Sensitive material of MQ-5 gas sensor is SnO2, which with lower conductivity in clean air. When the target flammable gas exist, the sensor’s conductivity gets higher along with the gas concentration rising. Users can convert the change of conductivity to correspond output signal of gas concentration through a simple circuit. MQ-5 gas sensor has high sensitivity to butane, propane, methane and can detect methane and propane at the same time. It also can detect kinds of flammable gases, especially LPG(propane).It is a kind of low–cost sensor for many applications.