Analog sensors
MQ‑2
• MQ‑2 gas sensor has high sensitity to LPG, Propane and Hydrogen, also could be used to Methane
and other combustible steam
board markings: KS123‑35Y00K
 LM393
MQ‑4
• High sensitivity to CH 4 ,Natural gas.
• Small sensitivity to alcohol, smoke.
board markings: LM‑22
 LM393
MQ‑7
• High sensitivity to carbon monoxide
board markings: LM‑22
 LM393
https://www.instructables.com/id/Arduino‑CO‑Monitor‑Using‑MQ‑7‑Sensor/ 
My changes for single‑line NAME=value output https://github.com/dpavlin/mq7‑co‑monitor