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Opening the case of made in China EXTECH CO100 air quality monitor reveals a very compact NDIR CO2 sensor made by SenseAir.com (Sweden) type model S8 004-0-0062. Connected to other electronics with just 3 wires: 5V, GND and output signal. On the sensor output signal is present a TTL pulse once a second (exactly every 1009ms) with PWM lineary proportional to CO2 concentration.
At 500 ppm CO2 the pulse width is 50ms. That is what I got today by opening the room window.
At 1200 ppm warning starts beeping and oscilloscope shows 120 ms pulse length.
At 10000 ppm the pulse width is 1s. Basically only a very short peak going down that is useable for triggering.
Visually one can see through the air diffusion membrane that a small lamp is turning on once a second for a short while, and it is not very bright, so emmision peak is in the Mid-IR and lamp can last for 15 years according to specs.
Schematics is nicely designed and assembled. On one side of the PCB following components can be identified. Large chip - LCD display driver. Small chip - touch sensor buttons driver. Moisture/temperature sensor model SHT. 3V Li cell for clock backup. 5V regulator working from 6.2 V at the input jack.
On the other PCB side is a MSP430F microcontroller. USB and FTDI chip place is not populated. No RX or TX signals coming from the microcontroller.
So the only way how to connect external electronics is to measure the output of the CO2 sensor directly.
Measuring precisely the pulse length on one second time scale should be an easy task to do with a CC3200 board (or TI MSP430 + ESP8266). It was nice to learn this methoid how to enclode analog signals using PWM.
Conclusion and recommendation to electronics guys is: Buy just the NDIR sensor!
Continue reading: David Giorni NDIR CO2 detector
L. Crawley 2008 Dr. Thesis about NDIR trace gas detection
