Household Alarm Notifier

I'm not sure this is something I want to pursue right now, but I stumbled upon a Reddit comment from [Mike Szczys] some time ago that stuck with me. He brings up a great point regarding utilitarian IoT devices; as an early purchaser and rapid abandon-er of a Fitbit, I'm definitely interested in cheap, connected sensors that have real value.

He used the example of the mandated CO/smoke detector--why didn't it send notifications to one's smartphone?

I started thinking about how I could get around this. Build a smoke/CO detector? Probably doable, provided I could find the parts. It would probably be easiest to use a reference circuit from the various sensor datasheets, assuming that sort of thing exists--maybe the First Alert company (and others) actually manufacture and own the supply chain for their sensors. Who knows?

I think a better approach could be to bypass the alarm guys entirely. Not that there isn't room for a product like that; last time I bought a combo detector I think it was in the neighborhood of $50, give or take. Seems like these units often come with other fancy features (like the dude that yells, "Fire, Fire, Fire" whenever I cook steaks), so maybe there is enough room in the budget to throw in an ESP8266 or something. But then you'd need to worry about the safety side of things--it's a smoke/CO alarm, and your life really does depend on it working reliably. At the very least, you'd want to install an unmodified factory original unless you somehow got the project through certification.

What about an ESP8266, a suitable microcontroller (or itself--from what I understand, folks have figured out how to program them), a microphone, and maybe a dedicated A/D chip? Or an analog bandpass filter, tuned to the audio frequency of a typical fire alarm? Put the system in some type of 'training' mode when it's near the alarm and trigger a smoke alarm test. Could a Trinket Pro handle the A/D part natively to minimize size and board count?

I don't want to spread myself too thin. I just picked a project up that I'd set aside for another project, which I've now set aside. But the Trinket Pro contest could be fun.

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Stryker295 wrote 12/15/2014 at 04:48

I think you could connect a wire to one of the leads of the beeper, and just watch for the voltage to change—set it up like a capacitance sensor, using one wire, rather than actually trying to measure the current/voltage. Throw in a transistor or something–beeper triggers on, pin gets pulled low, no need for the microphone.

Disclaimer: I've never actually worked with pulling pins hi/low via capacitance sensing... so I may have things backwards -.-

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zakqwy wrote 12/15/2014 at 14:51

I think that's a good thought. Even just looking for a voltage across the piezo speaker would probably work--the input impedance should be such that you wouldn't affect the functionality of the alarm. However, I'm also just generally concerned about _anything_ that involves cracking open the case on a smoke detector, just because it's such a safety critical piece of equipment. Maybe it's the paranoid homeowner in me saying this, but a hacked smoke detector seems like a goldmine for an insurance company looking to skip a claim.

But... smoke detectors are cheap, and you could always put in two. I think a good amount of time and energy needs to go in to making the interface setup dead-simple too so anyone can use the system--you shouldn't need an FDTI cable to set up the email target for notifications, for example.

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