1. Circuit principle: The thermistor RT is connected to the input end of U1A, it is adjusted with the voltage divider of resistors R2 and RP, so that the input voltage of U1A is high level and the output voltage is low level. When the temperature exceeds the set temperature limit, because the resistance of RT becomes smaller, the input terminal of U1A is low level through the voltage division of the voltage divider circuit, and the inverted output is high level. On the one hand, the high level is added to the 8-pin of the control terminal of the multivibrator composed of U1C, U1D and the resistance-capacitance circuit, so that the multivibrator starts to vibrate. Add to the base of the triode VT1 to make it conduct, the relay is energized and pulled in; on the other hand, after the U1B inverting output is low, the light-emitting diode LED lights up to indicate. 2. Kit parameters: Supply voltage: 4.5-5.0V (DC voltage) Board size: 32*72mm 3. Function introduction: The over-temperature monitoring automatic control circuit is composed of four 2-input NAND gate digital integrated circuits CD4011 and thermistor RT to form a monitoring and control circuit and a siren sound generating circuit, and the relay K is used as the execution circuit. The thermistor RT is used as a temperature sensor. When the temperature exceeds the set temperature, the relay K is driven by the triode VT1 to act. It can be used for over-temperature monitoring and alarming, and the normally open and normally closed contacts of the relay can drive a variety of controlled devices, etc.