Circuit Symbols

 

Circuit symbols are used in circuit diagrams which show how a circuit is connected together electrically. They are used when designing and testing circuits, and for understanding how they work.

The actual layout of the components is usually quite different from the circuit diagram. To build a circuit you need a different diagram showing the layout of the parts on stripboard or printed circuit board.

The circuit symbol images are not copyright so please feel free to use them for schoolwork for example. There are complete sets of all the circuit symbols to download, please see below for details.

 

Circuit Symbols

Wires and connections

Component

Circuit Symbol

Function of Component

Wire

wire symbol

To pass current very easily from one part of a circuit to another.

Wires joined

wires joined symbol

This symbol is used in circuit diagrams where wires cross to show that they are connected (joined). The 'blob' is often omitted at T-junctions, but it is vital to include it at crossings.

Wires not joined

wires crossing but not joined symbol

In complex circuit diagrams it is often necessary to draw wires crossing even though they are not connected. I prefer to use the 'hump' symbol shown on the right because the simple crossing on the left looks like a join where you have forgotten to add the 'blob'!

Power Supplies

Component

Circuit Symbol

Function of Component

Cell

cell symbol

Supplies electrical energy.
Single cells are often wrongly called a battery, but strictly a battery is two or more cells joined together.

Battery

battery symbol

Supplies electrical energy. A battery is more than one cell.

DC supply

DC power supply symbol

Supplies electrical energy.

AC supply

AC power supply symbol

Supplies electrical energy.

Lamps, Heater, Motor, Bell, Buzzer

Component

Circuit Symbol

Function of Component

Lamp (lighting)

lamp (lighting) symbol

A transducer which converts electrical energy to light. This symbol is used for a lamp providing illumination, for example a car headlamp or torch bulb.

Lamp (indicator)

lamp (indicator) symbol

A transducer which converts electrical energy to light. This symbol is used for a lamp which is an indicator, for example a warning light on a car dashboard.

Heater

heater symbol

A transducer which converts electrical energy to heat.

Motor

motor symbol

A transducer which converts electrical energy to kinetic energy (motion).

Bell

bell symbol

A transducer which converts electrical energy to sound.

Buzzer

buzzer symbol

A transducer which converts electrical energy to sound.

Switches

Component

Circuit Symbol

Function of Component

Push Switch
(push-to-make)

push-to-make switch symbol

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

push-to-break switch symbol

This type of push switch is normally closed (on), it is open (off) only when the button is pressed.

On-Off Switch
(SPST)

SPST on-off switch symbol

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 switch symbol

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 switch symbol

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 switch symbol

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.

Resistors

Component

Circuit Symbol

Function of Component

Resistor

resistor symbol

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.

Variable Resistor
(Rheostat)

rheostat symbol

This type of variable resistor with 2 contacts (a rheostat) is usually used to control current. Examples include: adjusting lamp brightness, adjusting motor speed, and adjusting the rate of flow of charge into a capacitor in a timing circuit.

Variable Resistor
(Potentiometer)

potentiometer symbol

This type of variable resistor with 3 contacts (a potentiometer) is usually used to control voltage. It can be used like this as a transducer converting position (angle of the control spindle) to an electrical signal.

Variable Resistor
(Preset)

preset symbol

This type of variable resistor (a preset) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment. Presets are cheaper than normal variable resistors so they are often used in projects to reduce the cost.

Capacitors

Component

Circuit Symbol

Function of Component

Capacitor

capacitor symbol

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

polarised capacitor symbol

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

variable capacitor symbol

A variable capacitor is used in a radio tuner.

Trimmer Capacitor

trimmer capacitor symbol

This type of variable capacitor (a trimmer) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment.

Diodes

Component

Circuit Symbol

Function of Component

Diode

diode symbol

A device which only allows current to flow in one direction.

LED
Light Emitting Diode

LED symbol

A transducer which converts electrical energy to light.

Zener Diode

zener diode symbol

A special diode which is used to maintain a fixed voltage across its terminals.

Photodiode

photodiode symbol

A light-sensitive diode.

Transistors

Component

Circuit Symbol

Function of Component

Transistor NPN

NPN transistor symbol

A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.

Transistor PNP

PNP transistor symbol

A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.

Phototransistor

Phototransistor symbol

A light-sensitive transistor.

Audio Devices

Component

Circuit Symbol

Function of Component

Microphone

microphone symbol

A transducer which converts sound to electrical energy.

Earphone

earphone symbol

A transducer which converts electrical energy to sound.

Loudspeaker

loudspeaker symbol

A transducer which converts electrical energy to sound.

Amplifier
(general symbol)

amplifier symbol

An amplifier circuit with one input. Really it is a block diagram symbol because it represents a circuit rather than just one component.

Meters and Oscilloscope

Component

Circuit Symbol

Function of Component

Voltmeter

voltmeter symbol

A voltmeter is used to measure voltage.
The proper name for voltage is 'potential difference', but most people prefer to say voltage!

Ammeter

ammeter symbol

An ammeter is used to measure current.

Galvanometer

galvanometer symbol

A galvanometer is a very sensitive meter which is used to measure tiny currents, usually 1mA or less.

Ohmmeter

ohmmeter symbol

An ohmmeter is used to measure resistance. Most multimeter's have an ohmmeter setting.

Oscilloscope

oscilloscope symbol

An oscilloscope is used to display the shape of electrical signals and it can be used to measure their voltage and time period.

Other Symbols

Component

Circuit Symbol

Function of Component

LDR
Light Dependent Resistor

LDR symbol

A transducer which converts brightness (light) to resistance (an electrical property).

Thermistor

thermistor symbol

A transducer which converts temperature (heat) to resistance (an electrical property).

Inductor
(Coil, Solenoid)

inductor symbol

A coil of wire which creates a magnetic field when current passes through it. It may have an iron core inside the coil. It can be used as a transducer converting electrical energy to mechanical energy by pulling on something.

Transformer

transformer symbol

Two coils of wire linked by an iron core. Transformers are used to step up (increase) and step down (decrease) AC voltages. Energy is transferred between the coils by the magnetic field in the core. There is no electrical connection between the coils.

Relay

relay symbol

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.

Fuse

fuse symbol

A safety device which will 'blow' (melt) if the current flowing through it exceeds a specified value.

Aerial
(Antenna)

aerial symbol

A device which is designed to receive or transmit radio signals. It is also known as an antenna.

Earth
(Ground)

earth symbol

A connection to earth. For many electronic circuits this is the 0V (zero volts) of the power supply, but for mains electricity and some radio circuits it really means the earth. It is also known as ground.