The MQ series of gas sensors use a small heater inside with an electrochemical sensor. They are sensitive to a range of gasses and are used indoors at room temperature. The output is an analog signal and can be read with an analog input of the Arduino.
The MQ-2 Gas Sensor module is useful for gas leakage detecting in home and industry. It can detect LPG, i-butane, propane, methane, alcohol, hydrogen and smoke.
Some modules have a built-in variable resistor to adjust the sensitivity of the sensor.
Note: The sensor becomes very hot after a while, don’t touch it!
The connections are pretty easy:
Vcc to Arduino 5V pin
GNG to Arduino GND pin
Output to Arduino Analog A0 pin