![Chinese CO Alarm](https://www.experimental-engineering.co.uk/wp-content/uploads/2016/02/20160208_104328-300x169.jpg)
I was looking around eBay for decent deals on a branded CO alarm, and came across these for next to no money, so I thought I’d grab one just to see how bad they could be.
![Alarm Opened](https://www.experimental-engineering.co.uk/wp-content/uploads/2016/02/20160208_103929-300x169.jpg)
Popping the casing open shows the very small circuit board inside, with the CO sensor cell on the right. I can’t find any manufacturer information on this cell, nor can I find a photo of anything similar on the intertubes, so no specifications there. The other parts are pretty standard, a Piezo sounder & it’s associated step-up transformer to increase the loudness.
![Sensor Closeup](https://www.experimental-engineering.co.uk/wp-content/uploads/2016/02/20160208_103941-300x169.jpg)
The sensor cell has the usual opening in the end to allow entry of gas.
![Main PCB](https://www.experimental-engineering.co.uk/wp-content/uploads/2016/02/20160208_104100-300x169.jpg)
The other side of the board doesn’t reveal much, just an LCD, a couple of LEDs, a pair of transistors, Op-Amp for the sensor & a main microcontroller.
![MCU](https://www.experimental-engineering.co.uk/wp-content/uploads/2016/02/20160208_104105-300x169.jpg)
The microcontroller isn’t marked unfortunately. It’s not had the number scrubbed off, it’s just never been laser marked with a part number. Above the micro is a SOT-23 LM321 low-power Op-Amp which does the signal conditioning for the CO sensor.
I tried to make this alarm trigger with the exhaust from the Eberspacher heater, which on a well-made branded alarm registered a reading of 154ppm after a few minutes. In the case of this alarm though, I couldn’t make it trigger at all, no matter how long I exposed it to hydrocarbon exhaust gases. I won’t be trusting this one then!
Nothing quite like a piece of safety equipment that doesn’t work correctly from new!