This paper presents a 32-bit control core based on the Cortex-M3 architecture, combined with an external controllable constant current source and a color sensor to create a simple yet effective device for testing LED characteristics. The system is designed to be cost-effective, easy to use, and highly practical for research purposes. It provides a foundation for understanding key parameters such as voltage-current relationships, brightness, and color properties of LEDs.
LEDs, or Light-Emitting Diodes, have emerged as a revolutionary lighting technology due to their high efficiency, energy savings, environmental friendliness, long lifespan, and reliability. Governments worldwide are supporting the development of white LEDs, and countries like the United States, Japan, and the European Union have actively promoted their adoption. As LED applications expand, user demands for product quality have increased. Not only do they require consistent optical performance—such as brightness and wavelength—but also strict control over forward voltage and current characteristics. Therefore, developing reliable and accurate test instruments for LED photoelectric parameters is essential for improving product quality and reducing manufacturing costs.
Traditional LED testing equipment often involves complex setups, including spectrometers and photometers, which offer high precision but come with limitations in terms of size, cost, and portability. These systems are typically used in specialized laboratories, making it difficult to integrate them into broader industrial or research settings. To address this, there is a growing need for compact, affordable, and less precise LED testing tools that meet general requirements.
While LEDs have various photoelectric characteristics, most users are primarily concerned with their voltage-current behavior and how brightness relates to current. This paper focuses on measuring these two key parameters. Additionally, with minor hardware and software modifications, the system can also assess other properties like color temperature, dominant wavelength, and light intensity distribution, making it valuable for LED research and drive circuit design.
The system is built around an STM32 microcontroller, an optical measurement module, a constant current driver, an LCD display, and a button control interface. The STM32 manages the control voltage via its internal DAC, which regulates the external constant current circuit. The resulting voltage drop across the LED is measured using the ADC within the microcontroller, allowing for accurate voltage-current analysis. Meanwhile, the optical module captures the light emitted by the LED and converts it into digital data for further processing.
The STM32F103RCT6 microcontroller, part of STMicroelectronics' enhanced series, features a 32-bit ARM Cortex-M3 core, operating at up to 72 MHz. It includes 128 KB of Flash memory, 20 KB of SRAM, and multiple I/O ports, timers, and communication interfaces. Its built-in ADC and DAC modules significantly reduce system complexity and cost.
The constant current drive circuit uses a V/I conversion setup, where the output current is controlled by a programmable resistor and a high-power operational amplifier (OPA548). This ensures stable and accurate current delivery to the LED under test, with compensation techniques used during calibration to maintain precision.
For optical measurements, the TCS3200 color sensor is employed. It measures the brightness of red, green, and blue components in LED light, converting them into digital signals. This eliminates the need for additional analog-to-digital conversion, simplifying the circuit and improving accuracy.
In terms of data processing, the system calculates tristimulus values (X, Y, Z) from RGB readings, enabling the determination of chromaticity coordinates, dominant wavelength, and correlated color temperature according to CIE standards. The STM32 handles all calculations, ensuring real-time and accurate results.
Experimental tests were conducted on various colored LEDs, including red, green, yellow, blue, and white. The results showed that the system’s accuracy is within 5% after hardware adjustments and software compensation, proving its effectiveness for practical applications.
In conclusion, this low-cost, intelligent LED testing system offers a viable alternative to expensive specialized equipment. It is suitable for both academic research and industrial applications, providing reliable and accurate measurements of key LED parameters. The design is simple, scalable, and well-suited for future enhancements.
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