The Automatic Resistance Tester Is Designed with AT89S52 Single Chip Microcomputer and ADC0809

For variable resistance, the tester can also display its resistance change curve by measuring its resistance. In addition, the tester also adds the function of automatically outputting resistance value through the color ring color of input resistance. The test results show that the resistance tester has high measurement speed, high resolution and high practicability.


With the progress of science and technology, the rapid increase of electronic components, resistance measurement has been widely used in measurement technology and product research and development. Measuring resistance with multimeter can not meet people's needs. Therefore, it is of great practical necessity to design a safe, reliable and convenient resistance tester. The measuring instrument also has the function of automatically screening resistance, automatically measuring and displaying the resistance value curve of potentiometer with rotation angle, and can directly read out the measured resistance value from LCD display screen, with the measurement accuracy of ± 0.1%. It not only has simple measurement and intuitive reading, but also has higher measurement accuracy and resolution than general bridges.

1. Theoretical analysis and calculation

1.1 resistance measurement principle

According to Kirchhoff's negative voltage law, two resistors in series are connected in series with the power supply, and the voltage at both ends of the two resistors is equal to the power supply voltage. When one of the resistance values is changed, the voltage at both ends changes linearly with the change of resistance value, that is:

Therefore, as long as the voltage at both ends of R1 is measured, the resistance value of R1 can be calculated.

1.2 automatic range conversion

In the automatic range conversion function, first compare the collected voltage signal with the voltage range of the gear. If the voltage signal is within this range, calculate the voltage signal collected again, convert it into resistance value for output display; If the voltage signal is not within this range, the measured gear will jump to the next gear through the control of the relay. Cycle until the resistance is pulled out or the measurement mode is jumped out.

1.3 screening function and principle

When the measurement mode is switched to the resistance screening mode, the screened resistance value and error range can be input through the keyboard to screen the resistance within the error range. The implementation method is:

When the measured resistance is within the range of preset resistance value, the "compliance" is displayed on the LCD; When the measured resistance is not within the preset resistance range, the "Non Conformance" is displayed on the LCD, so as to screen the resistance.

1.4 potentiometer resistance change curve device

The device drives the rotation of the stepping motor through the single chip microcomputer control ULN2003, so as to drive the rotating shaft of the potentiometer to rotate and change the resistance value of the potentiometer.

2. Hardware design and Implementation

The automatic resistance measuring instrument is mainly composed of controller, power supply, 4 * 4 keyboard, display signal acquisition circuit and gear control circuit. The block diagram is shown in Figure 1.

2.1 power supply

The switching power supply with 5V / 3A output and strong stability is used to supply power to the whole system, which ensures the stable operation of the system and improves the reliability of the instrument.

2.2 keyboard

The 4 * 4 keyboard is adopted. When the mode switching key is pressed and the input mode is started, the corresponding resistance value can be input through the keyboard. The interface between the keyboard port and the minimum system is port P1, in which key C is the mode switching key and keys a, e and F are the mode setting keys.

2.3 display part

12864a-1 Chinese character graphic dot matrix LCD module is adopted to display Chinese characters and graphics. 8192 Chinese characters (16x16 dot matrix), 128 characters (8x16 dot matrix) and 64x256 dot matrix display RAM (gdram) are built. The interface is connected with P0 port and P2 port of MCU, and the parallel data transmission method is used to control the LCD. The display speed of LCD is obviously improved, the screen display is more stable, and the page display is more comfortable and beautiful.

2.4 AD sampling part

Ad0809 analog-to-digital conversion chip with accuracy of verf * 255 / 256 is selected as the main sampling equipment. The clock is provided for ad0809 through 74ls74 frequency division, and the sampling rate is 200kbpms. It is equipped with appropriate resistance test channel to sample the voltage at both ends of the measured resistance, and the converted data is transmitted to the single chip microcomputer. The interface is P3. After calculation, the control relay selects the appropriate gear for measurement again, The resistance value of the measured resistance can be measured more accurately.

3. Software design (flow chart is as follows)

4. Test scheme and test results

4.1 test scheme

(1) Test method

Compare the resistance of the adjustable resistance with the measured value of the standard multimeter.

(2) Test data

4.2 analysis of test results

The test results show that 10K and 1K gears can better meet the design requirements, and the 100 Ω gear error is large.

It can be seen from the test results that when the measured resistance value is larger, the measurement error is smaller. Due to the internal resistance and noise interference of the tester itself, when the measured resistance value is smaller, the error is larger.

5. Summary

The automatic resistance tester has four gears of 100 Ω, 1K Ω, 10K Ω and 10m Ω, automatic conversion range and three digit display. It can automatically screen the resistance and display the corresponding parameters according to the requirements of keyboard input; It can automatically and accurately measure and display the resistance curve of potentiometer with angle within 10 seconds; It also has the intelligent function of automatically outputting the resistance value through the color ring color of the input resistance.

The Automatic Resistance Tester Is Designed with AT89S52 Single Chip Microcomputer and ADC0809 1

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