Automotive-style miniature relay, dust cover is taken off A relay is an electrically operated switch . Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays . Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits as amplifiers: they repeated the signal coming in from one circuit and re-transmitted it on another circuit. Relays were used extensively in telephone exchanges and early computers to perform logical operations. A type of relay that can handle the high power required to directly control an electric motor or other loads is called a contactor . Solid-state relays control power circuits with no moving parts , instead using a semiconductor device to perf
