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My British Gas supplied carbon monoxide sensor started bleeping at me, not the usual battery-dying bleep code, but a rather more serious detector-malfunction bleep code. This makes it fair game for a teardown!



The whole of the alarm is implemented on a single PCB, the two most obvious parts being the electrochemical sensor and the piezo sounder. Just peaking out from under the sensor is something that looks rather like a Microchip PIC ISCP port header.

The Sensor

The carbon monoxide sensor is a Figaro TGS5042, this is an electrochemical device that generates an electrical current that depends upon the concentration of carbon monoxide that defuses into the body of the device. As with all sensors of this type, the device has a limited working life - failure being sure to the liquid electrolytes drying out.


The 2-D barcode encodes quite a bit of information about this sensor, but I've not found anything that can read it. The number under the standard barcode is the output current in nano-amps / parts per million (nA/ppm), so this sensor produces 1.697 nA when exposed to 1ppm CO. A pretty high gain current to voltage amplifier is required to turn this tiny current into something a micro-controller or logic device can deal with.


The analog amplification and processing sits right under the sensor. The active device is a Microchip MCP6042 low power dual operational amplifier, likely wired as a trans-impedance amplifier (not checked the circuit yet).

The Processor

The presence of an ICSP header is a give away that the alarm uses a Microchip PIC type processor, and when we remove the piezo sounder we spot a PIC16F688. There is also a Microchip RE46C105 piezo sounder driver. There's no other voltage regulation or voltage dropping on the board, so at first I couldn't see how the pic ran from the 9V battery supply, a closer reading of the datasheet for the piezo driver mentions a built in voltage regulator selectable at either 5V or 3.3V, depending on pin 12 (low = 3.3V, high = 5V).


Reading the PIC

With my trusty PICkit-2 programmer, I was able to dump out the code from the processor via the ICSP port. There was no code protection and the hex dump doesn't seem to have any encryption or obfuscation. There are no obvious text strings in the code, but this device never interfaces to the external world apart from some bleeps and an LED flash, so there's no need for any human readable data.

I ran the hex though a simple disassembler, nothing very odd leaps out, but it is quite a few years since I programmed a 16-series PIC in assembler - I could well be missing something interesting. The hexdump and assembly code are in an archive for downloading - have a play and let me know if you discover how it works.


There was rather more in this alarm than I expected. I expected to see some kind of electrochemical sensor, but I thought the output would either go into a very simple comparator circuit or into a device designed specifically for this type of alarm. I wasn't expecting an accessible microcontroller in here. All in all, this seems a pretty well designed device, good named manufacturer components (not Chinese knock-offs) and some thought about the component choice (low power op amps and micros); they even use a specific piezo driver rather than just pulsing it from a spare pin on the PIC.

I must now buy a replacement carbon monoxide alarm.