Solid state relay (SSR) is an electronic switching device that switches states when an external voltage is applied along its n-type and p-type junctions.
SSR has a small control signal that controls a larger load current or voltage. It consists of a sensor which responds to an appropriate input (control signal),
a solid-state electronic switching device which switches power to the load circuitry, and some coupling mechanism to enable the control signal to activate this switch without mechanical parts. The relay may be designed to switch either AC or DC to the load. It serves the same function as an electromechanical relay, but has no moving parts.