A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch. Another principle—solid-state—is also used in relays. Relays are used where it is necessary to control a circuit by a separate low-power signal, or where several circuits must be controlled by one signal. A solid-state relay (SSR) is a solid-state electronic component that provides a function similar to an electromechanical relay, but does not have any moving components, increasing long-term reliability.
Time-delay relays, or timers, are essentially a relay with a built-in timing circuit to delay the opening or closing of the relay contacts. Timers are often used in industrial controls to sequence various portions of the process. Common timing functions are on-delay, off-delay, one shot, interval, and flashing. Multi-function timers combine multiple functions into one device. Timer outputs can be either electromechanical or solid-state.