Sensing 12v & Switching on/off 12v device?

I have two arduino's and the accompanying ethernet shields (Arduino Ethernet shield R3 with micro SD connector - Assembled : ID 201 : $45.00 : Adafruit Industries, Unique & fun DIY electronics and kits).

On one arduino I want to sense when a 12v wire has power applied to it (e.g. a security system siren), send a notification over the LAN to the second arduino to tell it to turn on power to a second 12v device (e.g. a second siren and the power would be provided from a separate power source).

My first question is, what is the best way to sense the 12v power on from the first arduino. And second, what would the best approach be to turn on the 12v device on the second arduino?

Thanks so much.

I would just use a resistor voltage drop to make it just about 5V (Ex. 10k and a 5k) in series to ground and picking the voltage at the middle point and applying to the input pin on the Arduino.

On the second one, I would use a relay of some sort (that would require a small amount of current like 20mA) or some sort of transistor that would then turn a relay on. I have used 2803A in the past to run relays with success.

Now, I don't know how much current this device you want to power uses, but pick a relay that can handle your current requirements.

As a hobbyist I just want to make sure and I'm not sure if it makes a difference. On the first arduino, it would be watching the line passively. The power to the siren would still be provided by the security system. Basically detecting when the siren is on.

You could use a small reed relay on both ends. Put the first 12v relay coil in parallel with the first siren so when the siren is activated the relayis also actuated. The arduino would monitor the relay contacts for open/close. No direct electrical connection between the two. The second arduino could operate a second relay to operate the second siren.

Zoomkat: That's a good idea.

Does the below schematic look correct?

12v+ ------------------------------- 12v- (1.5amps)
| |
8 (resistor) |
| |
| |
| |
| Arduino Pin |
| | |
--- (RR coil)---------

first 12v relay coil in parallel

In parallel that means the currents are different!!!

The remote siren will be the same, 1-1.5 amps with independent power supplies.