Smart energy-saving devices can detect when a room is empty using motion sensors and then turn off appliances through a control switch if no one is present. Some sensors detect body heat, while others send out waves (ultrasonic, microwave or radio) that reflect off any moving object. The former are generally preferred for energysaving devices, as they can distinguish whether someone is standing or sitting still. When no person is detected, a signal is sent to a control box that is attached to an appliance – normally a plug socket or light switch – to turn it off. For convenience, most devices also have a time delay, so leaving the room briefly will not instantly turn off all your appliances.