| 摘要 |
An infrared intruder detection system operable under severe weather conditions including fog, rain and snow over great distances exceeding 500 feet in the severest weather comprises a pulsed infrared transmitter and a self-contained spaced apart self-synchronized infrared receiver. Upon interruption of the transmitted beam for a predetermined period, the receiver generates an alarm in response to the remaining ever present signals due to ambient light and thermal instrument noise. When the beam is not interrupted, a local synchronous oscillator will track the incoming signal pulses. In the presence of an intruder, the pulsed beam is no longer received and the oscillator is unable to track the resulting amplified noise signal which arises from ambient light and thermal instrument noise. This results in the generation of an error signal at the output of the demodulator to which the oscillator is coupled. By the inclusion of appropriate bandpass amplifiers, filters and integrating circuits set to respond to predetermined thresholds coupled to the output of the demodulator, the receiver is able to discriminate between an intruder and such momentary interrupts as are caused by birds, falling leaves and blowing bits of paper which otherwise might cause false alarms.
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