Ultrasonic burglar alarm units - use oscillator transmitted frequency signal and Doppler shift detection to trigger alarm

摘要

The burglar alarm pair of elements consists of an ultrasonic transmitter unit and a matching receiver unit which detects Doppler shift in the received signal. The units are supplied from the mains and may be plugged in when required. The transmitting unit (10), which carries a plug for direct mains connection (28) contains an oscillator (19) transmitter circuit and an ultrasonic signal transducer. The receiver unit contains an ultrasonic transducer, a signal receiver, a demodulator (38), a low frequency amplifier and an alarm circuit. The transmitter inject a reference signal into the mains which the receiver unit detects and compares with the incoming ultrasonically transmitted signal. Any Doppler shift in this triggers the alarm circuit.