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| Miniature piezo transducer |
@25°C.
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| Miniature piezo transducer |
@25°C.
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| Snap-top plastic bags |
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| Drawer cabinet
Photographs © Rapid Electronics |
Probably the best storage system is a cabinet of plastic drawers. These can be expensive, but you do not need many drawers because there is no need to have a drawer for every single component value. Many parts can be grouped together, such as decades of resistor values. For example you could organise a 15-drawer cabinet like this:
+ (third band black) only a few, but they tend to be large
+ (third band brown)
+ (third band red)
+ (third band orange)
+ (third band yellow)
+ (third band green) and 10M (third band blue)
Audio & communication: Jack | Phono | Coax | BNC | DIN | D | IDC & RJ45
Cables: Single-core | Stranded | 'Figure 8' | Signal | Screened | Co-axial | Mains flex
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| Photographs © Rapid Electronics |
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.
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| PCB terminal block |
Terminal block
Photographs © Rapid Electronics |
PCB mounting terminal blocks provide an easy way of making semi-permanent connections to PCBs. Many are designed to interlock to provide more connections.
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| Crocodile clips Photographs © Rapid Electronics | |
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4mm terminal |
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| Photographs © Rapid Electronics | |
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.
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| Photograph © Rapid Electronics |
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| Photographs © Rapid Electronics | |
Miniature versions with a 1.3mm diameter plug are used where small size is essential, such as for personal cassette players.
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| ¼" (6.3mm) jack plug and socket | |
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| 3.5mm jack plug and socket | |
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| 3.5mm jack line socket (for fitting to a cable) Photographs © Rapid Electronics | |
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.
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| 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)
Do not use jack plugs for power supply connections because the contacts may be briefly shorted as the plug is inserted. Use DC power connectors for this.
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| Photographs © Rapid Electronics |
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| Construction of a screened cable |
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| Photographs © Rapid Electronics | |
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| BNC plug, photograph © Rapid Electronics |
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.
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| DIN plug |
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| 5 way 180° DIN socket (chassis mounting) |
| Photographs © Rapid Electronics |
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.
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| Photographs © Rapid Electronics |
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!
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| Photographs © Rapid Electronics |
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 project.
Standard UK telephone connectors are similar in style but a slightly different shape. They are called BT (British Telecom) connectors.
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.
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.
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| Photograph © Rapid Electronics |
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| Photograph © Rapid Electronics |
The picture shows 6-core cable, but 4-core and 8-core are also readily available.
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| Screened cable (mono) |
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| Screened cable (stereo) |
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| Screened cable (stereo) |
| Photographs © Rapid Electronics |
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| Construction of a screened cable |
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| Photograph © Rapid Electronics |
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| Photograph © Rapid Electronics |
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.
Rapid Electronics stock a wide range of connectors and they have kindly allowed me to use their photographs on this page. The photographs are from their Image Gallery CD-ROM
Construction | LIN & LOG | Rheostat | Potentiometer | Presets
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| Standard Variable Resistor Photograph © Rapid Electronics |
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.
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.
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| Rheostat Symbol |
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.
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| Potentiometer Symbol |
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.
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| Preset Symbol |
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.
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| Preset (open style) |
Presets (closed style) |
Multiturn preset |
Pin numbers | IC holders | Static | Datasheets | Sinking/sourcing | Combining outputs |
555 and 556 Timers | Logic ICs | 4000 Series | 74 Series | PIC microcontrollers
Also see: 4000 Series ICs | 74 Series ICs | 555 and 556 Timer Circuits
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.
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.
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| Antistatic bags for ICs Photograph © Rapid Electronics |
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.
| PDF files To view and print PDF files you need an Acrobat Reader which may be downloaded free for Windows, Mac, RISC OS, or UNIX/Linux computers. If you are not sure which type of computer you have it is probably Windows. |
Datasheets are available as PDF files from:
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 project to make the red LEDs flash alternately.
The maximum sinking and sourcing currents for an IC output are usually the same but there are some exceptions, for example 74LS TTL logic ICs can sink up to 16mA but only source 2mA.
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 1-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.
Example projects: Traffic Light | Dice | Model Lighthouse
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.
For further information please see the page on 555 and 556 timer circuits.
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. | |
| Outputs | Can sink and source about 5mA (10mA with 9V supply), enough to light an LED. To switch larger currents use a transistor. | Can sink and source about 20mA, enough to light an LED. To switch larger currents use a transistor. | Can sink and source about 20mA, enough to light an LED. To switch larger currents use a transistor. | Can sink up to 16mA (enough to light an LED), but source only about 2mA. To switch larger currents use a transistor. |
| 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. |
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| Driving 4000 or 74HC inputs from a 74LS output using a pull-up resistor. |
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.
| Quick links to individual ICs 4000 4060 |
However the CMOS circuitry also means that they are static sensitive. Touching 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.
For further information, including pin connections, please use the quick links on the right or go to 4000 Series ICs.
| Quick links to individual ICs 7400 7432 |
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!
For further information, including pin connections, please use the quick links on the right or go to 74 series ICs.
