Infrared intruder alarm system - uses pulsed 10 to 100 kilohertz high frequency modulated signal transmitted using IR LED

摘要

The infrared detector senses the presence of a person using a signal which is modulated at high frequency - pref. between ten and one hundred kilohertz. Using a high frequency signal, and a square pulse signal, a signal is produced comprising high frequency pulses at intervals of less than 100ms, and duration less than 20ms. A 12 volt supply (1) is connected to a voltage regulator (2) and two circuits (3,4) provide the carrier wave signal and the pulse signal. A multiplier (5) combines these signals. A power stage (6) amplifies the signal for application to an infrared light emitting diode (7), transmitting radiation via a lens (8) towards a receiver. The receiver includes a phase locked loop circuit, with integrator and comparator removing parasitic signals before detection.