The time is done by counting AC cycles; in this case likely through D7 and the R24/R25 divider. 10 years comes to 18,921,600,000 counts on that input pin so you only need a 35 bit counter. My guess is a RAM based counter counts off big chunk of time and then updates an EEPROM value periodically to make the timer non-volatile.
I had one of these in a house I rented. It went off overnight and scared the hell out of me, because there really WAS a CO risk there. .I took it out to the unheated garage and it didn't stop beeping so I knew it wasn't CO. Then I read the fine print. I wasn't yet schooled by IMSAI or EEVblog then, so I never looked inside....
I got a carbon monoxide detector for the house here and for the first few years I had placed about a metre from the ground... (I know I'm an absolute idiot) When I realised it should be up high I placed it to the proper height. I kept it beside the fire place in our sitting room as I didn't trust the chimney. Anyways 2 days after placing it up to the right height it started going crazy.
everything has a use by date nowadays. i expect that as there is no testing or calibration required every year, they have taken sensor life and the drift of the component values and decided that it won't detect Co correctly. Folk forget that if you have a resistive voltage divider supplied from a very good and stable supply to define a trigger point, after a few years it will be drifting away from it set point. So with very high stab components any analogue references will have drifted in that time.
The PIC16F883 has some nonvolatile memory in it. They probably increment a counter in that memory area every hour or day or so. Nothing fancy, and it doesn't need to be precise.
Almost all PIC controllers have an internal oscillator and flash memory, so no need for a crystal, and the flash can remember counts of whatever to get to 10 years.
Smoke detectors which are supposed to last for 10 years only just put a lithium ion battery in there rated for 10 years. Don't think I've ever seen a mains backup carbon monoxide device like this before. Wild of them to do a 10 year timer in only software.
My guess is that it divides the power grid frequency by 60 to get a second, and then counts a day count to 86 400 seconds before adding one to the year count, resetting the day count to 0 at the end. This keeps going to the point where the year count reaches 3653, then breaks itself and forces you to buy a new one. Chances are this is going to be plugged in all the time, the battery is just a backup if the power goes out. So the counter should be accurate. Additionally it could use it's internal oscillator as a backup clock to keep counting while battery powered.
Ten years is easy, even with the PIC's internal oscillator. It certainly doesn't have to be accurate, and the datasheet says it's accurate to 1%...plenty good enough for this application.
It sure has a bunch of zero-ohm resistors as jumpers to avoid a double-sided PCB and vias... Interesting view on the economic trade-offs at work for volume manufacturing.
First time I've seen a single-sided PCB with dozens of 0 Ohm, SMT resistors used as jumpers. The look like 1206, long enough to get a couple of traces under.
the CO sensor is based on a catalyst that deteriorates at a specific rate. in your device it around 10 years. if you have other installed at the same time failure is soon
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"Just the right size to poke myself and cause great damage..." We all know that one well.
The time is done by counting AC cycles; in this case likely through D7 and the R24/R25 divider. 10 years comes to 18,921,600,000 counts on that input pin so you only need a 35 bit counter. My guess is a RAM based counter counts off big chunk of time and then updates an EEPROM value periodically to make the timer non-volatile.
Put the CO sensor element in the curve tracer (then feed it some CO and see what happens)!
I had one of these in a house I rented. It went off overnight and scared the hell out of me, because there really WAS a CO risk there. .I took it out to the unheated garage and it didn't stop beeping so I knew it wasn't CO. Then I read the fine print. I wasn't yet schooled by IMSAI or EEVblog then, so I never looked inside....
A real transformer! Well, salvage the LEDs anyway.
I had two identical non-replaceable battery detectors which failed within a day of each other. They used a cheapo 32KHz watch crystal.
I got a carbon monoxide detector for the house here and for the first few years I had placed about a metre from the ground... (I know I'm an absolute idiot) When I realised it should be up high I placed it to the proper height. I kept it beside the fire place in our sitting room as I didn't trust the chimney. Anyways 2 days after placing it up to the right height it started going crazy.
Some PIC microcontrollers have a Zero-crossing detector also used for counting AC cycles.
everything has a use by date nowadays. i expect that as there is no testing or calibration required every year, they have taken sensor life and the drift of the component values and decided that it won't detect Co correctly. Folk forget that if you have a resistive voltage divider supplied from a very good and stable supply to define a trigger point, after a few years it will be drifting away from it set point. So with very high stab components any analogue references will have drifted in that time.
The PIC16F883 has some nonvolatile memory in it. They probably increment a counter in that memory area every hour or day or so. Nothing fancy, and it doesn't need to be precise.
Almost all PIC controllers have an internal oscillator and flash memory, so no need for a crystal, and the flash can remember counts of whatever to get to 10 years.
Smoke detectors which are supposed to last for 10 years only just put a lithium ion battery in there rated for 10 years. Don't think I've ever seen a mains backup carbon monoxide device like this before. Wild of them to do a 10 year timer in only software.
My guess is that it divides the power grid frequency by 60 to get a second, and then counts a day count to 86 400 seconds before adding one to the year count, resetting the day count to 0 at the end. This keeps going to the point where the year count reaches 3653, then breaks itself and forces you to buy a new one. Chances are this is going to be plugged in all the time, the battery is just a backup if the power goes out. So the counter should be accurate. Additionally it could use it's internal oscillator as a backup clock to keep counting while battery powered.
Ten years is easy, even with the PIC's internal oscillator. It certainly doesn't have to be accurate, and the datasheet says it's accurate to 1%...plenty good enough for this application.
You were nice to it. Big Clive would have put it in the "vice of enlightenment".
It sure has a bunch of zero-ohm resistors as jumpers to avoid a double-sided PCB and vias... Interesting view on the economic trade-offs at work for volume manufacturing.
First time I've seen a single-sided PCB with dozens of 0 Ohm, SMT resistors used as jumpers. The look like 1206, long enough to get a couple of traces under.
the CO sensor is based on a catalyst that deteriorates at a specific rate. in your device it around 10 years. if you have other installed at the same time failure is soon
Good of you to rotate the board to share your confusion about the ICs. 🙂
I disabled my CO2 detectors, because the constant beeping was giving me a headache 🤣🤣🤣