The CMOS circuitry used in the 74HC and 74HCT series ICs means that they are static sensitive. Touching 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.
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| www.picaxe.co.uk |
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 stocked by Rapid Electronics.
Types | Connecting | Soldering | Heat sinks | Testing | Codes | Choosing | Darlington pair
Also see: Heat sinks | Transistor Circuits
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 hFE.
For further information please see the Transistor Circuits page.
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| Transistor circuit symbols | |
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.
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| Transistor leads for some common case styles. |
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.
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| Crocodile clip Photograph © Rapid Electronics. |
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.
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| Heat sink
Photograph © Rapid Electronics |
For further information please see the Heat sinks page.
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| Testing an NPN transistor |
Test each pair of leads both ways (six tests in total):
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| A simple switching circuit to test an NPN transistor |
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.
Some multimeters have a 'transistor test' function which provides a known base current and measures the collector current so as to display the transistor's DC current gain hFE.
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. The quantities shown in the table are explained below.
| NPN transistors | ||||||||
| Code | Structure | Case style |
IC max. |
VCE max. |
hFE min. |
Ptot 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 | |
| Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data. | ||||||||
| PNP transistors | ||||||||
| Code | Structure | Case style |
IC max. |
VCE max. |
hFE min. |
Ptot 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 |
| Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data. | ||||||||
| 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. |
| Case style | There is a diagram showing the leads for some of the most common case styles in the Connecting section above. This information is also available in suppliers' catalogues. |
| IC max. | Maximum collector current. |
| VCE max. | Maximum voltage across the collector-emitter junction. You can ignore this rating in low voltage circuits. |
| hFE | 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 IC 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 hFE? It is one of a whole series of parameters for transistors, each with their own symbol. There are too many to explain here. |
| Ptot 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 IC × VCE. For transistors operating as switches the maximum collector current (IC 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. |
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:
Colours | Sizes and shapes | Resistor value | LEDs in series | LED data | Flashing | Displays
Example: 
Circuit symbol: 
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.
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!
For an accurate value please see Calculating an LED resistor value below.
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.
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.
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Photograph © Rapid Electronics |
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.
Rapid Electronics stock a wide selection of LEDs and their catalogue is a good guide to the range available.
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 = (VS - VL) / I |
VS = supply voltage
VL = LED voltage (usually 2V, but 4V for blue and white LEDs)
I = LED current (e.g. 10mA = 0.01A, or 20mA = 0.02A)
Make sure the LED current you choose is less than the maximum permitted and convert the current to amps (A) so the calculation will give the resistor value in ohms ().
To convert mA to A divide the current in mA by 1000 because 1mA = 0.001A.
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.
, so choose 390 (the nearest standard value which is greater).
So R = (VS - VL) / I
For more information on the calculations please see the Ohm's Law page.
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 VL.
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.
VL = 2V + 2V + 2V = 6V (the three LED voltages added up).
If the supply voltage VS is 9V and the current I must be 15mA = 0.015A,
Resistor R = (VS - VL) / I = (9 - 6) / 0.015 = 3 / 0.015 = 200,
so choose R = 220 (the nearest standard value which is greater).
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.
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 | IF max. |
VF typ. |
VF max. |
VR 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 |
| IF max. | Maximum forward current, forward just means with the LED connected correctly. |
| VF typ. | Typical forward voltage, VL in the LED resistor calculation. This is about 2V, except for blue and white LEDs for which it is about 4V. |
| VF max. | Maximum forward voltage. |
| VR 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. |
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| Bargraph | 7-segment | Starburst | Dot matrix |
| Photographs © Rapid Electronics | |||
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| Pin connections diagram © Rapid Electronics |
Also see: Display Drivers.
Also see: LEDs | AC and DC | Power Supplies
Example: 
Circuit symbol: 
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.
Ordinary diodes can be split into two types: Signal diodes which pass small currents of 100mA or less and Rectifier diodes which can pass large currents. In addition there are LEDs (which have their own page) and Zener diodes (at the bottom of this page).
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.
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.
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 |
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.
Also see: Power Supplies
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The diagram shows the operation of a bridge rectifier as it converts AC to DC. Notice how alternate pairs of diodes conduct.
Also see: Power Supplies
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| Various types of Bridge Rectifiers Note that some have a hole through their centre for attaching to a heat sink Photographs © Rapid Electronics | ||||
Circuit symbol: 
Zener diodes are used to maintain a fixed voltage. They are designed to 'breakdown' in a reliable and non-destructive way so that they can be used in reverse to maintain a fixed voltage across their terminals. The diagram shows how they are connected, with a resistor in series to limit the current.
Zener diodes can be distinguished from ordinary diodes by their code and breakdown voltage which are printed on them. Zener diode codes begin BZX... or BZY... Their breakdown voltage is printed with V in place of a decimal point, so 4V7 means 4.7V for example.
Zener diodes are rated by their breakdown voltage and maximum power:
Polarised (> 1µF) | Unpolarised (< 1µF) | Real Values | Variable & trimmers
Also see: Capacitance and Uses of Capacitors
Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):
Capacitor values can be very difficult to find because there are many types of capacitor with different labelling systems!
