The MP-702 ammonia gas sensor adopts a multi-layer thick film manufacturing process. The heater and the metal oxide semiconductor gas sensing layer are respectively made on both sides of the micro Al2O3 ceramic substrate, and are packaged in a metal shell. When the detected gas exists in the ambient air, the conductivity of the sensor changes. The higher the concentration of the gas, the higher the conductivity of the sensor. This change in conductivity can be converted into an output signal corresponding to the gas concentration using a simple circuit.