Design of Hardware Module of Temperature Measurement and Control System Based on AT89C51 Single Chip Microcomputer

This article mainly introduces the most important part of the temperature measurement and control system based on AT89C51 microcontroller: hardware module design. The following are the various modules and their working principles in the temperature measurement and control system.

1 Temperature detection module

The temperature measurement part of this system uses the one-line digital temperature sensor DS18B20 produced by DALLAS. It has the advantages of miniaturization, low power consumption, high performance anti-interference ability, wide measurement range, strong and easy to match processor. DS18B20 can directly convert temperature into a serial digital signal for microcontroller processing. It has a three-pin TO-92 small volume package. The temperature measurement range is -55~+125℃, and the temperature measurement resolution can reach 0.062 5℃.

2 4×4 determinant key module

The system uses a 4×4 determinant matrix key input. In addition to the 10 corresponding digital temperature keys from 0 to 9, temperature reset keys, confirmation keys, sub-zero temperature selection keys and decimal point keys are also designed to control temperature. The system uses a non-encoded keyboard, and the key recognition adopts the global scanning method.

3 Main control module

This design uses AT89C51 microcontroller as the main control chip. This chip is a high-performance CMOS 8-bit microprocessor with 4 KB flash memory. It has high cost performance and meets the design requirements of the system. The main control chip AT89C51 in this system continuously compares the collected temperature signal with the input control temperature. If it exceeds the set range, the heating or cooling device is started until the temperature reaches the set range.

4 Temperature control module

In the temperature control circuit of this system, the microcontroller controls the relay through the on-off of the transistor to achieve the purpose of controlling the electric heater. When the temperature is lower than the set temperature lower limit, the microcontroller sends a low-level signal through the 74LS04 NOT gate circuit and then changes to a high level, turning on the NPN transistor, the relay connects the power supply to the electric heater, the electric heater heats up, and the temperature rises slowly. When the temperature is higher than the set temperature upper limit, the microcontroller sends a high-level signal through the 74LS04 NOT gate circuit and then changes to a low level, turning off the NPN transistor, the relay connects the power supply to the refrigeration system, the refrigeration system works, and the temperature slowly decreases. When the relay is suddenly powered off, a large reverse current will be generated. The diodes connected in reverse at both ends of the transistor can shunt the reverse current to protect the transistor.

5 Temperature measurement and control display module

The display part of this design uses two 4-bit 8-segment common anode digital tubes 7SEG-MPX4-CA. One digital tube is used to display the current ambient temperature, and the other digital tube is used to display the set temperature. In order to save I/O ports, this design connects the bit codes of the two digital tubes through the 74LS04 dual 2-wire-4-wire decoder.
The temperature measurement and control system designed in this paper uses the DS18B20 digital temperature sensor to collect the ambient temperature in real time, and uses the 4×4 matrix buttons to freely set the upper and lower limits of the temperature. The AT89C51 microcontroller is responsible for processing the collected temperature data and sending the control temperature signal, and the ambient temperature and the set temperature are displayed in real time through the digital tube 7SEG-MPX4-CA.