Run Multiple Sirens With a Cheap Automotive 12V Relay.

Turns out, the alarm system in my house had a second, larger siren up in the attic.  What I thought was a 15 Watt interior siren turned out to be a 45 Watt siren system.  Needless to say, I damaged my board.

So now that I have a new board, and I still need to drive 45W worth of sirens.

Lots of ways to do this, but it seems to me the easiest way is to is a  $9 12V automotive-type relay.  These are cheap, durable, and literally every auto parts store has them.

I'm using the 12v siren  signal from the board to energize the relay.  (Typical resistance for an automotive 12V relay is 150 Ohms, so they typically pull 80 milliamps or so at 12V-- well under the board's 1.4A limit.)  

Power to the siren comes from a beefy 5A 12VDC power supply that needs enough capacity to power  all the sirens, the relay, and the board itself.  The board, relay and sirens all share the same 12V ground, and siren power runs through the switched side if the relay.

Many automotive relays have 5 terminals.  Two are for energizing the relay's coil.  One is to provide power to the switched circuit, and the remaining two are for the switched load, (the sirens in our case.) Why two?  One is normally open and gets power only when the coil is energized.  The other is normally CLOSED and opens then the coil is energized.  Don't use this one. ;)

Simple and relatively straightforward.  And these relays are generally rated at 30 or 40 Amps.  You can run a LOT of sirens with 30A, but you'd need a seriously big 12VDC power supply.

Power Supply:

  • Hello Robert,

    This is very informative. I am in same boat. Will it be possible for to share wiring details?

    So the Konnected panel comes with 12v 2A power supply, should we not use that power supply and instead use the 12V 5A as you suggested? I have two boards main board and add-on board.

    I am not able to understand how did you wire them all. Any details you can share will be greatly appreciated.

    Thank you,



  • Is there a reason the single relays Konnected sells wouldn't work for this?  I would think you'd connect the relay module to the 5v +, -, and out pins on an add-on board to power and trigger the relay then run the + from the power supply to the sirens + input and run the - from the power supply to Com on the relay  then the - from the siren to the NO terminal on the relay.

  •  Something like this?


  • @Troy Thank you for posting this idea. I just got the relay kit form winter sale and seems to be a workable option. However, I have used the out to drive my piezo buzzers so in this case, I think I would have to use a zone 1-5 as out which I don't mind as long as it works.

    I will try that later this week.

  • Yeah, that should work, good luck.

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