A complete sensor consists of a motion sensor, an electronic control unit, and a controllable switch/relay. The detector senses motion and determines whether there are occupants in the space.[7] It also has a timer that signals the electronic control unit after a set period of inactivity. The control unit uses this signal to activate the switch/relay to turn equipment on or off. For lighting applications, there are three main sensor types: passive infrared, ultrasonic,[8] and hybrid