Then put up two fire alarms, one unmodified and one where you replace the speaker with an Arduino that does whatever you want with the signal. They only cost $1

Then put up two fire alarms, one unmodified and one where you replace the speaker with an Arduino that does whatever you want with the signal. They only cost $10 a piece. Or I guess buy one with wifi or zigbee connectivity, but then you don't have a project anymore.