Complete circuit diagram of digital display thermometer design (DS18B20/89S51 microcontroller/LCD)

Digital display thermometer design circuit diagram (1): Digital computer thermometer circuit with LCD display

The picture shows the digital computer thermometer circuit with LCD display. This eight-segment four-digit LCD display has a built-in driver, serial data transmission, and is easy to use.

Digital computer thermometer circuit with LCD display

Digital display thermometer design circuit diagram (2)

Introducing a temperature control circuit with simple artificial intelligence. When using this circuit for temperature control, you only need to turn the switch to the 2 position, control the temperature by setting it, and display the temperature value displayed by the three-and-a-half-digit meter. The temperature can be accurately controlled, making temperature control operation very convenient. LM35 is an integrated temperature sensor with calibrated internal circuit. Its output voltage is proportional to the Celsius temperature. It has good linearity, high sensitivity and moderate accuracy. Its output sensitivity is 10.0MV/℃, and its accuracy reaches 0.5℃. Its measuring range is -55-150℃. The self-heating effect is low at rest temperature. The working voltage is wide and can work normally within the power supply voltage range of 4-20V, and consumes very little power. The working current is generally less than 60uA. The output impedance is low, 0.1Ω at 1MA load. According to the output characteristics of LM35, when the temperature changes between 0-150℃, the corresponding voltage at the output terminal is 0-150V. This voltage is divided by the potentiometer W3 and then sent to the detection of the 3.5-digit digital display meter. Signal input terminal. When the voltage input at the input terminal is 150V, adjust the potentiometer so that the displayed value is 150.0. After adjustment, the value displayed on the digital display head is the actual measured temperature value.

Temperature control selection can be achieved through potentiometer W2. By adjusting W2, the voltage of the intermediate head can be changed within the range of 0-1.65V, and the corresponding control temperature range is 0-165°C, which can fully meet the general heating needs. Set the switch K to the 2 position, and the voltage at the middle head of the potentiometer W2 passes through the voltage follower A and then is sent to the input end of the digital display head to display the control temperature value. Adjust the potentiometer W2, and the value displayed on the digital display will change accordingly. The temperature value displayed is the control temperature value. Potentiometer W1 is for pre-control temperature adjustment, its voltage adjustment range is 0-0.27V, and the corresponding adjustable temperature range is 0-27℃. After the potentiometer is adjusted, the voltage of its intermediate head and the voltage of the intermediate head of potentiometer W2 are respectively sent to the inverting and non-inverting input terminals of comparison amplifier B. The voltage at the output terminal of B is the difference between the two input voltages. This voltage corresponds to the difference between the two set temperature values. For example, if W1 is adjusted to 0.10V, the corresponding temperature is 10°C; if W is adjusted to 0.80V, the corresponding temperature is 80°C. The output voltage of B is 0.70V, which means the temperature is 70℃. This voltage and the voltage output by the integrated temperature sensor are sent to the voltage comparator C for voltage comparison.

When the voltage output by LM35 is less than the output voltage of B, C outputs high voltage, and the thyristor T1 is always turned on due to the bias current. AC 220V is directly applied to both ends of the electric heating element for high-power rapid heating. When the voltage output by LM35 is greater than the output voltage of B but less than the output voltage of A, it indicates that the actual temperature is close to the control temperature, C outputs low voltage, thyristor T1 is in a cut-off state due to no bias current, and voltage comparator D outputs high voltage. flat, the thyristor T2 is still in the conducting state, and AC 220V needs to be added to both ends of the electric heating element through the diode D2 for low-power slow heating (the heating power at this time is only 25% of the original). When the actual temperature rises above 80°C, the output voltage of LM35 is greater than 0.80V, the voltage comparator D outputs a low level, the thyristor T2 is also cut off, and the electric heating element is powered off.

Digital display thermometer design circuit diagram (3): LCD display thermometer circuit diagram

The picture shows the circuit diagram of the LCD thermometer. The working principle of this circuit is: the DS18B20 temperature sensor chip measures the current temperature and sends the result to the microcontroller. Then, the measured temperature readings sent are calculated and converted through the 89C205I microcontroller chip, and the results are sent to the liquid crystal display module. Finally, the SMC1602A chip displays the sent value on the display. This circuit is mainly composed of DSl8820 temperature sensor chip, SMCl602A liquid crystal display module chip and 89C2051 microcontroller chip. Among them, the DSI8B20 temperature sensor chip is connected to the microcontroller using a "one-line system". It independently completes the temperature measurement and sends the temperature measurement results to the microcontroller.

