FireAngel Smoke Alarm ST 620

I bought several of these smoke alarm units about 5 years ago. They were sold with a 10 year battery life.

Recently, all units started bleeping with a low battery condition (annoying blip every 45 seconds).
I did some googling and found that there was a manufacturer recall on these apparently due to defective battery life of 5 years.

I was going to return the batch for replacement units, but unfortunately could not find the proof of purchase receipt.

Interestingly, twisting the unit from the base plate and re-inserting, kept it silent for a few days, and then they started blipping again. This seemed to indicate some programming trickery at work. Needs further investigation!

So I thought I'd do a teardown analysis, and try and establish a root cause whether it's a hardware or software fault (or planned obsolescence?)

The fix is simply to switch it off and on again. However, easier said than done, as the wires are soldered directly from battery to board. Twisting the unit from its base does not do a power reset. So we need to break and reconnect the power.

Prise apart the 4 edge clips by pressing in on the lug with a narrow flatblade screwdriver.

Cut off the heat shrink on the red wire using a craft knife.

Desolder the wire. Leave it for a few minutes to discharge the capacitors, or press the button in the middle to speed things up.

Now is a good opportunity to measure battery voltage. This battery measured 2.92 volts. Anything less than 2.1 volts would be suspect, as 2v is the minimum microcontroller voltage.

Cut a new section of heat shrink insulation. Insert and resolder. Shrink the heat shrink ensuring minimal metal tag is exposed.

Insert the round plastic button locating the 2 legs in the home. The notch cutout points towards the piezo siren.

Snap back the 4 lugs.

Insert the back plate and press the test button.

Inside is a lithium 3v battery. It has a simple switch which closes when the unit is removed from the base fixing plate. I was expecting the switch to disconnect power on detaching from base. For some reason the manufacturer wants to detect removal but not do a power reset.

The smoke detector appears to be an ionization chamber type. It also has an led, test button, piezo siren and programmable microcontroller.

Measuring the voltage read just under 3 volts on all units (2.98 volts or higher). The battery voltage seemed acceptable for a 3 volt battery, so I was surprised it was beeping due to a low battery condition.

Giving it the benefit of doubt, maybe the battery was slowly deteriorating over the days, and then recovering voltage. However I measured it immediately after it started bleeping, and it was still sitting at a stable 3 volts.

I tried doing a load test by briefly shorting the battery using a multimeter. This measured several amps, and the battery retained 3v after removing the load. This seemed to indicate a healthy battery.

I suspect a software timer at work, after the unit is reattached to the base fixing plate.

Inside the unit is a programmable microcontroller (PIC MICROCHIP 16F785). These have capability for saving settings in RAM and ROM. RAM settings are lost on battery disconnect. ROM settings are retained after battery disconnect. ROM writes are generally not used as they can be unreliable, and have a limited number of write operations.

The microcontroller also works up to a minimum voltage of 2 volts, according to the datasheet.

The 3v lithium battery is soldered direct to the board making a simple disconnect/reconnect not possible. Hence the need to desolder the positive battery lug, to discharge, and reconnect the battery to try and do a factory reset.

Surprisingly, the unit appears to have done a hard reset, and returned to normal operating behaviour. Several weeks have passed, and it tests fine with no annoying low battery bleeps (until another 5 years?).

This strongly seems to indicate a software defect in the microcontroller code, or worse yet, planned obsolescence which times out after 5 years, even though the battery has plenty of life.

I would love to get my hands on the machine code to see what's really going on under the bonnet, and identify the root cause. However, the PIC micro is likely to be code protected with an internal chip fuse. There are ways to get around this to recover the code, but this would be illegal under the manufacturers terms and conditions. Nice and convenient for them, but not good for the planet and landfill.

Smoke alarms are a safety device and should not be tampered with.

Faulty devices within warranty should be returned to the manufacturer for a replacement, or replaced with new.