There are many types of capacitor but they can be split into two groups, polarised and unpolarised. Each group has its own circuit symbol.
Examples: 
Circuit symbol: 
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.
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
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.
For example: 472J means 4700pF = 4.7nF (J means 5% tolerance).
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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.
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.
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.
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| Variable Capacitor Symbol |
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| Variable Capacitor Photograph © Rapid Electronics |
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.
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| Trimmer Capacitor Symbol |
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| Trimmer Capacitor Photograph © Rapid Electronics |
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.
Example: 
Circuit symbol: 
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. is quite small so resistor values are often given in k and M. = 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:
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.
Find out how to make your own Resistor Colour Code Calculator
. 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
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.
For example:
= 2700 = 1000 k 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.
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| High power resistors (5W top, 25W bottom) Photographs © Rapid Electronics |
Power ratings of resistors are rarely quoted in parts lists because for most circuits the standard power ratings of 0.25W or 0.5W are suitable. For the rare cases where a higher power is required it should be clearly specified in the parts list, these will be circuits using low value resistors (less than about 300) or high voltages (more than 15V).
The power, P, developed in a resistor is given by:
| P = I² × R or P = V² / R |
where: | P = power developed in the resistor in watts (W) I = current through the resistor in amps (A) R = resistance of the resistor in ohms ( ) V = voltage across the resistor in volts (V) |
resistor with 10V across it, needs a power rating P = V²/R = 10²/470 = 0.21W. resistor with 10V across it, needs a power rating P = V²/R = 10²/27 = 3.7W. The picture that you can see is known as a circuit diagram or a schematic diagram. This is the standard way of showing an electronic circuit so that you can see how the circuit should work. Each component has its own symbol which indicates what it's function is. There are a number of different electronic component circuit symbols in the electronics reference section. The symbol on the left is for a battery, at the top there is the symbol for a switch and at the right the circle with a cross in it is the symbol for an lamp (or indicator lamp).
Note that when a circuit is created there is often a component layout diagram which shows how the components are installed onto the circuit board. This is useful if you are creating a replica of a circuit that has already been designed, but it is the schematic (circuit diagram) that is most useful for understanding how and why a circuit works as it does.
Unfortunately circuit diagrams / schematics do not always look exactly the same as there are differences in the circuit symbols used depending upon region and preference. For example the resistor in the IEC circuit symbols is shown as a rectangle, but in the US a resistor is normally shown as a zig-zag line. In most cases the differences are only small and it's still possible to recognise the symbol even if it is not the one you are familiar with, but in the worst case it usually means there are a couple of extra symbols to remember / look-up.
The picture representation of the switch is not part of the circuit diagram but is provided as a means to interact with the circuit. Also circuit diagrams are normally static and the switch symbol would not normally change to the closed position, or the lamp change colour.
This interactive circuit diagram is written in the Processing programming language. You need a recent working Java plug-in to be able to view this demonstration. If this does not load correctly look for "Active Content" blocked and enable if necessary.
This is an example of a real circuit used in battery operated torches.
For any electronic circuit to work there must be a complete circuit. This means that there must be a connection made out of conducting material that goes in a circle from one terminal of the battery through the equipment and then back to the other terminal of the battery. If there is a gap at any point then we have air which is a bad conductor and as a result nothing will happen.
This is how a switch works.
When the switch is in the open position then it creates a break in the circuit and the light is off. When the switch is closed the metal contacts inside the switch join and complete the circuit.
Press the switch button using your mouse to see the lamp light up.
Some common conductors and insulators are listed in the following table:
| Conductor | Insulator |
|---|---|
| Copper wire | Most plastics |
| Other metals | Dry wood |
| Tap / rain water* | Glass and ceramics |
| People! | Air |
Note that I specifically mention tap and rain water rather than just water. Pure distilled water is actually an insulator, but the impurities in most water turns it into a conductor. Never operate live mains equipment near water or outside in the rain, unless the equipment is specifically designed for that purpose
There is another type of material called a semi-conductor whose properties can change between an insulator and a conductor under certain conditions, but we'll look more at semiconductors later when we get on to active components.
Lets first look at some basics about electricity and how it works. I've kept this to the very minimum so that we can get on to creating our first circuit.
At some stage you will probably need to desolder a joint to remove or re-position a wire or component. There are two ways to remove the solder:
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| Using a desoldering pump (solder sucker) |
1. With a desoldering pump (solder sucker)
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| Solder remover wick Photograph © Rapid Electronics |
2. With solder remover wick (copper braid)
After removing most of the solder from the joint(s) you may be able to remove the wire or component lead straight away (allow a few seconds for it to cool). If the joint will not come apart easily apply your soldering iron to melt the remaining traces of solder at the same time as pulling the joint apart, taking care to avoid burning yourself.
Top of page | How to Solder | Advice for Components | What is Solder? | Desoldering | First Aid
Most burns from soldering are likely to be minor and treatment is simple:
To reduce the risk of burns:
It is very tempting to start soldering components onto the circuit board straight away, but please take time to identify all the parts first. You are much less likely to make a mistake if you do this!
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