This is the MQ-2 Gas and Smoke Analog Sensor Breakout Board. It's most common use is domestic gas leakage alarms and detectors with a high sensitivity to propane and smoke. The sensing material is SNO₂ (tin oxide) for which the conductivity is lower in clean environments and higher when smoke or flammable gas is present. This is a low-cost sensor for use in a variety of applications. NOTE: After the sensor is powered on it needs to warm up around 20S.