I need to put an alarm siren close to the location of one of my audio speakers. But I don't have a dedicated cable towards this siren. Because the alarm control unit is close to my speaker amplifier I want to use the speaker cable for transmitting a trigger signal for the siren as well.
At the alarm-control-unit/amplifier side I can use an audio relay to connect the speaker cable either with the amplifier or with the siren signal based on whether the alarm is armed or not. (When armed, I don't need audio.)
But on the speaker/siren side I also need to drive a relay to switch between siren and speaker connection. I was thinking on using an Arduino to detect a DC signal on the speaker cable for this purpose, via one of its analog input pins.
Any suggestion for the electronics to use, in particular on the speaker/siren side? Or other cheap and simple mechanisms to multiplex the siren trigger signal on the audio signal?
My first thoughts: Reduce the siren signal level via a voltage divider to a lower level that still can be detected by the Arduino but does not cause a "plop" on the speaker. And a diode in series with a zener diode in order to clip the AC audio signal between 0 and 5V, before feeding it to an analog input pin of the Arduino?