Not sure if others would have interest in this, but I'm designing a PCB for an easy-to-solder TH node. As presently conceived, it would have 3 surface mount components (an LED on the front and a resistor and capacitor on the back), plus a DIP atmega328p, a header to accept an inexpensive si7021 TH breakout board, an FTDI header, and an RFM69HW. The idea is that it would run at 8Mhz and be powered by two AA batteries, so it's sized to be compact but still easy to solder. The same thing could be achieved with wires and some of the other boards out there, but this might be a little tidier if a TH mote is what you want as either the starting point or the end-point.

PCB dimensions are 0.65x2.55 inches. i.e. it is narrower than a typical AA battery holder, but roughly the same length.

The LED and resistor are optional, and you could forego the capacitor as well if you wanted a truly bare bones TH solution. However, the pads are there if you wanted to utilize them. Also, you could skip the si7021 BoB if you wanted just a generic mote.

I had hoped that someone would make an easy to solder board like this for the RFM69HW, but I got tired of waiting and finally decided to just make my own.

I found a good place to do range testing.... too bad I didn't bring the gear!

Good news! Last night I did some accelerated load testing on the supercap. First I charged it to 3.6v and then I hooked up an RFM69HW mote which woke up once a second to do 3 things: 1. check the voltage level, 2. turn on an LED for 1ms to simulate a sensor load, and 3. transmit a packet containing the voltage data using the RFM69HW.. Bottom line: 14,111 packets transmitted before running out of juice.

Not bad for a first attempt.

Yesterday received the PCB. Today assembled for testing this battery-powered nRF52-based passive infrared motion detector:

Made this 12 button keypad. Requires only one analog pin to read which button is pressed, and any button press can also wake an arduino from sleep:

Consumes no power when no button is pressed.

A pair of lithium AA primaries is hard to beat because:

1. Unlike alkaline's, they don't leak.
2. Have a look at the discharge curve: https://data.energizer.com/pdfs/l91.pdf By the time they drop to 2.4v, if not before, you'll want to replace them.
3. Obviously much longer life, both shelf life (20 years!) and energy capacity.

I think running 8Mhz from the internal RC is a no-brainer: wake up time is less than 4us. So, if your node wakes up often, you'll save a ton of energy over time.

The best time to take your battery measurement is immediately after a Tx. That will give you the most conservative reading. Save that measurement in a variable and then send it in your next transmission. Switch on your ADC just before Tx and take your first ADC measurement during Tx, because you have to throw out the first measurement anyway. That way you can take a fresh (and valid) ADC measurement just after Tx before the voltage rebounds.

Hope that helps!

Here's the version that I most recently assembled:

As you can see, the 15F supercap is now on the board itself. It works fine.

I've since made a few refinements and have sent the new files off to be fabbed. The newest version of the PCB will measure roughly 22mm x 22mm.

Put together this pro mini nRF24 shield for testing...

I have a number of sensor projects that sit within the nice footprint of a Pro Mini. Now I want to find a kind of universal power platform--hopefully small--to power them. So far, what I came up with was this, which runs the Pro Mini from 2xAAA batteries, and where the Pro Mini (and whatever the project built on it) simply "plugs in" to it:

Is there anything better? Ideas? Comments?

@vecnar Nice job! I think that I too will mostly stick with my current smoke alarm setup until either prices on the Nest drop to parity levels or something better comes along. Kinda the "if it isn't broken, don't fix it" philosophy. If it turns out that de-dusting my smoke alarms on a regular basis provides 100% anti-false alarm protection, then I may never feel the need to upgrade at all. Meanwhile, I think I will replace my ionizing smoke alarms with photoelectric ones, since photoelectric smokes alarms are thought to have many fewer false alarms.

@vecnar If I'm understanding right, your device listens for a loud sound and then triggers on that and only that. So, then the tradeoffs are you want to set the threshold low enough that it triggers whenever the alarm siren sounds, but high enough that it doesn't trigger on false events, like a car honking in the distance or possibly someone dropping something on the floor or a baby crying. Well, I imagine that would work, though you might get occasional false positives. If your device could be dialed in to only trigger on sustained loud noise, rather than bursty noise, it might cut-out the false positive of someone dropping a vase on the floor, or the like. One thing to possibly be wary of is whether there's drift in the sound threshold over time or temperature of the components in your detection device.

Depending on how your alarm sounds, you could maybe improve on that. For instance, if it has some kind of repeating on-off-on-off pattern to the siren, then you could program an arduino to listen for that pattern specifically, instead of just purely level of loudness. I'd wager that would be quite accurate at triggering if and only if your smoke alarm sounds. Otherwise, there are projects out there where you can allegedly "teach" a sensor to identify your particular alarm's sounds.using classifiers. I happened to notice such a project was published just a few days ago: https://www.hackster.io/news/sandeep-mistry-walks-through-tinyml-audio-classification-using-tensorflow-lite-raspberry-pi-rp2040-f4065b654565

Worthy of note is that bCNC now supports camera alignment for two sided PCB boards. You just need a spindle mounted camera, and then you simply show bCNC a couple of points (by centering the camera cross-hairs on them) around which to do the alignment for the opposite side. This replaces the need to use physical alignment pins to align the two sides, as flatcam does. I haven't yet tried it, but it sounds like a very nice convenience. The "Ant PCB Maker" project has been leveraging this feature of bCNC for one or two years now. I don't believe the existence of bCNC's camera alignment capability is widely known, though, as I only just recently heard about it, and even then only by chance.

