Relay

A relay is an electromagnetic switch operated by a small electric current. The current can turn on or off a much larger electric current. The basic part of a relay is an electromagnet. It consists of a coil of wire. It becomes a temporary magnet when electricity flows through it. When power flows through the circuit with the coil, it activates the electromagnet, generating a magnetic field that attracts a contact and activates the second circuit. When the power is switched off, a spring pulls the contact back up to its original position. When power flows in the opposite direction, a magnetic field is changed and the other contact can be connected which activates an another circuit.

Many sensors in electronic equipment produce only small electric currents. But often we need them to drive bigger pieces of apparatus that use bigger currents. That is why relays are needed. Relays can work either as switches or as amplifiers.