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Sharing "Add External Relay to Smoke Detector"

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rnorman3

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I have a basement where a smoke detector is, but you would never hear it if it went off...so I made this simple circuit to connect an external horn to it. I just soldered a wire from the detector's Piezo buzzer to a diode which feeds into a MOSFET which closes the relay. The cap and diode is there to hold the positive voltage on the gate, since the piezo output appears sinusoidal which would turn the mosfet on and off, and the resistor is there to discharge the cap when the detector turns off. I added a 120dB siren with a separate 12V 1A adapter across the relay. The battery seen in the schematic is the 9V battery inside the smoke detector (I just tapped into the red and black wires). I used a 6V relay in case the smoke detectors battery is running low when it were to go off. Works quite well with insignificant power drain from the 9V battery inside the detector (mosfet "Idss" in the nano-amps).

This is just to share. I've seen similar more complex designs with a comparator but new detectors use IC's with sleep mode that wakes up and create a micro surge which sets of the comparator ever 20 seconds or so.
 
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