Thinking about it now, two known points are required when nothing is pinned because that's how many points are needed to re-establish the origin as well as correct for any rotation that may have occurred after the board is flipped over. So, presumably, first a new origin needs to be established, and then all of the g-code for the second side needs to be amended after-the-fact to account for any rotation that may have happened when the board was flipped.

Seems like only way to be sure is to test it with canned smoke or similar. If there's a way for a smoke detector to automatically test itself without that, then that would be awesome. The Nest Protect says it tests itself 400 times a day, but as I wrote earlier: what does that really mean? For instance, only just that it can communicate with the sensor? If so, that would be a pretty weak test. Ideally you'd like to know that the sensor is actually working as intended and able to detect smoke in accordance with its specs. Maybe by detecting dust particles in the air it would have some insight into whether it's functioning or not? So, maybe a particle detector would make a superior smoke detector? It probably would have to run off of mains power though.

No problem. I had to look into this for my own situation anyway.

I'm not sure what to think of the Consumer Reports test results. It may be true that some detectors detect certain kinds of fires faster than others, but if the price of that turned out to be enduring a high number of false alarms.... is that something that you'd really want? For instance, ionization smoke detectors are known to be much better at detecting fast developing fires, but they are also known to be much more prone to false alarms. AFAIK, Consumer Reports doesn't test the propensity of different smoke detectors to have false alarms. From what I've read, Nest Protect's 2nd generation smoke detector was designed to avoid some of the frequent causes of false alarms (such as insects and shower steam) that other smoke alarms may falsely trigger on, and that's something that the Consumer Reports rating doesn't factor into its numeric rating. i think I like the nest approach of giving a gentle "heads up" when it first starts to detect low levels of smoke rather than doing what most other smoke detectors do, which is doing nothing at all until a higher threshold is reached and then going straight to emergency alarm mode with sirens blaring.

Much to my surprise, Cosumer Reports gave its highest rating of 93 to the First Alert SCO501CN, whereas it scored the Nest Protect at merely 62, because it did not perform well on either carbon monoxide or fast flame spread detection. The SCO501CN is the battery powered, wirelessly interconnected smoke and carbon monoxide detector with Voice location that I had referenced above. I think if the trade-off is between better sensors (First Alert) or better firmware (Nest Protect), I'd lean toward the better sensors, because that is more mission critical.

Except... Gotcha. The SCO501CN it warranteed for only 7 years, and the listing says it should be replaced every 5 years to ensure an effective CO sensor: https://www.firstalertstore.com/store/products/sco501cn-3st-onelink-wireless-talking-battery-operated-smoke-and-co-alarm.htm
which means buying two SCO501CN over a 10 year period instead one Nest Protect.

Also, as you would expect, there are quite a few arduino alarm listening projects out there on the internet, such as:
Simple Smoke Alarm Listening Circuit for Arduino – 11:13
— Kevin Darrah

I suppose the one thing to be sure about is that it triggers on the frequency of your smoke/CO alarm and not something else, like a car alarm or leak detector. If mounted nearby to the alarm siren on an interconnected system, then I'm guessing you might also want to include audio loudness as a discriminator.

I see that you already found the mysensors opto-isolator project:
How To - Smoke Alarm Monitoring with Arduino and MySensors – 11:01
— Pete B

Lastly, this is Nest Protect's argument for why they think their wireless interconnect is better than wired interconnection: https://support.google.com/googlenest/answer/9231654?hl=en

So, though I haven't yet checked, it occurs to me that the following might work: have a single hardwired smoke alarm with wireless interconnect (BRK does make these), and maybe replace all the other smoke alarms with with battery powered wireles interconnect (BRK also makes those). Then, the question is: would the wirelessly interconnected smoke alarms trigger a BRK relay that's connected to the one hardwired smoke alarm that is also wirelessly interconnected? I'm pretty sure the answer to that would be "Yes". One possible gotcha (?) from that is that perhaps the one hardwired BRK that includes wireless interconection maybe wouldn't relay its location to the other smoke alarms if it were triggered, but, if so, maybe you could utilize that information to identify it as the triggering alarm since it would be the one occasion when the other alarms might not be able to verbally identify the source. Obviously, this is speculative and would need to be confirmed. It would be a shame if BRK disabled all the location information just because there was a single hardwired (but wirelessly interconnected) smoke detector in the mix.

