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