This is a DIY project, and here's what I used:
- Arduino Uno clone
- Ethernet Shield
- 12VDC 1A Horn (by Elk)
- SLA-12VDC-SL-A board
I wrote firmware on the Arduino to (a) provide a temperature analog value, a digital value for a door sensor, and the siren state, and (b) control the siren via a digital output going to the SLA-12VDC-SL-A board. The firmware uses a tiny web server to serve up the values as well as receive on/off commands via HTTP GET.
I also wrote a custom Indigo plug-in that exposes these values.
I mounted the horn on on the side of my garage door to my shop building:
With the plug-in, I'm able to turn on/off the horn as well as toggle it both in the main Indigo window or in a control page. I'm also able to set the state of the horn based on the state of any variable or as part of a script.
Neat!