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

Switches

Component	Circuit Symbol	Function
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.

Selecting a Switch

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

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.

Switch Contact Ratings

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.

Resistors

Component	Circuit Symbol	Function
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:
Resistor (Variable)		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.
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.
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.

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.

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

Resistance is measured in ohms, the symbol for ohm is an omega Ω.
1Ω is quite small so resistor values are often given in kΩ and MΩ
1Ωk = 1000Ω     1ΩM = 1000000Ω.

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

Most resistors have 4 bands:

1. The first band gives the first digit.
2. The second band gives the second digit.
3. The third band indicates the number of zeros.
4. 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 (which has the value of 2), Violet (which has the value of 7), Yellow (which means that we need to add 4 zeros to the amount) and gold bands.

So its value is 270000Ω = 270kΩ.

On circuit diagrams the is usually omitted and the value is written 270K.

Small value resistors (less than 10 ohm)

For example:

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

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 390Ω resistor with a tolerance of ±10% will have a value within 10% of 390Ω, between 390 - 39 = 351Ω and 390 + 39 = 429Ω (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.

Capacitors

Component	Circuit Symbol	Function
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.

Capacitors store electric charge. They are used with resistors in timing circuits because it takes time for a capacitor to fill with charge. They are used to smooth varying 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.

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 to the power of 6 (millionth), so 1000000µF = 1F

n means 10 to the power of 9 (thousand-millionth), so 1000nF = 1µF

p means 10 to the power of 12 (million-millionth), so 1000pF = 1nF

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

Diodes

Component	Circuit Symbol	Function
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.
Photo diode		A light-sensitive diode.

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

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 LED's and Zener diodes.

Connecting and soldering

Diodes must be connected the correct way round, the diagram may be labeled 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 labeled 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.

Transistors

Component	Circuit Symbol	Function
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.
Photo transistor		A light-sensitive transistor.

Transistors amplify current, for example they can be used to amplify the small output current from a logic chip 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.

Types of Transistor

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 labeled 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!

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 strip board 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.

Audio and Radio Devices

Component	Circuit Symbol	Function
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.
Antenna (Aerial)		A device which is designed to receive or transmit radio signals. It is also known as an antenna.

Meters and Oscilloscope

Component	Circuit Symbol	Function
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 multi meters 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.

Voltmeters

Voltmeters measure voltage.

Voltage is measured in volts, V.

Voltmeters are connected in parallel across components.

Voltmeters have a very high resistance.

Measuring voltage at a point

Connect the black (negative -) voltmeter lead to 0V, normally the negative terminal of the battery or power supply.

Connect the red (positive +) voltmeter lead to the point you where you need to measure the voltage.

The black lead can be left permanently connected to 0V while you use the red lead as a probe to measure voltages at various points.

You may wish to use a crocodile clip on the black lead to hold it in place.

Voltage at a point really means the voltage difference between that point and 0V (zero volts) which is normally the negative terminal of the battery or power supply. Usually 0V will be labeled on the circuit diagram as a reminder.

Analogue meters take a little power from the circuit under test to operate their pointer. This may upset the circuit and give an incorrect reading. To avoid this voltmeters should have a resistance of at least 10 times the circuit resistance (take this to be the highest resistor value near where the meter is connected).

Ammeters

Ammeters measure current.

Current is measured in amps (amperes), A.
1A is quite large, so mA (milliamps) and µA (micro amps) are often used. 1000mA = 1A, 1000µA = 1mA, 1000000µA = 1A.

Ammeters are connected in series.
To connect in series you must break the circuit and put the ammeter across the gap, as shown in the diagram.

Ammeters have a very low resistance.

The need to break the circuit to connect in series means that ammeters are difficult to use on soldered circuits. Most testing in electronics is done with voltmeters which can be easily connected without disturbing circuits.

Galvanometers

Galvanometers are very sensitive meters which are used to measure tiny currents, usually 1mA or less. They are used to make all types of analogue meters by adding suitable resistors as shown in the diagrams below. The photograph shows an educational 100µA galvanometer for which various multipliers and shunts are available.

Ohmmeters

An ohmmeter is used to measure resistance in ohms (Ω). Ohmmeters are rarely found as separate meters but all standard multi meters have an ohmmeter setting.

1Ω is quite small so kΩ and MΩ are often used.

1kΩ = 1000Ω

1MΩ = 1000kΩ = 1000000Ω.

Multi meters

Multi meters are very useful test instruments. By operating a multi-position switch on the meter they can be quickly and easily set to be a voltmeter, an ammeter or an ohmmeter. They have several settings (called 'ranges') for each type of meter and the choice of AC or DC.

Some multi meters have additional features such as transistor testing and ranges for measuring capacitance and frequency.

Analogue multi meters consist of a galvanometer with various resistors which can be switched in as multipliers (voltmeter ranges) and shunts (ammeter ranges).

Sensors

Component	Circuit Symbol	Function
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).

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 multi meter 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 1MΩ.
• Very bright light: minimum resistance, about 100Ω.

An LDR 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 thermistor's have a negative temperature coefficient (NTC) which means their resistance decreases as their temperature increases. It is possible to make thermistor's with a positive temperature coefficient (resistance increases as temperature increases) but these are rarely used. Always assume NTC if no information is given.

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

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

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.

Logic Gates

Component	Circuit Symbol		Function
	(Traditional)	(IEC Symbol)
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).

Logic gates process signals which represent true (1, high, +Vs, on) or false (0, low, 0V, off).

Logic gates process signals which represent true or false. Normally the positive supply voltage +Vs represents true and 0V represents false. Other terms which are used for the true and false states are shown in the table on the right. It is best to be familiar with them all.

Gates are identified by their function: NOT, AND, NAND, OR, NOR, EX-OR and EX-NOR. Capital letters are normally used to make it clear that the term refers to a logic gate.

Note that logic gates are not always required because simple logic functions can be performed with switches or diodes:

Switches in series (AND function)

Switches in parallel (OR function)

Combining chip outputs with diodes (OR function)

Inputs and Outputs

Gates have two or more inputs, except a NOT gate which has only one input. All gates have only one output. Usually the letters A, B, C and so on are used to label inputs, and Q is used to label the output. On this page the inputs are shown on the left and the output on the right.

The Inverting 'O'

Some gate symbols have a circle on their output which means that their function includes inverting of the output. It is equivalent to feeding the output through a NOT gate. For example the NAND (Not AND) gate symbol shown on the right is the same as an AND gate symbol but with the addition of an inverting circle on the output.

Truth Tables

A truth table is a good way to show the function of a logic gate. It shows the output states for every possible combination of input states. The symbols 0 (false) and 1 (true) are usually used in truth tables. The example truth table on the right shows the inputs and output of an AND gate.

Logic IC's

Logic gates are available on special ICs (chips) which usually contain several gates of the same type, for example the 4001 IC contains four 2-input NOR gates. There are several families of logic ICs and they can be split into two groups:

4000 Series

74 Series

The 4000 and 74HC families are the best for battery powered projects because they will work with a good range of supply voltages and they use very little power. However, if you are using them to design circuits and investigate logic gates please remember that all unused inputs MUST be connected to the power supply (either +Vs or 0V), this applies even if that part of the IC is not being used in the circuit!