Abstract

An improved apparatus for switching off power to an electric load in the absence of the detection of movement from the doppler shift of a transmission signal. An oscillator generates an ultrasonic transmission frequency which is transmitted via a transmission plate coupled to the oscillator. A plurality of receivers detect reflections of the ultrasonic sound signal and supply the detected signal to a bandpass filter which passes the transmission frequency. A low-pass demodulator will detect any doppler signal which modulates the transmission signal and will supply it to an amplifier for amplification. The amplified signal is then provided to a narrow band filter which will pass only a narrow band of doppler-shift signals which correspond to the frequencies of human movement. The signal is then supplied to a switching means which, when activated, will discharge a first node. The first node is charged via a resistor and a capacitor coupled to a voltage source. A comparator is coupled to the first node and is also coupled to a voltage reference. The comparator will produce an output signal when the first node has a higher voltage than the voltage reference, thereby enabling a control means which will supply current to the electric load. The detected doppler signal discharges the capacitor voltage thus insuring that an output signal is supplied while movement is detected in the room to keep the electric load supplied with current.