Solid state relay is an electronic switch having no moving parts. Most often the solid-state relay circuit is en-capsulated either in a plug-in module or as an integral part of the photoelectric control circuitry. It consists of an input circuit, an optocoupler, a power output circuit, and power protection components. One of the primary properties of a relay is its ability to isolate the control circuit from the output circuit, which may be subject to high voltage and transient voltage spikes, which could easily disrupt or destroy the control circuit. The key to circuit isolation in solid-state relay is the optocoupler, a small device consisting of an LED that shines light through an optically transperent electrical insulator toward a photosensitive semiconductor device that is activated by the light.