Design of a simple LED photoelectric characteristics test device

Abstract: With the 32-bit STM32 microprocessor based on the Cortex-M3 core as the control core, external controllable constant current source circuit and color sensor and other components, a simple LED photoelectric characteristics test device is formed. It has a simple structure and low cost, and has high practical value in the study of LED characteristics.

1. Introduction

LED (Light-Emitting-Diode), or light-emitting diode, is gradually replacing traditional incandescent lamps and fluorescent lamps with its advantages of high efficiency, energy saving, environmental protection, long life and high reliability, becoming a new generation of lighting sources. Governments of various countries have vigorously supported the development of white light LEDs. Developed countries such as the United States, Japan, and the European Union have set up special projects by their governments to actively promote it. With the expansion of the application scope of LEDs, users have higher requirements for product quality, requiring not only the consistency of its optical characteristics such as luminous brightness and wavelength, but also strict requirements for its electrical characteristics such as forward working voltage and current. Therefore, research on the test instruments of LED optoelectronic parameters is of great significance to improving product quality and reducing production costs.

The structure of dedicated LED photoelectric characteristic testing equipment is complex, especially the optical characteristic test requires the use of spectrometers, photometers, etc. Although the instruments have the characteristics of high precision, they still have the disadvantages of complex structure, high cost, large size, inconvenient to carry and use, and limited system stability. Therefore, this kind of instrument can only remain in large-scale analytical testing experiments, and its application range is difficult to expand.

Developing a small-sized, low-priced LED photoelectric measuring instrument that is not very accurate but can meet general requirements has become a trend in the current instrument development.

Although there are many photoelectric characteristic parameters of LED, most users are mainly concerned about the LED volt-ampere characteristics and the relationship between LED brightness and current. Therefore, the system designed in this article is mainly used to complete the test of these two parameters. In addition, through simple software and hardware expansion, the LED's related color temperature, main wavelength, light intensity distribution and other parameters can also be tested for LED characteristic research and drive circuit design and other application development.

2. System principles and composition

The whole system is mainly composed of STM32 microprocessor, optical measurement module, constant current drive module, LCD liquid crystal display module, key control module, etc., as shown in Figure 1.

The STM32 microprocessor is controlled by buttons or the touch screen on the LCD. The internal D/A converter generates a control voltage, which controls the external constant current drive circuit to generate the constant current required for the LED to work and add it to the LED to be tested. The voltage drop generated on the LED is collected by the A/D converter inside the STM32 after passing through the signal amplification and adjustment circuit, thereby measuring the volt-ampere characteristics of the LED. In addition, the light emitted by the LED is measured by the optical measurement module and converted into a digital signal, which is collected and processed by the STM32 to measure the optical characteristics of the LED such as the luminous brightness.

2.1 STM32 microprocessor

The microprocessor is the core of the entire control system. It controls the constant current drive circuit to output the set current, collects the voltage of the LED, measures the data of the optical measurement module, performs data processing, control algorithm calculations, display control, etc. In order to ensure the practicality and scalability of the system, this control system uses the "enhanced" series STM32F103RCT6 launched by STMicroelectronics, a 32-bit ARM Cortex-M3 core, an operating frequency of up to 72MHz, built-in high-speed memory (up to 128K bytes of flash memory and 20K bytes of SRAM), rich enhanced I/O ports and peripherals connected to two APB buses, 16-channel 12-bit ADC and 2-channel 12-bit DAC, 3 general 16-bit timers and a PWM timer, and also includes standard and advanced communication interfaces: up to 2 I2C and SPI, 3 USARTs, a USB and a CAN, which meet the requirements in terms of storage capacity and operation speed. In this design, the ADC and DAC modules of STM32 itself are used, which greatly reduces the system cost.

2.2 Constant current drive circuit

The core of the constant current drive circuit is the V / I conversion circuit, as shown in Figure 2. Vin is the voltage output by the internal D/A of STM32, RL is the load, that is, the LED to be tested, Rs is the current sampling resistor, which is used to control the output current, and U1 is a high-power operational amplifier.

As shown in Figure 2, if the resistors R1 and R2 are much larger than Rs and RL, then ideally

From formula 2, it can be seen that the output current is independent of the load. When the sampling resistor Rs is fixed, the output current is proportional to the input control voltage. However, in the application, it should be noted that the two input resistors R3 and R4 and the two feedback resistors R1 and R2 must be strictly matched, otherwise it will cause a large error. Rs should also use a precision power resistor. In addition, compensation calibration can also be performed in the software during system debugging to ensure the output current accuracy.

