With the rapid development of electronics and sensor technology, temperature measurement and control have been widely used in civil, industrial, and aerospace technology fields. Small, low-power, low-cost, and highly reliable temperature sensors have attracted widespread attention. In the actual production, life and other fields, temperature is an indispensable part of environmental factors, and timely and accurate temperature control and detection are particularly important. This text is based on AT89S51 single-chip microcomputer, adopts LM35 temperature sensor, has designed a kind of high sensitivity, strong anti-interference ability, stable and reliable temperature acquisition and display system.
1. System structure and working principleThe temperature acquisition and display system circuit is composed of a temperature acquisition module, an A/D conversion module, a single-chip microcomputer control module, a digital tube display module and a download module. The working principle of the circuit is: First, the temperature of the external environment is collected by the LM35 temperature sensor, and then amplified by LM358 by 10 times, and then input to the A/D sampling circuit in the form of voltage. The A/D Converter TLC549 transmits the digital value of the temperature to the single-chip microcomputer system. , And then a single-chip microcomputer system drives the digital tube to display the temperature. The temperature measurement range of the LM35-based single-chip temperature acquisition and display system designed in this paper is 25°C to 80°C. The temperature acquisition and display system circuit is an open-loop control system. The system principle block diagram is shown in Figure 1:
The core hardware circuit design of the system mainly includes the design of the temperature acquisition module, the design of the A/D conversion module, the design of the single-chip control module, the design of the digital tube display module and the design of the download module.
2.1, the design of the acquisition module
The sensor is the first link in signal input and one of the key links in the performance of the entire test system. Therefore, the correct selection of the sensor is particularly important. In this system, the core hardware of the temperature acquisition module uses the LM35 temperature sensor. This device has high working accuracy and a wide linear working range. Its output voltage is linearly proportional to the temperature in Celsius. The voltage rises every time the temperature rises by 1°C. 10ms. LM35 does not require external calibration and can provide a common room temperature accuracy of ±1/4°C. Consider from many aspects such as economy and applicability, the system adopts LM35 temperature sensor and LM358 amplifying circuit to carry on the design of the temperature acquisition module, the design principle diagram is shown as in Fig. 2. In Figure 2, the weak voltage collected by the LM35 sensor is amplified 10 times by the LM358 amplifier circuit and then sent to the microcontroller.
2.2 Design of /D conversion module
The core hardware of the A/D conversion module uses TLC549, which is a CMOS serial 8-bit A/D converter. The integrated circuit has a built-in 8-bit switched capacitor successive approximation ADC. The conversion time of the A/D chip is 17us and supports voltage. It is 3V ~ 6V. TLC549 only uses input/output clock, and input data is controlled by chip select (CS). The input frequency of the input/output clock of TLC549 is up to 1.1MHz. The design principle diagram of the A/D conversion module is shown in Figure 3:
2, 3 Design of the single-chip microcomputer control module
Because 8031 ​​chip has no internal program memory, it needs to be expanded externally, which not only occupies a large space, but also increases circuit complexity. The 51 series microprocessor is based on a simplified embedded control system structure, which should be widely used in personal PCs, automatic control, and even military fields, and it is prominent as the core advantage of control. The core hardware of the single-chip control module adopts AT89S51. The single-chip is a low-power, high-performance CMOS 8-bit microcontroller with 8K system programmable Flash memory, which can be read and written. It has the function of online programming, and can debug the software online to match the hardware. The system drives the digital tube through the single-chip microcomputer to display the measured temperature. The design principle diagram of the control module of the one-chip computer is shown as in Fig. 4.
2.4, the design of the digital tube display module
The display system is an important part of the single-chip control system, mainly used to display the values ​​of various parameters, so that the staff can grasp the production process in time. The nixie tube display module adopts an ordinary 8 as nixie tube, and uses a single chip microcomputer to drive the nixie tube to display the measured temperature. The design principle diagram of the digital tube display module is shown in Figure 5:
2.5, the design of the download module
The download module can realize single-computer communication, multi-computer communication between single-chip computers, and communication with the computer. This system mainly realizes downloading programs from the computer to the single-chip computer to drive the work of the entire circuit board. The core hardware of the download module adopts serial port MAX232 and RS232. MAX232 is a chip that converts between TTL and RS232 level. Single chip microcomputer through internal universal asynchronous receiver/transmitter
(UART) to communicate with MAX232. The design schematic diagram of the download module is shown in Figure 6.
System software design adopts C language programming and modular structure for development. The program mainly includes the main function, delay sub-function, A/D conversion sub-function and temperature display sub-function. Among them, the delay sub-function and the A/D conversion sub-function are shown in Figure 7-a, and the temperature display sub-function and the system main function are shown in Figure 7-b.
The performance test shows that it moves at a constant speed indoors and the room temperature does not change significantly. Similarly, when the temperature is collected outdoors, the change is not very obvious. When covering the chip with your fingers or using a heater or fan to heat the chamber. The temperature changes rapidly and the temperature rises sharply. The test result shows that this system has short response time and high sensitivity.
5 ConclusionTemperature acquisition and control systems are widely used in modern life. The system has realized the functions of temperature collection and display. After repeated tests, it has been shown that the system has the characteristics of short response time, strong anti-interference ability, stable and reliable work, etc. At the same time, it has the advantages of small size and low cost, and has high engineering value. There are broad application prospects in areas such as daily life and academic research. The temperature range collected by this system is between 25°C and 80°C, and the accuracy of the temperature value is an integer. The next step will be to study how to use frequency detection and control technology to further improve the temperature measurement accuracy and range. At the same time, it can also be used as a multifunctional product integrating temperature measurement, temperature warning, and control after expansion.
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