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.

Hey guys!  I hate to dig this thread back up, but my searching for my situation brought me here (thanks to Troy on a different thread).  I too bought the winter sale kit (6-zone add-on board, 5v relay) and am trying to replace my ADT system, but that add-on board obviously doesn't have the 12v alarm-out.

I guess my main question is: can I just use the 12v AUX out (not alarm out) on the original ADT alarm panel to provide power to the siren?  If yes, I assume the rest of the wiring would look similar to Troy's image above, but instead of the "plug" image in the lower-right, it'd be that ADT AUX out panel?  I'm literally trying to just re-use that ADT panel as regulated power to the siren only since the add-on board doesn't have it.

Also, I suspect that I have a dumb siren anyway because it was capable of two-way conversations with the ADT dispatchers in the event the alarm went off.  I will say that it looks exactly like the "DSC Indoor Surface mount" siren that this Konnected website sells, but I don't know if that means it will work w/o a siren driver board or replacement.  Any thoughts on this would be appreciated to.


Your board should have a 12v aux output it's just not switched so you could wire your siren like the following, just make sure the siren draws less than an amp or 12 watts  or you risk damaging the board.  To eliminate the risk to the board you could use one of the splitters Konnected sells and one of the DC Power Screw Terminal Adapters to convert that to the red and black 12volt wires.  Lots of options.


Thanks, Troy!  The ADEMCO schematic on the original alarm panel door shows "10.5-13.8VDC, 2A max. (600mA MAX for UL usage)..." for the Alarm Output terminals.    So is it technically stating the siren will draw no more the 600mA and the rest is some overhead amperage?


Troy - Here was my original idea (as I gratuitously steal your drawing) to use the existing alarm panel aux-out to power the siren as it's already a "known good" configuration as it comes down to proper voltage/amperage to the siren:


  Does this appear viable?  This will likely be a short-term, proof-of-concept until I determine whether I'll need a siren driver in addition to the splitters and terminals.


 That looks like it should work as long as the siren is 600ma max and if that is the case it should work fine through the Konnected Aux as well.  Have you opened the siren to see if there is power requirement info in it or a model number you could lookup to see what the power requirements are?  From your message yesterday it seemed like you were looking in the alarm panel rather than in the siren case.

Thanks again Troy!  So I went up there and pulled the siren off the wall.  It didn't have any identifying information on it or the cover.  It does look exactly like model "DSC SC 15W-ULF" which going by DSC's specs show "12VDC/380mA/90dB".  It's wired black(-) and red(+) "YELP", with the yellow(+) "STEADY" wire simply clipped.  So you said as long as it doesn't draw an amp or 12 watts, that the Konnected board should drive it.  This says 15W, but it's 380mA?  So, it would appear too much of one (the wattage), but OK on the other (380mA)?  Please excuse my lack of electronics knowledge.  :-)

It does "yelp" when it goes off, but I'll be darned if I can figure out how ADT heard me say my "safe word" out loud when I set it off accidentally and they came across the speaker asking me to identify myself, unless they're able to switch that speaker to a async microphone when they're not transmitting their voice.

 The Konnected board I believe is actually rated at 1.2 amps which is just under 15 watts, so even if the first split second of draw took 15amps then went down to the average 380ma it shouldn't be a problem for the board.

If you are worried it might overload your board you could eliminate that risk by using the splitter and the DC screw terminal to connect it.  That way the load is solely on the 2.5 amp transformer they supply by default.


That seems like an inexpensive route to take to protect the board.  I think I'll go that direction.  Thanks Troy!