These projects exist because modding life-critical safety equipment is usually not the best idea. You invalidate the testing originally put into the product, and it may fail when you most need it.

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.

Yeah, everything that touches them needs to be UL listed. Commercial alarm panels have approved relay modules that can be used to output to various unregulated devices without affecting the electronics inside the panel for this reason. They also use cellular now and have to use a special modem that is UL-listed to connect.

I agree ! But I have a few fire alarms, and 2 water leak alarms, that would be a lot of wires lying around, and this solution is wireless :)

Then what? Run 50-200 feet of wire back to an Arduino? Or buy an Arduino and provision it for every smoke detector in the house? How do you power them? Another wall wart?

If audio fingerprinting can identify Miley Cyrus in background noise in a crowded bar, certainly it can identify the symmetrical piercing monotone of an alarm.

An arduino BLE, 18650 battery will probably run you about 50 bucks (retail, one-off). From experience, that will run for at least 4+ years in low-power mode -- maybe even decades. Announce a BLE service with the alarm status.

Agreed, I have no idea what the sibling comments are on about with safety. A binary sensor reading the wire going to the speaker isn't some magic modification that invalidates the safety and you can get teeny tiny sensors with zigbee off the shelf.

> A binary sensor reading the wire going to the speaker isn't some magic modification that invalidates the safety

That’s where you have to be careful about what you’re calling “safety”. There’s “does this, even a little bit, make it less likely that my fire alarm will go off at the appropriate time?” and there’s “will this modification invalidate the UL certification of the fire alarm?”

If reasonably designed (e.g. not low impedance) you could pretty readily conclude that the answer to the first question is No. However… the answer to the second question is almost certainly Yes, which can transition from irrelevant to “a very big deal” if there’s an incident that gets fire marshals and insurance companies doing an investigation.

It'd be even simpler to set up a piezoelectric sensor or a mic right on the device. If the device is vibrating, there's a 99.9% chance its going off.