Polarised Capacitors (Large Values, 1Μf +)

LED

Example: Circuit symbol:

LEDs must be connected the correct way round, the diagram may be labelled a+ for anode and k or - for cathode (yes, it really is k, not c, for cathode!).

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

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
Photographs © Rapid Electronics

ICs 555 Timer

Example circuit symbol (above)

Actual pin arrangements

Polarised capacitors (large values, 1µF +)

Examples: Circuit symbol:

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.

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!

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.

Trimmer capacitors

Trimmer Capacitor Symbol
Trimmer Capacitor
Photograph © Rapid Electronics

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.

Resistors

Example: Circuit symbol:

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.

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

Resistor shorthand

560R means 560
2K7 means 2.7 k = 2700
39K means 39 k
1M0 means 1.0 M = 1000 k

Power Ratings of Resistors

Photographs © Rapid Electronics

High power resistors
(5W , 25W )

Variable Resistors

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.

Standard Variable Resistor
Photograph © Rapid Electronics

Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along

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.

Rheostat Symbol

This is the simplest way of using a variable resistor. Two terminals are used: one connected to an end of the

Diodes

Example: Circuit symbol:

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!

Zener diodes

Example: Circuit symbol:
a = anode, k = cathode

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.

Transistors

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 Darlingtonpair is two transistors connected together to give a very high current gain.

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.

Transistor codes

There are three main series of transistor codes used in the UK:

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.

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.

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.

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.

Thermistor

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, 400.

Standard Switches

Type of Switch / CircuitSymbol / 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.
Photograph © Rapid Electronics / /
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.
Photograph © Rapid Electronics / /
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.
Photograph © Rapid Electronics / /
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.
Photographs © Rapid Electronics / /
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.
Photograph © Rapid Electronics / /
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.
Photograph © Rapid Electronics / /
DPDT slide switch

Wiring for Reversing Switch

Op Amps