@vecnar said in Connecting smoke detectors to house alarm:

@NeverDie
Everything is now being driven by batteries, unfortunately lithium batteries is just another thing that can create fire, that is just my association with lithium batteries after watching too many liveleak videos.
I think the X-Sense comes with "3 V CR123A lithium battery" and they say should last for 5 years. Below is example of worst case scenario and doesn't look too bad compared to phone batteries.
CR123A Lithium Battery Fire – 00:30
— HabboX

Interesting find. I haven't heard of any actual non-abused lithium primary cells doing that in real life though. Have you? It has me wondering now. There's no dendrites that I know of like there are with rechargeable lithium batteries, which is the main culprit in those disasters.

Having smoke detector on battery for 5-10 years is good way of doing it as it allows you to place sensor anywhere you want and not to depend on AC wiring.
The only thing to check if it is possible to link more than 1 remote control, 1 for each floor/location. Possibly as you would need to get to remote first before going to check originating sensor.
According to below website it is best to use a combination of ionization and photoelectric sensors but not sure if any have 2, I looked on a few and only photoelectric was listed.
https://www.nfpa.org/Public-Education/Staying-safe/Safety-equipment/Smoke-alarms/Ionization-vs-photoelectric
X-sense also has smart wifi smoke detector, I assume it still operates internally between other smoke sensors over RF 868 or 915MHZ and uses 2.4GHZ wifi network and internet connection to get to their servers and notify you on the phone. If it would depend on wifi alone power loss to Access Point would cause communication breakdown between the sensors.
Let me know which one you will go for and your findings.

I suppose the argument in favor of hardwired smoke alarms is that people are lazy and may not replace batteries, or might remove the batteries altogether if they were battery powered to avoid the chirping but then remount the smoke alarm (for aesthetic reasons) and then forget about it. However, as long as one doesn't do that, I think an argument can be made that battery powered smokes are actually safer because they're immune to power surges that might damage them if they were wired to mains. If that were to occur, when would it be discovered? During a weekly test? Well, if laziness is a concern, I don't think doing a weekly test is very likely. In fact, I doubt if more than a tiny minority of people do a weekly smoke alarm test, and I'm not at all sure how thorough those tests really are in terms of discovering failure modes.

So, out of the battery powered smoke alarms, the BRK battery powered wireless interconnected smoke alarm with voice location does seem to work as expected, even if the hardwired ones don't. The Nest Protect looks even better, but the price is more than 2x as much. If I could convince myself that the Nest Protect actually did a good job of self testing itself (as it claims to), then I probably would pay up for that. So far, though, all the reviews I've seen have been far too superficial to address that question other than to say something like "It tests itself 400 times a day" or some such. Yeah, but what does that really mean? How thorough is the test, and is it really good enough? It might just be marketing puffery.

Neither of them work with a relay that I'm aware of. However, there is a Ring Listener that listens for a smoke alarm going off. It has the virtue of being rather inexpensive:

https://www.amazon.com/Ring-Alarm-Smoke-CO-Listener/dp/B07M93Z1NT/ref=sr_1_5?dchild=1&keywords=ring+listener&qid=1631294313&s=hi&sr=1-5

I'm not sure if there are other listeners available. I suspect so, but I haven't looked. It might save the bother of DIYing a custom solution. Alternatively, the Nest probably sends a text to your phone in the event of a smoke detection, and that could perhaps be used to activate a separate home alarm through ITTT or similar. Maybe it can trigger something less indirect in the event of a smoke detection? If so, if it could avoid needing to DIY a solution, perhaps that might tilt the decision in favor of a Nest. However, I'd rather a solution didn't blithely assume that internet connectivity is working. Something more direct would certainly be preferable.

P.S. BRK has OneLink, but the reviews on amazon, including nearly all recent reviews, have made it look like a terrible choice--far worse than I would have imagined, especially given its high price. For that reason, I wouldn't consider a OneLink until well after the issues raised are known to be fixed. For the price, people should be getting top quality, and it doesn't read like they are. Nearly all the most recent reviews are pretty consistently extremely negative and in stark contrast to the amazon reviews of the second generation Nest Protect, which has been on the market for quite some time now.

P.P.S. This article refers to a number of other alarm listeners:https://www.cnet.com/home/smart-home/devices-to-help-your-smart-home-guard-against-fire/ The article is from 2016, though, and I haven't confirmed whether all of those products are still on the market or not. I did check on Halo, and they went out of business in 2018. In contrast, the Roost Battery thing actually sounds kind of interesting: https://getroost.com/sensors/ [But is it even still available? Their webstore says it's sold out] Some ebay sellers have it, but at a much higher price. And if it hinges on using a special battery.....I don't know that I'd want to be chained to that except as maybe a short-term stopgap. ]

There's probably somethig out there that I haven't seen that might do the job. Overall, it seems like a strangely stagnant market filled with products that aren't user friendly. I presume the stranglehold that BRK and Kidde have on the American market has probably stifled innovation, at least as far as what's available here that has regulatory approval. I'd be surprised if Europe doesn't have better options than we do here in the US.

Unfortunately, it appears that the x-sense smokes I referenced may not be adequately tested:
https://www.firemarshals.org/resources/Documents/NASFM Documents/NASFM News Release on Non UL 217 Listed Alarms 6 8 2016.pdf

There don't seem to be many good choices available. It might be best to wait a few years and see if anything better comes onto the market.