ELECTRICAL SYMBOLS – K Hinds

Circuit symbols are used in circuit diagrams which show 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 stripboard or printed circuit board.

Wires and connections Component Circuit Symbol
Wire

Function of Component
To pass current very easily from one part of a circuit to another. 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 Tjunctions, as shown on the right. In complex diagrams it is often necessary to draw wires crossing even though they are not connected. I prefer the 'bridge' symbol shown on the right because the simple crossing on the left may be misread as a join where you have forgotten to add a 'blob'!

Wires joined

Wires not joined

Power Supplies Component
Cell

Circuit Symbol

Function of Component
Supplies electrical energy. The larger terminal (on the left) is positive (+).
A single cell is often called a battery, but strictly a battery is two or more cells joined together.

Battery

DC supply

AC supply

Fuse

Transformer

Earth (Ground)

Supplies electrical energy. A battery is more than one cell. The larger terminal (on the left) is positive (+). Supplies electrical energy. DC = Direct Current, always flowing in one direction. Supplies electrical energy. AC = Alternating Current, continually changing direction. A safety device which will 'blow' (melt) if the current flowing through it exceeds a specified value. 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. 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.

ELECTRICAL SYMBOLS – K Hinds

Output Devices: Lamps, Heater, Motor, etc. Component Circuit Symbol
Lamp (lighting)

Function of Component

Lamp (indicator)

Heater Motor

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

Bell

Buzzer

A transducer which converts electrical energy to sound.

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.

Switches Component
Push Switch (push-to-make) Push-to-Break Switch On-Off Switch (SPST) 2-way Switch (SPDT)

Circuit Symbol

Function of Component
A push switch allows current to flow only when the button is pressed. This is the switch used to operate a doorbell. This type of push switch is normally closed (on), it is open (off) only when the button is pressed. SPST = Single Pole, Single Throw. An on-off switch allows current to flow only when it is in the closed (on) position. 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 = 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.

Dual On-Off Switch (DPST)

ELECTRICAL SYMBOLS – K Hinds

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.

Resistors Component
Resistor

Circuit Symbol

Function of Component
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:

Variable Resistor (Rheostat)

Variable Resistor (Potentiometer)

Variable Resistor (Preset)

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. 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. 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
Capacitor

Circuit Symbol

Function of Component
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. 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.

Capacitor, polarised

ELECTRICAL SYMBOLS – K Hinds

Variable Capacitor

A variable capacitor is used in a radio tuner. 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.

Trimmer Capacitor

Diodes Component
Diode LED
Light Emitting Diode

Circuit Symbol

Function of Component
A device which only allows current to flow in one direction. A transducer which converts electrical energy to light. A special diode which is used to maintain a fixed voltage across its terminals. A light-sensitive diode.

Zener Diode Photodiode

Transistors Component
Transistor NPN

Circuit Symbol

Function of Component
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 and Radio Devices Component Circuit Symbol
Microphone

Function of Component
A transducer which converts sound to electrical energy. A transducer which converts electrical energy to sound.

Earphone

ELECTRICAL SYMBOLS – K Hinds

Loudspeaker

A transducer which converts electrical energy to sound.

Piezo Transducer

A transducer which converts electrical energy to sound. An amplifier circuit with one input. Really it is a block diagram symbol because it represents a circuit rather than just one component. A device which is designed to receive or transmit radio signals. It is also known as an antenna.

Amplifier
(general symbol)

Aerial (Antenna)

Meters and Oscilloscope Component Circuit Symbol
Voltmeter

Function of Component
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

Ohmmeter

A galvanometer is a very sensitive meter which is used to measure tiny currents, usually 1mA or less. An ohmmeter is used to measure resistance. Most multimeters have an ohmmeter setting. An oscilloscope is used to display the shape of electrical signals and it can be used to measure their voltage and time period.

Oscilloscope

Sensors (input devices) Component Circuit Symbol
LDR

Function of Component
A transducer which converts brightness (light) to resistance (an electrical property). LDR = Light Dependent Resistor A transducer which converts temperature (heat) to resistance (an electrical property).

Thermistor

ELECTRICAL SYMBOLS – K Hinds

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

Gate Type
NOT

Traditional Symbol

IEC Symbol

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

AND

NAND

OR

NOR

EXOR

EXNOR