Circuits Symbols

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.

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.

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.

Diode

A device that permits current to flow through it in only one direction.

Fuse

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

Motor

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

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.

Light Emitting Diode (LED)

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

Earphone

A transducer which converts electrical energy to sound.

Loudspeaker

A transducer which converts electrical energy to sound.

Microphone

A transducer which converts sound to electrical energy

wire

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

Solar Cell

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

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!

Ammeter

Measures current.

Voltmeter

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

SPST, on-off switch

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

Battery

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

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.

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

Capacitor, unpolarised

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.

Capacitor, polarised

stores electric charge. Polarised capacitors must be connected the correct way round. They usually have larger values, 1µF and greater.