Circuit Symbols

Next Page: Electricity and the Electron

Also see: Circuit Diagrams

Circuit symbols are used in circuit diagrams showing how a circuit is connected together. 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 breadboard (for temporary circuits), stripboard or printed circuit board.


Wire and connection symbols

wire symbol

Wire

Connects components and passes current easily from one part of a circuit to another.

wires joined symbol

Wires joined

A 'blob' should be drawn where wires are connected (joined), but it is sometimes omitted. Wires connected at 'crossroads' should be staggered slightly to form two T-junctions, as shown on the right.

wires crossing but not joined symbol

Wires not joined

In complex diagrams it is often necessary to draw wires crossing even though they are not connected. The simple crossing on the left is correct but may be misread as a join where the 'blob' has been forgotten. The bridge symbol on the right leaves no doubt!


Power supply symbols

cell symbol

Cell

Supplies electrical energy. The larger line is positive (+). A single cell is often called a battery, but strictly speaking a battery is two or more cells joined together.

battery symbol

Battery

Supplies electrical energy. A battery is more than one cell. The larger line is positive (+).

solar cell symbol

Solar Cell

Converts light to electrical energy.
The larger line is positive (+).

DC power supply symbol

DC supply

Supplies electrical energy.
DC = Direct Current, always flowing in one direction.

AC supply

AC power supply symbol

Supplies electrical energy.
AC = Alternating Current, continually changing direction.

fuse symbol

Fuse

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

transformer symbol

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.

earth symbol

Earth (Ground)

A connection to earth. For some electronic circuits this symbol is used for 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.


Output device symbols

lamp (lighting) symbol

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) symbol

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 symbol

Heater

A transducer which converts electrical energy to heat.

motor symbol

Motor

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

bell symbol

Bell

A transducer which converts electrical energy to sound.

buzzer symbol

Buzzer

A transducer which converts electrical energy to sound.

inductor symbol

Inductor, Coil, Solenoid

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


Switch symbols

push-to-make switch symbol

Push-to-make switch

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 symbol

Push-to-break switch

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

SPST on-off switch symbol

SPST, on-off switch

SPST = Single Pole, Single Throw. Current flows only when the switch is in the closed (on) position.

SPDT switch symbol

SPDT, 2-way switch

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'.

DPST switch symbol

DPST switch

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.

DPDT switch symbol

DPDT switch

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 symbol

Relay

An electrically operated switch, for example a 9V battery circuit connected to the coil can switch an AC mains circuit. The rectangle represents the coil.
NO = Normally Open, COM = Common, NC = Normally  Closed.


Resistor symbols

resistor symbol

Resistor

A resistor restricts the flow of charge. Uses include limiting the current passing through an LED, and slowly charging a capacitor in a timing circuit.
Some publications use the old resistor symbol: old zig-zag resistor symbol

rheostat symbol

Rheostat variable resistor

A rheostat has 2 contacts and is usually used to control current. Uses include controlling lamp brightness or motor speed and changing the rate of flow of charge into a capacitor in a timing circuit.

potentiometer symbol

Potentiometer variable resistor

A potentiometer has 3 contacts and 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.

preset symbol

Preset 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 standard variable resistors so they are sometimes used in projects to reduce the cost.


Capacitor symbols

Capacitor, unpolarised

capacitor symbol

A capacitor stores electric charge. It can be used with a resistor in a timing circuit, for smoothing a supply (it provides a reservoir of charge) and can be used as a filter (blocking DC signals but passing AC signals). Unpolarised capacitors usually have small values, less than 1µF.

polarised capacitor symbol

Capacitor, polarised

A capacitor stores electric charge. Polarised capacitors must be connected the correct way round. They usually have larger values, 1µF and greater. See above for uses.

variable capacitor symbol

Variable capacitor

A variable capacitor is used in a radio tuner.

trimmer capacitor symbol

Trimmer variable capacitor

This type of variable capacitor is designed to be set when a circuit is made and then left without further adjustment.


Diode symbols

diode symbol

Diode

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

LED symbol

Light Emitting Diode

A transducer which converts electrical energy to light. Usually abbreviated to LED.

zener diode symbol

Zener diode

A zener diode can be used to maintain a fixed voltage.

photodiode symbol

Photodiode

A light-sensitive diode.


Transistor symbols

NPN transistor symbol

Transistor NPN

A transistor amplifies current and can be used with other components to make an amplifier or switching circuit. This symbol is for a bipolar junction transistor (BJT), the type you are most likely to use at first.

PNP transistor symbol

Transistor PNP

A transistor amplifies current and can be used with other components to make an amplifier or switching circuit. This symbol is for a bipolar junction transistor (BJT), the type you are most likely to use at first.

Phototransistor symbol

Phototransistor

A light-sensitive transistor.


Audio and Radio symbols

microphone symbol

Microphone

A transducer which converts sound to electrical energy.

earphone symbol

Earphone

A transducer which converts electrical energy to sound.

loudspeaker symbol

Loudspeaker

A transducer which converts electrical energy to sound.

piezo transducer symbol

Piezo Transducer

A transducer which converts electrical energy to sound.

amplifier symbol

Amplifier (general symbol)

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

aerial symbol

Aerial (Antenna)

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


Meters and Oscilloscope

voltmeter symbol

Voltmeter

Measures voltage. The proper name for voltage is 'potential difference' but voltage is more widely used.

ammeter symbol

Ammeter

Measures current.

galvanometer symbol

Galvanometer

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

ohmmeter symbol

Ohmmeter

Measures resistance. Most multimeters have an ohmmeter setting.

oscilloscope symbol

Oscilloscope

An oscilloscope is used to display the 'shape' of electrical signals - showing how they vary with time. It can be used to measure voltage and time periods.


Sensors (input devices)

LDR symbol

LDR

A transducer which converts brightness (light) to resistance (an electrical property). LDR = Light Dependent Resistor

thermistor symbol

Thermistor

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


Logic gate symbols

Logic gates process signals which represent true (1, high, +Vs, on) or false (0, low, 0V, off). For more information please see the page on logic gates. There are two sets of symbols: traditional and IEC (International Electrotechnical Commission).

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.

NOT gate traditional symbol
NOT gate IEC symbol

Traditional

IEC

AND

An AND gate can have two or more inputs. The output of an AND gate is true when all its inputs are true.

AND gate traditional symbol
AND gate IEC symbol

Traditional

IEC

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.

NAND gate traditional symbol
NAND gate IEC symbol

Traditional

IEC

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.

OR gate traditional symbol
OR gate IEC symbol

Traditional

IEC

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.

NOR gate traditional symbol
NOR gate IEC symbol

Traditional

IEC

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-OR gate traditional symbol
EX-OR gate IEC symbol

Traditional

IEC

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).

EX-NOR gate traditional symbol
EX-NOR gate IEC symbol

Traditional

IEC


Set of circuit symbols

Download a set of the circuit symbols shown above: circuit symbols (zipped)

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