectors

These multi-pole insulation displacement connectors are used for computer and telecommunications equipment. They automatically cut through the insulation on wires when installed and special tools are required to fit them. They are available as 4, 6 and 8-way versions.

The 8-way RJ45 is the standard connector for modern computer networks. If you regularly use these you may be interested in our Network lead tester .

Standard UK telephone connectors are similar in style but a slightly different shape. They are called BT (British Telecom) connectors.

Cables

Cable... flex... lead... wire... what do all these terms mean?

A cable is an assembly of one or more conductors (wires) with some flexibility.
A flex is the proper name for the flexible cable fitted to mains electrical appliances.
A lead is a complete assembly of cable and connectors.
A wire is a single conductor which may have an outer layer of insulation (usually plastic).

Single core equipment wire

This is one solid wire with a plastic coating available in a wide variety of colours. It can be bent to shape but will break if repeatedly flexed. Use it for connections which will not be disturbed, for example links between points of a circuit board.

Typical specification: 1/0.6mm (1 strand of 0.6mm diameter), maximum current 1.8A.

Stranded wire

This consists of many fine strands of wire covered by an outer plastic coating. It is flexible and can withstand repeated bending without breaking. Use it for connections which may be disturbed, for example wires outside cases to sensors and switches. A very flexible version ('extra-flex') is used for test leads.

Typical specifications:
10/0.1mm (10 strands of 0.1mm diameter), maximum current 0.5A.
7/0.2mm (7 strands of 0.2mm diameter), maximum current 1.4A.
16/0.2mm (16 strands of 0.2mm diameter), maximum current 3A.
24/0.2mm (24 strands of 0.2mm diameter), maximum current 4.5A.
55/0.1mm (55 strands of 0.1mm diameter), maximum current 6A, used for test leads.

'Figure 8' (speaker) cable

'Figure 8' cable consists of two stranded wires arranged in a figure of 8 shape. One wire is usually marked with a line. It is suitable for low voltage, low current (maximum 1A) signals where screening from electrical interference is not required. It is a popular choice for connecting loudspeakers and is often called 'speaker cable'.

Signal cable

Signal cable consists of several colour-coded cores of stranded wire housed within an outer plastic sheath. With a typical maximum current of 1A per core it is suitable for low voltage, low current signals where screening from electrical interference is not required.

The picture shows 6-core cable, but 4-core and 8-core are also readily available.

Screened cable (mono)

Screened cable (stereo)

Screened cable

The diagram shows the construction of screened cable. The central wire carries the signal and the screen is connected to 0V (common) to shield the signal from electrical interference. Screened cable is used for audio signals and dual versions are available for stereo.

Construction of a screened cable

Co-axial cable

This type of screened cable (see above) is designed to carry high frequency signals such as those found in TV aerials and ociloscope leads.

Mains flex

Flex is the proper name for the flexible cable used to connect appliances to the mains supply. It contains 2 cores (for live and neutral) or 3 cores (for live, neutral and earth). Mains flex has thick insulation for the high voltage (230V in UK) and it is available with various current ratings: 3A, 6A and 13A are popular sizes in the UK. Mains flex is sometimes used for low voltage circuits which pass a high current, but please think carefully before using it in this way. The distinctive colours of mains flex should act as a warning of the mains high voltage which can be lethal; using mains flex for low voltage circuits can undermine this warning.

Relays

2009-06-11T12:35:00.003+05:30

Relays

Circuit symbol for a relay

Relay showing coil and switch contacts

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches.

Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical.

The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification.

Relays are usuallly SPDT or DPDT but they can have many more sets of switch contacts, for example relays with 4 sets of changeover contacts are readily available. For further information about switch contacts and the terms used to describe them please see the page on switches.

Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay.

The supplier's catalogue should show you the relay's connections. The coil will be obvious and it may be connected either way round. Relay coils produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. To prevent damage you must connect a across the relay coil.

The animated picture shows a working relay with its coil and switch contacts. You can see a lever on the left being attracted by magnetism when the coil is switched on. This lever moves the switch contacts. There is one set of contacts (SPDT) in the foreground and another behind them, making the relay DPDT.

The relay's switch connections are usually labelled COM, NC and NO:

COM = Common, always connect to this, it is the moving part of the switch.
NC = Normally Closed, COM is connected to this when the relay coil is off.
NO = Normally Open, COM is connected to this when the relay coil is on.
Connect to COM and NO if you want the switched circuit to be on when the relay coil is on.
Connect to COM and NC if you want the switched circuit to be on when the relay coil is off.

