The temperature sensor inside the car is on the air duct of the car air conditioner. It is a thermistor with a negative temperature coefficient. The resistance of the thermistor will gradually decrease as the temperature increases. Its function is to detect whether the temperature in the car reaches the set value to control the operation of the air conditioning system: according to the level of the engine speed, the cooling capacity of the car's air conditioning compressor is also different. According to the set temperature, supply air temperature (air conditioning outlet temperature) and return air temperature (actual interior temperature), these three temperature values can be calculated and controlled by the controller program "variable air volume".