The attraction of a First Alert model CO1210 was the promise of a ten year life on battery. It was installed on 1 May 2015 and declared defeat on 26 March 2017. That was a disappointment. The battery is a cr17335 Manganese Dioxide-Li/Organic Electrolyte (Maxell Datasheet) for 3 volts and 1.75 Ah. The Sanyo (datasheet catalog) indicates a ten year life at 23C at 20 microAmps with a final voltage of 2.5 volts. The battery on this detector after almost 2 years was down to 1.7 volts so something was wrong (overnight it recovered to 2.1 volts after being pulled from circuit despite the device being turned off). Here’s the disassembled detector:
and a closer view of the circuit board:
The brown AA sized thing in the picture is a Figaro TGS5042 is the CO detector (datasheet). It generates 1.2~2.4nA/ppm CO and you have to be careful not to apply anything more than 10 mV to avoid damage to the sensor.
The big chip is a PIC16LF1937. It’s an 8-Bit CMOS Microcontroller with LCD Driver and nanoWatt XLP Technology. To its left is is the RE46C107 for driving a piezoelectric horn. The op amp to get the nanoVolt signal up to the microcontroller’s ADC levels looks to be a MCP6V01.
So, it’s a straightforward microcontroller with sensor, display, and noise maker. The special thing about it is the use of low power components. In this area, the CO detector is the key item. The operating principles describes it this way:
Figaro Electrochemical-type gas sensors are amperometric fuel cells with two electrodes. The basic components of two electrode gas sensors are a working (sensing) electrode, a counter electrode, and an ion conductor in between them. When toxic gas such as carbon monoxide (CO) comes in contact with the working electrode, oxidation of CO gas will occur on the working electrode through chemical reaction with water molecules in the air (see Equation 1).
Two other types are also available. These are the MOS and Catalytic types. The Electrochemical-type is very low power but that means a very weak signal. Now the question is whether or not to put in another battery or just replace the device.