Choosing a relay

You need to consider several features when choosing a relay:

Physical size and pin arrangement
If you are choosing a relay for an existing PCB you will need to ensure that its dimensions and pin arrangement are suitable. You should find this information in the supplier's catalogue.
Coil voltage
The relay's coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value.
Coil resistance
The circuit must be able to supply the current required by the relay coil. You can use to calculate the current:For example: A 12V supply relay with a coil resistance of 400 passes a current of 30mA. This is OK for a 555 timer IC (maximum output current 200mA), but it is too much for most ICs and they will require a transistor to amplify the current.
Switch ratings (voltage and current)
The relay's switch contacts must be suitable for the circuit they are to control. You will need to check the voltage and current ratings. Note that the voltage rating is usually higher for AC, for example: "5A at 24V DC or 125V AC".
Switch contact arrangement (SPDT, DPDT etc)
Most relays are SPDT or DPDT which are often described as "single pole changeover" (SPCO) or "double pole changeover" (DPCO). For further information please see the page on switches.

Protection diodes for relays

Transistors and ICs must be protected from the brief high voltage produced when a relay coil is switched off. The diagram shows how a signal diode (eg 1N4148) is connected 'backwards' across the relay coil to provide this protection.

Reed relays

Reed Relay

Photograp

eed relays consist of a coil surrounding a reed switch. Reed switches are normally operated with a magnet, but in a reed relay current flows through the coil to create a magnetic field and close the reed switch.

Reed relays generally have higher coil resistances than standard relays (1000 for example) and a wide range of supply voltages (9-20V for example). They are capable of switching much more rapidly than standard relays, up to several hundred times per second; but they can only switch low currents (500mA maximum for example).

The reed relay shown in the photograph will plug into a standard 14-pin ('IC holder').

Relays and transistors compared

Like relays treasistor can be used as an electrically operated switch. For switching small DC currents (<> 5A). In these cases a relay will be needed, but note that a low power transistor may still be needed to switch the current for the relay's coil! The main advantages and disadvantages of relays are listed below:

Capacitors

2009-06-11T12:04:00.001+05:30

Capacitors

Function

Capacitors store electric charge. They are used with resistors in tining cir because it takes time for a capacitor to fill with charge. They are used to smooth DC supplies by acting as a reservoir of charge. They are also used in filter circuits because capacitors easily pass AC (changing) signals but they block DC (constant) signals.

Capacitance

This is a measure of a capacitor's ability to store charge. A large capacitance means that more charge can be stored. Capacitance is measured in farads, symbol F. However 1F is very large, so prefixes are used to show the smaller values.

Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):

µ means 10^-6 (millionth), so 1000000µF = 1F
n means 10^-9 (thousand-millionth), so 1000nF = 1µF
p means 10^-12 (million-millionth), so 1000pF = 1nF

Capacitor values can be very difficult to find because there are many types of capacitor with different labelling systems!

Polarised capacitors (large values, 1µF +)

Examples: Circuit symbol:

Electrolytic Capacitors

Electrolytic capacitors are polarised and they must be connected the correct way round, at least one of their leads will be marked + or -. They are not damaged by heat when soldering.

There are two designs of electrolytic capacitors; axial where the leads are attached to each end (220µF in picture) and radial where both leads are at the same end (10µF in picture). Radial capacitors tend to be a little smaller and they stand upright on the circuit board.

It is easy to find the value of electrolytic capacitors because they are clearly printed with their capacitance and voltage rating. The voltage rating can be quite low (6V for example) and it should always be checked when selecting an electrolytic capacitor. If the project parts list does not specify a voltage, choose a capacitor with a rating which is greater than the project's power supply voltage. 25V is a sensible minimum for most battery circuits.

Tantalum Bead Capacitors

Tantalum bead capacitors are polarised and have low voltage ratings like electrolytic capacitors. They are expensive but very small, so they are used where a large capacitance is needed in a small size.

Modern tantalum bead capacitors are printed with their capacitance, voltage and polarity in full. However older ones use a colour-code system which has two stripes (for the two digits) and a spot of colour for the number of zeros to give the value in µF. The standard colour code is used, but for the spot, grey is used to mean × 0.01 and white means × 0.1 so that values of less than 10µF can be shown. A third colour stripe near the leads shows the voltage (yellow 6.3V, black 10V, green 16V, blue 20V, grey 25V, white 30V, pink 35V). The positive (+) lead is to the right when the spot is facing you: 'when the spot is in sight, the positive is to the right'.

