Circuit Symbols

Next Page: Electricity and the Electron
See also: Circuit Diagrams

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. Please see the projects for some examples.

The circuit symbol images are not copyright. Please feel free to download and use them for schoolwork etc.

Wires and connections

Component Circuit Symbol Function of Component

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

Wires joined 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 Supplies electrical energy.

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

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

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

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

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

For more information please see the Components page or click on the component name in the table.

Next Page: Electricity and the Electron | Studying Electronics

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Wires and connections
Component	Circuit Symbol	Function of Component
Wire		To pass current very easily from one part of a circuit to another.
Wires joined		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		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		Supplies electrical energy. Single cells are often wrongly called a battery, but strictly a battery is two or more cells joined together.
Battery		Supplies electrical energy. A battery is more than one cell.
DC supply		Supplies electrical energy.
AC supply		Supplies electrical energy.
Lamps, Heater, Motor, Bell, Buzzer
Component	Circuit Symbol	Function of Component
Lamp (lighting)		A transducer which converts electrical energy to light. This symbol is used for a lamp providing illumination, for example a car headlamp or torch bulb.
Lamp (indicator)		A transducer which converts electrical energy to light. This symbol is used for a lamp which is an indicator, for example a warning light on a car dashboard.
Heater		A transducer which converts electrical energy to heat.
Motor		A transducer which converts electrical energy to kinetic energy (motion).
Bell		A transducer which converts electrical energy to sound.
Buzzer		A transducer which converts electrical energy to sound.
Switches
Component	Circuit Symbol	Function of Component
Push Switch (push-to-make)		A push switch allows current to flow only when the button is pressed. This is the switch used to operate a doorbell.
Push-to-Break Switch		This type of push switch is normally closed (on), it is open (off) only when the button is pressed.
On-Off Switch (SPST)		SPST = Single Pole, Single Throw. An on-off switch allows current to flow only when it is in the closed (on) position.
2-way Switch (SPDT)		SPDT = Single Pole, Double Throw. A 2-way changeover switch directs the flow of current to one of two routes according to its position. Some SPDT switches have a central off position and are described as 'on-off-on'.
Dual On-Off Switch (DPST)		DPST = Double Pole, Single Throw. A dual on-off switch which is often used to switch mains electricity because it can isolate both the live and neutral connections.
Reversing Switch (DPDT)		DPDT = Double Pole, Double Throw. This switch can be wired up as a reversing switch for a motor. Some DPDT switches have a central off position.
Resistors
Component	Circuit Symbol	Function of Component
Resistor		A resistor restricts the flow of current, for example to limit the current passing through an LED. A resistor is used with a capacitor in a timing circuit.
Variable Resistor (Rheostat)		This type of variable resistor with 2 contacts (a rheostat) is usually used to control current. Examples include: adjusting lamp brightness, adjusting motor speed, and adjusting the rate of flow of charge into a capacitor in a timing circuit.
Variable Resistor (Potentiometer)		This type of variable resistor with 3 contacts (a potentiometer) is usually used to control voltage. It can be used like this as a transducer converting position (angle of the control spindle) to an electrical signal.
Variable Resistor (Preset)		This type of variable resistor (a preset) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment. Presets are cheaper than normal variable resistors so they are often used in projects to reduce the cost.
Capacitors
Component	Circuit Symbol	Function of Component
Capacitor		A capacitor stores electric charge. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.
Capacitor, polarised		A capacitor stores electric charge. This type must be connected the correct way round. A capacitor is used with a resistor in a timing circuit. It can also be used as a filter, to block DC signals but pass AC signals.
Variable Capacitor		A variable capacitor is used in a radio tuner.
Trimmer Capacitor		This type of variable capacitor (a trimmer) is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment.
Diodes
Component	Circuit Symbol	Function of Component
Diode		A device which only allows current to flow in one direction.
LED Light Emitting Diode		A transducer which converts electrical energy to light.
Zener Diode		A special diode which is used to maintain a fixed voltage across its terminals.
Photodiode		A light-sensitive diode.
Transistors
Component	Circuit Symbol	Function of Component
Transistor NPN		A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.
Transistor PNP		A transistor amplifies current. It can be used with other components to make an amplifier or switching circuit.
Phototransistor		A light-sensitive transistor.
Audio Devices
Component	Circuit Symbol	Function of Component
Microphone		A transducer which converts sound to electrical energy.
Earphone		A transducer which converts electrical energy to sound.
Loudspeaker		A transducer which converts electrical energy to sound.
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.
Meters and Oscilloscope
Component	Circuit Symbol	Function of Component
Voltmeter		A voltmeter is used to measure voltage. The proper name for voltage is 'potential difference', but most people prefer to say voltage!
Ammeter		An ammeter is used to measure current.
Galvanometer		A galvanometer is a very sensitive meter which is used to measure tiny currents, usually 1mA or less.
Ohmmeter		An ohmmeter is used to measure resistance. Most multimeters have an ohmmeter setting.
Oscilloscope		An oscilloscope is used to display the shape of electrical signals and it can be used to measure their voltage and time period.
Other Symbols
Component	Circuit Symbol	Function of Component
LDR Light Dependent Resistor		A transducer which converts brightness (light) to resistance (an electrical property).
Thermistor		A transducer which converts temperature (heat) to resistance (an electrical property).
Inductor (Coil, Solenoid)		A coil of wire which creates a magnetic field when current passes through it. It may have an iron core inside the coil. It can be used as a transducer converting electrical energy to mechanical energy by pulling on something.
Transformer		Two coils of wire linked by an iron core. Transformers are used to step up (increase) and step down (decrease) AC voltages. Energy is transferred between the coils by the magnetic field in the core. There is no electrical connection between the coils.
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.
Fuse		A safety device which will 'blow' (melt) if the current flowing through it exceeds a specified value.
Aerial (Antenna)		A device which is designed to receive or transmit radio signals. It is also known as an antenna.
Earth (Ground)		A connection to earth. For many electronic circuits this is the 0V (zero volts) of the power supply, but for mains electricity and some radio circuits it really means the earth. It is also known as ground.