Digital electronic thermometer using semiconductor diode as temperature sensor

The circuit shown in the attached figure is a digital electronic thermometer that uses a semiconductor diode as a temperature sensor. Its temperature measurement range is -50 to +150°C, and the temperature measurement accuracy is ±0.1°C.

How the Circuit Works

The forward voltage drop of a semiconductor diode depends on the size of the forward current and the ambient temperature. When the forward current is constant, the forward voltage drop decreases with the increase of temperature. For the ordinary silicon diode 1N4148, it has a temperature coefficient of about -2.1mV/℃. When two 1N4148s are connected in series, the total forward voltage drop has a relationship with temperature of about -4.2mV/℃. Both theory and practice have proved that within the range of -50～+150℃, the temperature measurement accuracy of the diode can reach ±0.1℃. Compared with other temperature sensors, in terms of low temperature measurement, the diode temperature sensor has the characteristics of high sensitivity, good linearity and simplicity.

In the attached circuit, resistors R6-R8, diodes VD1-VD3, and transistor Vl constitute a temperature sensor circuit. Among them, VD1 and VD2 are connected in series as a temperature probe; R6-R8, VD3, and Vl constitute a constant current source circuit to provide a constant forward current to the temperature probe. By calculation, it can be obtained that the constant current provided by the constant current source to VD1 and VD2 is about 0.6mA. Diode VD3 plays the role of temperature compensation to ensure the temperature stability of the constant current.

The digital thermometer circuit shown in the figure is based on the A/D converter ICL7107 produced by INTERSIL, USA. ICL7107 is a monolithic CMOS 31/2-bit dual-integral A/D converter, which includes linear amplifiers, analog switches, clock oscillators, seven-segment decoders, display drivers and other components, and can directly drive common anode LED digital tubes. ICL7107 is designed to work with dual power supplies (+5V, -5V) and has automatic zero calibration and polarity automatic conversion functions. ICL7107 is a commonly used A/D converter with strong performance and low power consumption, and the retail price is around 10 yuan.

In the figure, R1 and Cl form the RC network of the oscillator. When R1 and Cl are taken as shown in the figure, the clock pulse frequency is fCLK=45kHz, and there are about three readings per second. C2 is the reference capacitor. C3 is the input filter capacitor, and R3 is the input current limiting resistor.

It should be noted that C5 is the integral capacitor and R2 is the integral resistor. The quality of both should be guaranteed, as they directly affect the measurement accuracy of the circuit. The integral capacitor should have low dielectric absorption performance, and a relatively cheap polypropylene (CBB) capacitor can be selected, while the integral resistor can be a metal film resistor. C4 is the zero calibration capacitor, and a non-inductive polyester capacitor is preferred.

The segment drive signals of the thousands, hundreds, tens and units output by IC1 are directly connected to four common anode LED digital tubes, among which the "b segment" and "c segment" of the thousands digital tube LED4 are driven by "AB4" of IC1; "g segment" is driven by the polarity display terminal POL of IC1 to display the minus sign "-", indicating the temperature below 0oC. The decimal point is located at the tens digital tube LED2, and the decimal point "dp" of LED2 is directly grounded. In order to simplify the circuit, each LED digital tube is connected to only one 200Ω current limiting resistor, instead of one current limiting resistor for each segment of the digital tube.

In the figure, IC2 and IC3 provide stable +5v and -5v power supplies for IC1 and the temperature sensor circuit. Potentiometer RP1 in the circuit is used to adjust the boiling point (100oC); RP2 is used to adjust the freezing point (0℃).

Production and debugging methods

IC1 is ICL7107, which can be directly replaced by TSC7107, CH7107, etc. Digital tubes LED1~LED4 use 0.5-inch common anode LED digital tubes. Other components can be selected according to the circuit shown in Figure 1.

The circuit in the figure is very simple and easy to make by amateurs. Because ICL7107 is a CMOS circuit, it is suitable to use an IC socket. After all components are soldered, insert ICL7107.

After welding and installing the circuit shown in the figure below, the digital thermometer needs to be debugged before it can be used normally. Before debugging, prepare 1000ml of ice water mixture at 00C and 100℃ boiling water. The debugging steps are as follows:

(1) Adjust RP1 to the uppermost position so that Vref is the highest voltage, place the diode temperature probe in ice water at 0°C, and adjust RP2 so that the digital tube displays "00.0".

(2) Place the diode temperature probe in boiling water at 100°C and adjust RP1 so that the digital tube displays "100.0".

After the above debugging, the digital thermometer can work normally.