Smoke Alarm Circuit

Posted on Feb 4, 2014

The original smoke alarm circuit has some problems and has left some hobbyists and students disappointed. Rather than to attempt to solve its many problems, I have created an update that uses the venerable LM741 op amp. It is simpler, more conventional, easier to understand and has much better battery life. The photo-interrupter is simply an opti

Smoke Alarm Circuit
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cal 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. Although, I recommend the Darlington device, do not despair if you have the non-Darlington version. See the smoke alarm schematic for alternative devices /circuits. These were not tested, but I see no reason why they are not viable. I had my doubts as to the effectiveness of the technique, so I tested a photo-interrupter with smoke. The oscillograph demonstrates what happens to the photo-transistor collector voltage when the gap is obscured with smoke from my soldering iron as it melted rosin core solder. Cool! It does work. The oscillograph also has a trace showing what happens when a transparent poly film is inserted into the gap. It has a lower signature. I figured if it could see this effectively, it is an easier test, so the remaining testing is with film rather than smoke. Ambient light also affects the output voltage. In my case, it was only about ±0. 1V, but the sensor may need...

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