In electrical systems, a static relay is a type of relay, an electrically operated switch, that has no moving parts. Static relays are contrasted with electromechanical relays, which use moving parts to create a switching action. Both types of relay control electrical circuits through a switch that is open or closed depending upon an electrical input. Static relays may be based on analog solid state circuits, digital logic circuits, or microprocessor-based designs. Some authors use the term "static relay" to refer only to solid state relays.