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Motion Detector Alrams

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Dialtone

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The term ambient light is somewhat misleading according to my understanding of the circuit. What they are really doing is using the infared beam as a carrier for a 5KC signal, much like a radio would do. The receiver has a tuned section to pass the 5KC signal to the detector stage. Loss of the signal would trigger an alarm output. If they just used an infared beam without a modulated signal, it is possible to fool the receiver with stray light in the proper frequency range of the detector.
Dialtone
 
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