Sensitive material of the MQ-9 gas sensor is SnO2, which with lower conductivity in clean air. It makes detection by method of cycle high and low temperature and detects CO when low temperature (heated by 1.5V). The sensor’s conductivity is higher along with the gas concentration rising. When the high temperature (heated by 5.0V), detects Methane, Propane, etc combustible gas and cleans the other gases adsorbed under low temperature. Please use simple electronic circuits, Convert change of conductivity to correspond output signal of gas concentration. MQ-9 gas sensor has a high sensitivity to Carbon Monoxide, Methane, and LPG. The sensor could be used to detect different gases contains CO and combustible gases, it is at low cost and suitable for different applications.