Ultrasonic Doppler detector sensing movement of person or object - uses two detectors to measure movement and trigger alarm when certain distance has been covered within set period

摘要

An ultrasonic square-wave is transmitted into the room and the signal received is processed to detect the Doppler shift. The received signal is taken to two synchronous detectors which are controlled by quarter frequency signals derived from the transmitter but ninety degrees out of phase relative to each other. The output of one of the two detectors is used to sample the other output on its positive edge and the inverse of the other output on its negative edge. A Doppler shift produced by movement causes deviation from a non-zero mean to these samples and is integrated to trigger the alarm.