Relays

Relays are electrically controlled switches.In the usual type ,a coil pulls in an armature when sufficient coil current flows.

Many  varieties are available, including "latching" and "stepping" relays;the letter provided the cornerstone for telephone switching  stations, and they are still populer in pinball machines.

Relays are available for dc and ac excitation,and coil voltages from 5 volts up to 110 volts are common . "Mercury-wetted " and "reed" are intended for high speed (~1ms) applications, and giant relays intended to switch thousands of amps are used by power companies.

Many previous relay applications  are now handled with transistor or FET switches,and  devices known as solid-state relays  are now available to handle ac switching  applications.

Because it is important to keep electronic circuits electrically isolated  from the ac power line , relays are useful to switch ac power line while keeping  the controle signals  electrically  isolated.Relays are very useful in electronics and very popular in electricians.