MH-Z14A, MH-Z19B CO2 ... Module
Arduino implementation for MH-Z CO2 sensors such as MH-Z14A, MH-Z18B (as I didn't find all info in one place).
The sensor is available for ~20 bucks at the usual places.
PPMuart: 602, PPMpwm: 595, Temperature: 23
C ppm = 5000 * (T_high - 2 ms) / (T_high + T_low - 4ms)
The implementation is mostly based on https://forum.arduino.cc/index.php?topic=525459.msg3587557#msg3587557
More info about the sensor: https://revspace.nl/MHZ19
(russuian, but google translate does a good job)