Abstract: A room temperature control system includes a transmitter-receiver unit for producing an acoustic standing wave pattern in a room and for producing a signal when that pattern is disturbed by motion in the room. A logic circuit provides timing signals and interprets a pattern disturbance signal to determine whether it evidences room occupancy. The timing and interpreting sequence is initiated by a switch which detects opening and closing of the room door. Temperature control by the room thermostat is permitted when the room is occupied and control within a wider range can be provided when it is not occupied.