MH-Z19b CO2 sensor

* https://www.winsen-sensor.com/d/files/infrared-gas-sensor/mh-z19b-co2-ver1_0.pdf
* https://revspace.nl/MH-Z19B

* https://github.com/WifWaf/MH-Z19
* https://en.ovcharov.me/2019/06/10/carbon-dioxide-sensor-mh-z19b-part-1/
* Selected [Arduino Library](https://github.com/crisap94/MHZ19)
* http://ihormelnyk.com/mh-z19_co2_meter
* [German Blog about how to use MH-Z19B with Arduino](https://www.blikk.it/forum/blog.php?bn=neuemedien_fb&lang=de&id=1575901160)
* https://git.unhb.de/smash/co2ampel

* fancy LCD with buttons:
  + https://www.adafruit.com/product/772
  + https://learn.adafruit.com/rgb-lcd-shield