Relay

A relay is an electromagnetic switch. In other words it is activated when a current is applied to it. Normally a relay is used in a circuit as a type of switch (as you will see below). There are different types of relays and they operate at different voltages. When you build your circuit you need to consider the voltage that will trigger it.
The main part of a relay is the coil at the center. A small current flowing through the coil in the relay creates a magnetic field that pulls one switch contact against or away from another. Putting it simply, when current is applied to the contacts at one side of the relay the coil allows the contacts at the other side to work. Usually relays are used to turn on a second circuit. The first circuit activates the relay which then ‘turns on’ the second circuit.