A low-cost and high-precision temperature measurement circuit

Using 3DG6 as the temperature sensor and the measurement amplification circuit composed of the LM324 operational amplifier is shown in Figure 1. The transistor 3DG6 is placed at the temperature measurement site and connected so that the base and collector are short-circuited, that is, the emitter junction is forward biased. The collector junction is used as a diode as a temperature sensor. The power supply provides about 45mA to 3DG6 through the resistor R1 (100K). electrode current. Its UBE is connected to the non-inverting terminal of LM324. R1, R2, R3, and R4 are all ordinary metal film resistors. If R2=R3 is selected, the amplifier outputs U0≈2UBE. This instrument uses 2 pieces of LM324 to detect 7 input signals at the same time.

The detection and processing circuit is shown in Figure 2. In Figure 2, 4051 is an eight-select one analog switch, its inputs I0~I6 are temperature detection inputs, and I7 is an automatic compensation input. The amplifier LM324 is connected as a follower amplifier, and its input is the VREF voltage output by pin 2 of the LM331 chip. LM331 is a voltage/frequency converter (V/F). Its output is divided by 74LS74 and then connected to the P3.2 terminal of the microcontroller AT89C2051. The microcontroller detects the pulse width and calculates the corresponding temperature value. AT24C02 is a serial I2C bus EEPROM circuit, used to store the pulse width corresponding to the two reference temperature values ​​during adjustment and the pulse width corresponding to the LM331 reference voltage VREF.