Isolated relay for smoke detector

Discussion in 'The Projects Forum' started by airflap, Oct 11, 2011.

  1. airflap

    Thread Starter New Member

    Oct 10, 2011
    Hi all,
    I want to have a smoke detector trigger an isolated set of relay contacts.
    Be able to power any 12 volt device like a louder alarm or light.

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

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

    The smoke detector is a common 9 volt battery type.
    I dont know what the draw is of a relay coil , but I would hope its under 100 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.
    Attached Images [​IMG] smoke-detector-circuit.gif (7.2 KB, 5 views)
  2. BMorse

    Senior Member

    Sep 26, 2009
    Buy a Do it yourself alarm system (all in one, all built into the horn), such as the ones you can get from an automotive store, connect it according to the directions, use say one of the door inputs as a way to trigger it via a relay, any 9volt low voltage relay will do, just use an NPN (2N3904) type transistor to activate the relay from the speaker output, use a 100 or 200 ohm resistor to connect the base to pin 4 of the output..... so when the smoke detector goes off, so will the alarm...... anyways, this is still relating to your "wiring" in a car, so don't be shocked if it gets closed again....
  3. Georacer


    Nov 25, 2009
    What type is your load? A speaker or a light? It affects your relay choice.
    Last edited: Oct 11, 2011
  4. iONic

    AAC Fanatic!

    Nov 16, 2007
    It's obvious that on this site we can not discuss Automotive alterations of any kind and we wont here.

    In my opinion, the smoke detector peizo alarm would be loud enough. If your not close enough to hear that alarm then you are being negligent as such charging should be done attended and not unattended. Placing Charging LiPo batteries, or any batteries, for that matter inside a car is subject to the exact outcome that you just went through or far worse. Moving the Piezo to outside the car would be one way to make the alarm louder and distinguish it from an all-to-common car horn alarm. But your best bet is to charge your batteries before you leave the house and top them off when you arrive....outside the car!

    sorry mods, if I stepped on your toes a bit. Send me a PM if you have any concerns.
  5. Georacer


    Nov 25, 2009

    I merged your thread with this one, as I believe it draws its content from it, and for now it seems that the situation is under control.
  6. SgtWookie


    Jul 17, 2007
    Actually, LiPo's can burst into flame some time after charging has stopped. If one knows that an item could burst into flames spontaneously, why take that risk?

    Make certain that the batteries are charged up several hours before leaving, and allow them to cool off in their safety container. If they are still OK, then transport them to the site, and finish topping them off OUTSIDE the vehicle prior to using them.

    There is no substitute for proper planning.
    Safety is no accident.
  7. bertus


    Apr 5, 2008
  8. airflap

    Thread Starter New Member

    Oct 10, 2011
    Thanks Bmorse for your suggestion. Would you mind if I pm you with a drawing on the transistor configuration. :)

    Thanks to the group for all your feedback and suggestions.
    I am very familiar with the safety and hazards of lipo battery's. I have been using them for years and know they can cause fires. I wont even charge on store them inside the house. Why take a chance? However most of the time they ride in a metal ammo box in the trunk of my car. I just wanted a better warning system than nothing.

    Example if a battery decided to smoke in the trunk of my car I would have a beter chance to know with a car alarm. Especially if I was in the house or away from the car. This might prevent further damage. No system is fool proof but at least this could monitor the battery box for smoke and give me a little piece of mind. ;)
  9. Georacer


    Nov 25, 2009
    Please don't try getting our sympathy in order to get an appealing answer. Drawing another car-related answer will result in closing the thread for good.

    I hope I judged your intention poorly, and all you meant was justifying your actions.
  10. airflap

    Thread Starter New Member

    Oct 10, 2011
    Moderator Please I have no motive or intent of altering my autos wiring. Perhaps I should have said this was to be used for the safety of my pull behind camper.
    Safety is my utmost priority here. Sorry if I ruffled any feathers.

    Also The (isolated) relay contacts would be good if they are rated for at least 3 amps 12volt.

    Last edited: Oct 11, 2011