Salvaging an Ionization Smoke Detector... Disabling End of Life Circuit?  - Page 1
Forum section: electronics / projects

author: doublec4
author_rank: Regular Contributor
timestamp: February 24, 2020, 04:00:04 pm
content: Hi all,I recently replaced one of the smoke detectors in my home (basic "First Alert" smoke detector 9120FA, has the interconnect feature) ... started making several beeps indicating that the detector has reached its "end of life." I would love to re-purpose this detector and use it for the piezo alarm inside, NOT as a smoke detector as I know this is unsafe. I was thinking something along the lines of a "door alarm."Firstly, I'd love to understand how these detectors know they have reached the end of life, and is there a way to disable this feature along with the ionization detection? I realize each brand and model is going to be different, but the bare bones detectors probably have a lot of similarities... hoping to get some general advice.If I re-purpose this detector, I do not want it constantly chirping from the end of life feature (annoying) and if I use it in my garage I do not want the alarm going off each time I start the car. I'm thinking I can simply trigger the interconnect line to set the alarm off... this way I can easily interface with the alarm (just need to pull the interconnect line high when the door opens).If anyone has any experience hacking into these things that would be great!(Also, I realize I can just pull out the piezo and build some kind of driving circuit for it, but the goal here is to learn about how the detector works, along with hopefully being able to easily modify a regular detector)Thanks!

author: james_s
author_rank: Super Contributor
timestamp: February 24, 2020, 05:20:59 pm
content: Usually it's just the battery voltage, is this one of those sealed units?The detector itself normally doesn't have a defined end of life. We still have an ionization detector from the early 80s in our cabin and it still works fine. The Americium used in the ionization chamber has a very, very long halflife. It is not like the chemical sensors used in many CO detectors which do wear out.

author: TomS_
author_rank: Frequent Contributor
timestamp: February 24, 2020, 05:24:12 pm
content: You may need to install your own microcontroller. Presumably the existing micro contains some EEPROM which it uses to store a count over it's operational life, and once it exceeds some value it starts chirping (if it is that smart). You won't get around that without replacing it with something of your own creation.Otherwise, I would probably start by reverse engineering the circuit, draw a new schematic out of it. That way you will know what it is doing with various parts of the circuit to function the way it does now, and you can more easily figure out how to build your own software to reproduce only the bits you need.I've put some FireAngel smoke alarms in my house, which I later discovered contain a (surface mount) PIC16F. While I may not be able to extract the code, it is prime for repurposing later on simply by switching out the existing PIC or if possible reprogramming it with my own code (I don't recall if there were any headers/pads on the board).

author: james_s
author_rank: Super Contributor
timestamp: February 24, 2020, 07:02:51 pm
content: I had a CO detector with a microcontroller. Something caused the EOL timer to expire way before it should have so I was poking around and found the pic firmware was unprotected which surprised me. I bought another identical unit, dumped the eeprom and wrote that to the other one and that fixed it so I put it downstairs as a backup unit.

author: floobydust
author_rank: Super Contributor
timestamp: February 25, 2020, 12:17:50 am
content: Check which smoke detector ASIC is used in the product.Microchiplineup RE46C200 photoelectric:"The EOL indicator is a user-programmable function. If the EOL indicator function is enabled, then approximately every 15 days of continuous operation, T_EOL, the circuit will read an age count stored in EEPROM, and will increment this age. After 10 years of operation, an audible indication will be given to signal that the unit should be replaced. The EOL indicator is the same as the chamber test failure warning.""A beep or chirp occurring virtually simultaneously with an LED flash indicates a low-supply condition. A beep occurring half-way between LED flashes indicates degraded chamber sensitivity."

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 25, 2020, 02:22:02 am
content: If you hold down the test button and then insert the battery, does it sound? If so, solder a jumper across the button and connect the (normally open) sensor in series with the battery.

author: doublec4
author_rank: Regular Contributor
timestamp: February 25, 2020, 12:25:12 pm
content: Thanks for all of the input guys! I'll have a closer look in the next few days and see what I can come up with based on your suggestions!