Electronic thermometer circuit

The electronic thermometer circuit is as shown. RT is the thermistor and A is the integrated operational amplifier. VD1 ~ VD4 form a standard flow
path and output a one-way signal .
VT1, VT2, etc. form a switching
circuit . When the resistance of the thermistor RT changes to a certain resistance value
with the temperature of the object being measured , a difference signal is output at points A and B. This signal is amplified by A, rectified by VD1~VD4, and then added to the base of VT1. pole, VT1 is on, VT2 is off, and the small light bulb is on. Not lit}At this time. Change the resistance of potentiometer RP2 so that there is no difference signal output between points A and B, then VT1 will be cut off, VT2 will be turned on, and the small light bulb will light up. If the potentiometer is marked with a temperature scale, then when the light is on, read the temperature of the object being measured.     Rr is a metal shell thermistor with a negative temperature coefficient. The model is RRG3J22. The resistance value is lOkfl at 20°C. It is more ideal to use a glass shell (a thermistor with a positive temperature coefficient can also be used). A uses the gain operational amplifier in the processing product. The leakage current of VT1 and VT2 should be less than lOOtjLA, and Lu should be greater than 30. Small light bulbs use voltages below 2.5V. Ei uses ZZ. 5v instrument with laminated battery, Bureau Yi uses one AA battery.