circuit for Smoke Detector to turn car horn on

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Thread Starter


Joined Oct 10, 2011
Hi all,
I had a fire in my car due to lipo battery that overcharged and caught fire. :eek:
I fly RC and charge the batterys in the trunk of my car on the way to the field. Be careful and always store and charge Lipos in a fire proof box.

I'm now using a metal battery box that is vented to a (battery operated) smoke detector.
This should work well if I am driving the car or near by. But what I really want is a way to switch a loud car horn if I am in the house or at the far side of the field.

OK so I am a bit of a nubee with correctly biasing a transistor to do the job.

I would like to use a common automotive relay.

Q1 Do I even need a transistor to drive the coil of a 12 volt common relay?

The smoke detector is a common 9 volt battery type.
I dont know what the draw is of a 12 volt relay coil , but I would hope its under 300 mA.

Q2 Can I take the 2 wires from the smoke detectors alarm/speaker.
and use a transistor to drive a relay?

Hope someone can chime in here with the correct parts needed. :)

Thanks very much.

Attached is a common wire diagram of a smoke detector.



Joined Nov 25, 2009
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