The MQ-2 smoke sensor is sensitive to smoke and to the following flammable gases:
LPG
Butane
Propane
Methane
Alcohol
Hydrogen
The resistance of the sensor is different depending on the type of gas. The smoke sensor has a built-in potentiometer that allows you to adjust the sensor digital output (D0) threshold. This threshold sets the value above which the digital pin will output a LOW signal.
 
				 
				
