Abstract: In this system, a receiver having adjustable sensitivity to movement by a room occupant picks up continuously generated compressional waves of ultrasonic frequency reflected from the occupant. Movements of the occupant cause changes in frequency of the reflected wave which are detected, and pulses are applied to a timer which applies a timed voltage to a logic circuit. A signal indicating a difference between a preset ambient temperature and that sensed by an electronic sensor is applied to the logic circuit. The logic circuit is arranged to actuate relays to turn a selected heating or cooling unit on if occupant movement is detected and ambient temperature difference exists, and to turn the selected unit off if the room becomes unoccupied. The logic circuit is effective to turn on the heating unit if ambient temperature falls below a preset, low other temperature, even if the room is unoccupied.