This module includes an MQ-2 Gas sensor. The MQ-2 Gas sensor detects LPG, butane, alcohol, hydrogen, smoke and other gases.
The 4 pins are GROUND, Vcc (5VDC), AO and DO. The AO pin connects to any Arduino analog input and the voltage detected is proportional to the amount of gas present. The DO (Digital Out) trigger threshold is set using the onboard potentiometer.