The operational amplifier OPA548 in Figure 2 is a high voltage, high current power operational amplifier with excellent small signal amplification performance. It is ideal for driving a variety of loads. The power supply voltage (+VS~-VS) is 60V, and it can work with a single power supply or dual power supplies. The input impedance is high and the bias current is small. It can continuously output a large current of 3A (peak current up to 5A), and it has internal over-temperature and current overload protection. Users can perform precise current limiting design according to their needs. 2.3 Optical Measurement Module

This design uses a new color sensor TCS3200 to measure the optical characteristics of LEDs, and can simultaneously measure the brightness of the three primary colors contained in the LED light. TCS3200 is a programmable color light to frequency converter launched by TAOS. It integrates a configurable silicon photodiode and a current-frequency converter on a single CMOS circuit, and integrates three red, green and blue (RGB) filters on a single chip. It is the industry's first RGB color sensor with a digital compatible interface.

The output signal of TCS3200 is a digital quantity, which can drive standard TTL or CMOS logic input, so it can be directly connected to a microprocessor or other logic circuit. Since the output is a digital quantity and can achieve a conversion accuracy of more than 10 bits for each color channel, the A/D conversion circuit is no longer required, making the circuit simpler. Figure 3 is the pin and function block diagram of TCS3200. 

When the incident light is projected onto TCS3200, different filters can be selected through different combinations of the photodiode control pins S2 and S3; after passing through the current-to-frequency converter, square waves of different frequencies are output (the duty cycle is 50%), and different colors and light intensities correspond to square waves of different frequencies; different output scale factors can also be selected through the output calibration control pins S0 and S1 to adjust the output frequency range to meet different needs.

Because the operating frequency of STM32 is relatively high, S0 and S1 are directly connected to high potential to make the output proportional factor 100%. S2 and S3 are controlled by the pins of STM32, and the output signal of the color sensor is programmed with STM32 to measure the frequency.

3. Measurement data calculation and processing

The software system design of STM32 mainly includes LCD display, button processing, DAC control, ADC control, color sensor control and counting measurement. These modules are relatively simple and will not be described here. The following mainly introduces the data calculation and processing procedures for LED optical characteristics measurement.

The quantitative measurement of object color is a complex problem involving many factors such as the observer's visual physiology, visual psychology, lighting conditions, and observation conditions. CIE (International Commission on Illumination) has published a series of colorimetric systems since 1931, stipulating a complete set of color measurement principles, data, and calculation methods, forming the CIE standard colorimetric system that laid the foundation for modern colorimetry.

According to CIE's recommendation, the chromaticity of a light source can be represented by a colorimetric system of three stimulus values ​​X, Y, Z and chromaticity coordinates.

The following formula can be used to convert RGB values ​​to XYZ values:

The relationship between chromaticity coordinates and tristimulus values ​​is:

The color sensor TCS3200 measures the ratio of the three primary colors that make up the chromaticity of the light source and obtains the R, G, B values. After calculation, the three stimulus values ​​X, Y, Z and chromaticity coordinates x, y, z of the chromaticity of the light source can be obtained. With the chromaticity coordinates, the brightness, dominant wavelength, color purity, correlated color temperature and other parameter values ​​of the light source can be obtained according to the CIE1931 standard colorimetric system. The Y value in formula 3 corresponds to the response of the human eye to brightness and can be used to calculate the brightness of the LED.

There are many methods to obtain the correlated color temperature (CCT) from the chromaticity coordinates. Among them, the approximate formula method is simple to calculate, easy to implement, and the accuracy can meet general requirements.

4. Experimental results

Using the above circuit, a simple LED photoelectric characteristic test device was designed, and some red, green, yellow, blue, white and other colored LEDs were used for testing. The results are shown in Figure 4. Figure 4 (a) is the volt-ampere characteristic curve of LEDs of different colors, and Figure 4 (b) is the relationship between the luminous brightness and current of LEDs of different colors. It can be seen from the figure that the LED current changes very quickly with the voltage, while the luminous brightness and current are basically linear. This can be used as a reference for designing LED drive circuits. In addition, the volt-ampere characteristic was calibrated with a Keithley2612 high-precision source meter, and the brightness test was calibrated with an illuminance meter. The experimental results show that through hardware adjustment and software compensation, the result error can be controlled within 5%, achieving the practical goal.

5. Conclusion

This paper uses the STM32 microprocessor as the core and the color sensor as the main component to design a simple LED photoelectric characteristic device to test the LED's volt-ampere characteristics, luminous intensity, correlated color temperature, dominant wavelength and other LED characteristics. The entire system is simple, intelligent, and low-cost. It can replace expensive special equipment such as spectrum analyzers in certain LED research and application fields. The experimental results show that the test accuracy meets practical requirements and the scheme in this paper is feasible.