Digital display thermometer design circuit diagram (4): Use 7136 to make LCD digital display thermometer circuit diagram

The circuit diagram of using 7136 to make an LCD digital display thermometer is as follows:

Digital display thermometer design circuit diagram (5)

During the measurement process, the thermocouple generally generates a temperature difference electromotive force relative to the cold end. Industrial standards generally stipulate that the temperature of the cold end is 0°C. In actual use, it is inconvenient to put the cold end into the ice-water mixture. If the local temperature is not 0℃, the temperature difference electromotive force may be too large or too small. Therefore, actual circuits usually require temperature compensation for the thermoelectromotive force. The entire system of this portable low-power, high-precision digital thermometer consists of four parts: the first is the thermocouple; the second is the data acquisition circuit composed of AD7705 ($5.1240) and AD589 ($2.0760), among which the A/D conversion circuit The function is to convert the thermal electromotive force generated by the thermocouple into a digital signal; the third part is AD7416 ($1.2000), which can measure the cold junction temperature and calculate the compensation voltage; the fourth part is MSP430F413 ($1.5188) A control and display circuit composed of a six-digit pen-segment liquid crystal display. The specific circuit schematic is shown in Figure 1. In order to achieve low power consumption and high accuracy, the chips selected in this design have low power consumption modes and can work in power saving mode between measurements. Each part of the circuit is described in detail below.

Figure 1 Schematic diagram of portable low-power high-precision digital thermometer

Thermocouple

K-type or J-type nickel-chromium-copper-nickel (constantan) thermocouples are selected in this design. They are more suitable for temperature measurement systems in oxidizing and weakly reducing environments. Their temperature measurement range is -200℃ ~ 1000℃, and the thermoelectromotive force range is -9.835mV ~ 76.358mV. Because these thermocouples have good stability and high sensitivity , low price and other advantages, so it is very suitable for the use of portable temperature measuring instruments. Figure 2 shows the thermoelectromotive force-temperature curve of a nickel-chromium-copper-nickel (constantan) thermocouple. After analysis, its accuracy can reach ±0.1℃, and its sensitivity can reach 38μV/℃ at -150℃.

Figure 2 Thermoelectromotive force-temperature curve of nickel-chromium-copper-nickel (constantan) thermocouple

Data acquisition circuit

In this part of the circuit, the AD7705 is a front-end device used in low-frequency measurement systems. It has high resolution and a power-saving mode, which can meet the requirements of high accuracy and low power consumption. In addition, the AD7705 also has a digital filter circuit, calibration circuit and compensation circuit on-chip, which can better ensure high-precision temperature measurement. AD7705 uses a single power supply of 2.7V ~ 3.3V. It has two analog differential input channels. When the power supply is 3V and the reference voltage is 1.235V, the maximum amplitude range of the bipolar input signal is 0 ~ ±10mV (Gain= 128) to 0~±1.235V (Gain=1). In addition, the AD7705 can also directly receive the small signal generated by the sensor to perform A/D conversion and output a serial digital signal. It uses Σ-Δ technology to achieve 16-bit A/D conversion. The sampling rate is determined by the master clock at MCLKIN and the variable gain of the amplifier. In fact, the AD7705 can perform on-chip amplification, modulation conversion and digital filtering of the input signal at the same time. The stopband of its digital filter is programmable to adjust the filter cutoff frequency and output data update rate.

The response of this filter is similar to that of the median filter, but the falling edge is steeper. Because the output rate of the digital filter is consistent with the frequency of the first notch of the filter's amplitude-frequency response. Therefore, when the output rate is 25Hz, the first notch of the filter is also 25Hz. In addition, the (sinx/x) 3 filter can also suppress the harmonic component of the first pit frequency, and the suppression amount is greater than 40dB. When FS0 and FS1 are 0 and 1 respectively, the output rate and first pit frequency are 25Hz, and the -3dB point is 6.55Hz. If the temperature of the measured environment changes slowly, then during the analog-to-digital conversion process, the circuit can effectively suppress interference signals greater than 6.55Hz, including interference signals of 50Hz.