1. Circuit principle:

The thermistor RT is connected to the input terminal of U1A, and the voltage division of the resistors R2 and RP is adjusted 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 , the resistance of the RT becomes smaller, and the voltage division of the voltage dividing circuit makes the U1A input terminal low level, and the inverted output is high level. The high level is added to the 8 pin of the control end of the multi-vibrator composed of U1C, U1D and the RC circuit, so that the multi-vibrator starts to oscillate, and after the transistor VT2 is amplified, the buzzer sounds an alarm, and also It is applied to the base of the transistor VT1 to be turned on, and the relay is energized to attract; on the other hand, after the inverted output of the U1B is low, the LED of the LED is illuminated.
 
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 consists of a four-input NAND digital integrated circuit CD4011 and a thermistor RT to form a measurement and control circuit and a siren sound generation circuit, and a relay K as an execution circuit. The thermistor RT is used as a temperature measuring sensor. When the temperature exceeds the set temperature, the relay K is driven by the transistor VT1. It can be used for over-temperature monitoring and alarm. The normally open and normally closed contacts of the relay can drive a variety of controlled devices.

4. Components List:

Resistor 1K x 1
Resistor 2K x 1
Resistor 100R x 1
Resistor 51K x 1
Potentiometer 10K x 1
Capacitor 22uF x 1
Diode 1N4007 x 1
Transistor 9013 x 2
Red LED x 1
Thermistor 10K x 1
Buzzer x 1
5VDC relay x 1
2P Terminal block x 1
3P Terminal block x 1
IC CD4011  x 1
14P IC socket x 1
PCB board x 1