Domestic burglar alarm using domestic remote television controls - uses ultrasonic tuning switches as detectors

摘要

The ultrasonic transmitting unit contains a coding network, an oscillator and a transmitting piezoelectric crystal, whilst the receiving unit contains another crystal, an amplifier and a decoder. A burglar alarm is constructed by supplying power continuously to the transmitting unit, and transmitting a steady sinusoidal ultrasonic signal indirectly via the room walls and so on, to the receiving unit. The amplified, received signal is compared with the transmitted one, using connecting cable, and any change of phase or frequency is used to trigger a loudspeaker drive circuit. Instead of ultrasound, an alternative arrangement uses a light beam reflected to and fro across the room by mirrors.