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Smoke Alarm Circuit(Photo-interrupter module)

2015-03-02 18:07  
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Smoke Alarm Circuit(Photo-interrupter module)
The photo-interrupter is simply an optical coupler with the elements separated with a slot. Anything that enters this slot reduces the current transfer ratio (output current /input current). I used the H21B1 Darlington device since I did not have the H21A1 on hand. I am glad I did, because I learned that it is a better device, requiring far less LED current. Since the H21A1 /H21B1 series is no longer available from DigiKey, I selected a currently available, more inexpensive device—the Sharp GP series—see schematic(s) for details. Sharp appears to be the leader in this product.I did not test the circuit with these devices.