Home > Circuit Bricks > Relays
UPDATED:.21:11 21 October 2013


A relay is essentially a switch operated by an electro-magnet and is very useful for switching on high current devices.

There is no electrical connection between the switch contacts and the coil, therefore a low voltage circuit (and coil) can switch a larger voltage or high current device.


Since a relay has an electromagnet, it produces EMF which can damage any electronic devices driving it. You must use a silicon diode D1 (1N4001 recommended) to absorb the EMF, connected in reverse bias. (in reverse).

The switch contacts are called changeover contacts and are labelled as:

CM = Common
NO = Normally open
NC = Normally closed.


Relay with Single Pole
Changeover contacts

Relay with Dual Pole
Changeover contacts

Coil is not energised, CM connects to NC

Coil is energised, CM connects to NO

There are many different types of relays but essentially they are all electrically operated switches.

Some have 2 sets of contacts acting as individual switches that switch at the same time but are totally independent electrically.

Other types have 3 or even 4 sets of contacts (poles)

A suitable driver for relays is a single transistor driver.

Usually the CM and the NO contacts are incorporated into another circuit to switch on a device of higher current or voltage. Below is an example of a 6v circuit controlling a 12volt lamp.


There are many types of relays, from PCB miniature, to large plug in types.

4-Pole Relay

Miniature SPDT Relay


PCB mount DPDT Relay
Written by Phil Townshend - 2008
www.edutek.ltd.uk - Working Electronics For Students & Teachers