Winsen MH-Z19

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MH-Z19B is one cheapest CO2 sensor, though not cheap as other sensors. They costs around $15 a piece. This one has been showing error code and either 0s or 400s reading. After trying with different codes and MCUs. Time to see what is inside.

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from the back, we can see a circuit board in green. The sensor is enclosed by a plastic cover with golden color.

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the top cover is plastic but painted with shiny gold color. Inside, it has an enclosure and a reflector

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this VERY shinny golden plastic has an old bulb (hair-wired style) sticks out and a photocell showing out.

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and here the circuit board with an STM chip, a bulb that pulses light on once when reading request made and a big photocell as the detector.