For example: blue, grey, black spot means 68µF
For example: blue, grey, white spot means 6.8µF
For example: blue, grey, grey spot means 0.68µF

Unpolarised capacitors (small values, up to 1µF)

Examples: Circuit symbol:

Small value capacitors are unpolarised and may be connected either way round. They are not damaged by heat when soldering, except for one unusual type (polystyrene). They have high voltage ratings of at least 50V, usually 250V or so. It can be difficult to find the values of these small capacitors because there are many types of them and several different labelling systems!

Many small value capacitors have their value printed but without a multiplier, so you need to use experience to work out what the multiplier should be!

For example 0.1 means 0.1µF = 100nF.

Sometimes the multiplier is used in place of the decimal point:
For example: 4n7 means 4.7nF.

Capacitor Number Code

A number code is often used on small capacitors where printing is difficult:

the 1st number is the 1st digit,
the 2nd number is the 2nd digit,
the 3rd number is the number of zeros to give the capacitance in pF.
Ignore any letters - they just indicate tolerance and voltage rating.

For example: 102 means 1000pF = 1nF (not 102pF!)

For example: 472J means 4700pF = 4.7nF (J means 5% tolerance).

Colour Code
Colour	Number
Black	0
Brown	1
Red	2
Orange	3
Yellow	4
Green	5
Blue	6
Violet	7
Grey	8
White	9

Capacitor Colour Code

A colour code was used on polyester capacitors for many years. It is now obsolete, but of course there are many still around. The colours should be read like the resistor code, the top three colour bands giving the value in pF. Ignore the 4th band (tolerance) and 5th band (voltage rating).

For example:

brown, black, orange means 10000pF = 10nF = 0.01µF.

Note that there are no gaps between the colour bands, so 2 identical bands actually appear as a wide band.

For example:

wide red, yellow means 220nF = 0.22µF.

Polystyrene Capacitors

This type is rarely used now. Their value (in pF) is normally printed without units. Polystyrene capacitors can be damaged by heat when soldering (it melts the polystyrene!) so you should use a heat sink (such as a crocodile clip). Clip the heat sink to the lead between the capacitor and the joint.

Real capacitor values (the E3 and E6 series)

You may have noticed that capacitors are not available with every possible value, for example 22µF and 47µF are readily available, but 25µF and 50µF are not!

Why is this? Imagine that you decided to make capacitors every 10µF giving 10, 20, 30, 40, 50 and so on. That seems fine, but what happens when you reach 1000? It would be pointless to make 1000, 1010, 1020, 1030 and so on because for these values 10 is a very small difference, too small to be noticeable in most circuits and capacitors cannot be made with that accuracy.

To produce a sensible range of capacitor values you need to increase the size of the 'step' as the value increases. The standard capacitor values are based on this idea and they form a series which follows the same pattern for every multiple of ten.

The E3 series (3 values for each multiple of ten)
10, 22, 47, ... then it continues 100, 220, 470, 1000, 2200, 4700, 10000 etc.
Notice how the step size increases as the value increases (values roughly double each time).

The E6 series (6 values for each multiple of ten)
10, 15, 22, 33, 47, 68, ... then it continues 100, 150, 220, 330, 470, 680, 1000 etc.
Notice how this is the E3 series with an extra value in the gaps.

The E3 series is the one most frequently used for capacitors because many types cannot be made with very accurate values.

Variable capacitors

Variable Capacitor Symbol

Variable Capacitor

Variable capacitors are mostly used in radio tuning circuits and they are sometimes called 'tuning capacitors'. They have very small capacitance values, typically between 100pF and 500pF (100pF = 0.0001µF). The type illustrated usually has trimmers built in (for making small adjustments - see below) as well as the main variable capacitor.

Many variable capacitors have very short spindles which are not suitable for the standard knobs used for variable resistors and rotary switches. It would be wise to check that a suitable knob is available before ordering a variable capacitor.

Variable capacitors are not normally used in timing circuits because their capacitance is too small to be practical and the range of values available is very limited. Instead timing circuits use a fixed capacitor and a variable resistor if it is necessary to vary the time period.

Trimmer capacitors

Trimmer Capacitor Symbol

Trimmer capacitors (trimmers) are miniature variable capacitors. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built.

