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:

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