Dobrica Pavlinušić's random unstructured stuff
MQ: Revision 2

Analog sensors

MQ-2

board markings: KS123-35Y00K
LM393

MQ-4

board markings: LM-22
LM393

MQ-7

board markings: LM-22
LM393

https://www.instructables.com/id/Arduino-CO-Monitor-Using-MQ-7-Sensor/