Relays

Relays are a special type of switch which is turned on and off by an electromagnet. Below is a diagram of a simple relay. When a current flows through the coil an electro-magnetic field is set up. The field attracts an iron armature, whose other end pushes the contacts together, completing the circuit. When the current is switched off, the contacts open again, switching the circuit off.

Relay switch

A useful property of relays is that the circuit powering the coil is completely separate from the circuit switched on by the relay. For this reason relays are often used where a safe low-voltage circuit is being used to control a high-voltage circuit. The symbol for a relay makes the separation of the two circuits clear by separating the coil symbol from the switch symbol:

Relay symbol

The relays used in schools generally a have voltage between 6V and 15V. When using a diode with a relay, be sure that it is connected across the poles (terminals) of the relay to prevent damage to the transistor from back EMF generated when the relay coil switches off.