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 detection in home and industry. It can detect LPG, i-butane, propane, methane, alcohol, hydrogen, and smoke.