A small screwdriver or similar tool is required to adjust trimmers. The process of adjusting them requires patience because the presence of your hand and the tool will slightly change the capacitance of the circuit in the region of the trimmer!

Trimmer capacitors are only available with very small capacitances, normally less than 100pF. It is impossible to reduce their capacitance to zero, so they are usually specified by their minimum and maximum values, for example 2-10pF.

Trimmers are the capacitor equivalent of presets which are miniature variable resistors.

Resistors

2009-06-11T11:59:00.002+05:30

Resistors

Example: Circuit symbol:

Function

Resistors restrict the flow of electric current, for example a resistor is placed in series with a light-emitting diode (LED) to limit the current passing through the LED.

Connecting and soldering

Resistors may be connected either way round. They are not damaged by heat when soldering.

The Resistor Colour Code
Colour	Number
Black	0
Brown	1
Red	2
Orange	3
Yellow	4
Green	5
Blue	6
Violet	7
Grey	8
White	9

Resistor values - the resistor colour code

Resistance is measured in ohms, the symbol for ohm is an omega .
1 is quite small so resistor values are often given in k and M.
1 k = 1000 1 M = 1000000 .

Resistor values are normally shown using coloured bands.
Each colour represents a number as shown in the table.

Most resistors have 4 bands:

The first band gives the first digit.
The second band gives the second digit.
The third band indicates the number of zeros.
The fourth band is used to shows the tolerance (precision) of the resistor, this may be ignored for almost all circuits but further details are given below.

This resistor has red (2), violet (7), yellow (4 zeros) and gold bands.
So its value is 270000 = 270 k.
On circuit diagrams the is usually omitted and the value is written 270K.

Small value resistors (less than 10 ohm)

The standard colour code cannot show values of less than 10. To show these small values two special colours are used for the third band: gold which means × 0.1 and silver which means × 0.01. The first and second bands represent the digits as normal.

For example:
red, violet, gold bands represent 27 × 0.1 = 2.7
green, blue, silver bands represent 56 × 0.01 = 0.56

Tolerance of resistors (fourth band of colour code)

The tolerance of a resistor is shown by the fourth band of the colour code. Tolerance is the precision of the resistor and it is given as a percentage. For example a 390 resistor with a tolerance of ±10% will have a value within 10% of 390, between 390 - 39 = 351 and 390 + 39 = 429 (39 is 10% of 390).

A special colour code is used for the fourth band tolerance:
silver ±10%, gold ±5%, red ±2%, brown ±1%.
If no fourth band is shown the tolerance is ±20%.

Tolerance may be ignored for almost all circuits because precise resistor values are rarely required.

Resistor shorthand

Resistor values are often written on circuit diagrams using a code system which avoids using a decimal point because it is easy to miss the small dot. Instead the letters R, K and M are used in place of the decimal point. To read the code: replace the letter with a decimal point, then multiply the value by 1000 if the letter was K, or 1000000 if the letter was M. The letter R means multiply by 1.

For example:

560R

2K7

39K

1M0

Real resistor values (the E6 and E12 series)

You may have noticed that resistors are not available with every possible value, for example 22k and 47k are readily available, but 25k and 50k are not!

Why is this? Imagine that you decided to make resistors every 10 giving 10, 20, 30, 40, 50 and so on. That seems fine, but what happens when you reach 1000? It would be pointless to make 1000, 1010, 1020, 1030 and so on because for these values 10 is a very small difference, too small to be noticeable in most circuits. In fact it would be difficult to make resistors sufficiently accurate.

To produce a sensible range of resistor values you need to increase the size of the 'step' as the value increases. The standard resistor values are based on this idea and they form a series which follows the same pattern for every multiple of ten.

The E6 series (6 values for each multiple of ten, for resistors with 20% tolerance)
10, 15, 22, 33, 47, 68, ... then it continues 100, 150, 220, 330, 470, 680, 1000 etc.
Notice how the step size increases as the value increases. For this series the step (to the next value) is roughly half the value.

The E12 series (12 values for each multiple of ten, for resistors with 10% tolerance)
10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, ... then it continues 100, 120, 150 etc.
Notice how this is the E6 series with an extra value in the gaps.

The E12 series is the one most frequently used for resistors. It allows you to choose a value within 10% of the precise value you need. This is sufficiently accurate for almost all projects and it is sensible because most resistors are only accurate to ±10% (called their 'tolerance'). For example a resistor marked 390 could vary by ±10% × 390 = ±39, so it could be any value between 351 and 429.