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Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
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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