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Trigger a Smoke Alarm with an emergency light

iseeall

Hey Everyone,

  I am fairly new to building raw circuits without the use of a micro controller.  I have a small project I am working on for my grandfather.  He is hellbent on the idea that if the power goes out at his place his c-pap machine will sufficate him.  He knows I tinker so he asked if I could put something together.  So I found 2 small cheap devices that I believe should be able to be hacked together to make an alarm loud enough to wake him if the power goes out.

 

I have a small wall pluggable emergency led flashlight.  One of those deals that when the power goes out the light immediately turns on. 

Second I have a simple smoke detector.   So what I was thinking is this.

 

I took the smoke detector apart and there are 2 leads that when bridged fire the "test" mode on the alarm.  Perfect.  Normally with a micro-controller i would just use a transistor or mosfet and wire it up to the leads and done.  However, not wanting to put a micro controller on it.  I was trying to figure out how I can use the current going to the led in the light to trip the transistor.  That way as soon as you flip the switch on the light, or the power comes back on, the alarm turns off. 

 

But I am lost at how to get the led current to trigger the transistor.

 

Any help would be hugely appreciated both by me and my grandfather.  Thanks!

  • Hi Jason,

    You could try to use an Opto-isolator. Use the power from the emergency light to turn on the optoisolator LED and use the other side of the opto-isolator to act like a switch to trigger the test on the smoke alarm. I am not sure which of many different Opto-isolators to recommend as I do not have the specs on your two units. It sounds like you have some experience so a little data sheet reading and testing of your unit should allow you to make a reasonably good choice.

    John

  • John's idea is good, but a more simple method would be a plug pack (or Wallart) that energises a relay.

    When the power goes out (or is unplugged) it triggers the smoke alarm.

    Use the contacts maked nc and common (the marking are in an unpowered state)

     

     

    I would test your smoke alarm to see what affect leaving the test button in does, it might timeout after a few seconds.

     

    Your alternative is a home alarm box that normall gets mounted outside, they contain a battery to power the siren.

     

     

    Mark

  • in reply to mcb1

    Thanks guys.  Definitely giving me some more ideas on how to accomplish this.  The idea of the relay may work since I am using that emergency light as the connection to the ac power.  I could wire up the relay so the circuit is open while power is there and close the circuit on power fail since the smoke alarm and emergency light have their own batteries this should be an issue.  I may try that this evening