Color control scheme of LED lighting based on small MCU

Under the general trend, LEDs are just around the corner for general lighting. LEDs have many advantages in general lighting, such as longer life and higher efficiency. However, LED technology still faces some challenges. One of the challenges is how to produce high-quality white light. The composition of the white LED includes a blue LED and a phosphor that can shift the light output to other wavelengths of the spectrum. Many white LEDs are unable to produce a high color rendering index (CRI). This parameter is used to measure the light source's ability to reproduce colors.

By mixing two or more colors of LED light, a higher quality white light system can be obtained. In these multi-color systems, the light output of each color source will drift with time and temperature. Light sensors and small microcontrollers (MCUs) can be used to maintain specific colors and correlated color temperatures (CorrELated Color Temperature, CCT). In this article, we will learn more about sensors, required MCU resources and software.

There are many affordable small light sensors on the market that can provide information for processing to the MCU. Typically, the sensor has some optional color filters for measuring red, green, blue, or white light (no filter). The light sensor output interface can be connected to the MCU through a series of methods. The light-to-voltage sensor is connected to the analog-to-digital converter (ADC) through the output voltage. The light-to-frequency sensor provides variable-frequency output. The output frequency is proportional to the amount of light. The pulse output of these sensors can be accumulated in the MCU timer to determine the light level. Light-to-digital sensors usually have a serial digital interface, such as I2C. Each type of sensor interface has unique advantages and requires different MCU resources. The system block diagram shown in Figure 1 shows a variety of MCU peripherals, which are very useful in color-adjustable LED lighting design.

In a complete closed-loop color control system, the MCU must read the color components from the light sensor, calibrate the light sensor output, and adjust the output of each LED driver to obtain the desired color. LEDs require constant current drivers to maintain the consistency of light output. This can be achieved using various driver technologies including linear and switch mode solutions. The final choice depends on factors such as efficiency requirements, input voltage range, and the number of LEDs used.

The driver output can be controlled using different methods. First, the MCU can generate an analog reference voltage through a digital-to-analog converter (DAC) or digital potentiometer. The reference voltage allows the driver output to vary from zero to maximum current. The MCU can also provide PWM signals for modulating the driver output. The PWM signal can be used to enable / disable the driver itself, or to control a switch that disconnects the LED from the driver output. If PWM control is used, the selected PWM frequency must be high enough so that the human eye cannot detect any flicker.

The designer must determine the degree of control resolution required by the color control system in order to select the MCU with the corresponding peripherals. For light-to-voltage sensors, the measurement resolution of the ADC on the MCU is important. The optical to frequency sensor requires an MCU time base that is incremented by an external clock. Light-to-digital sensors require corresponding serial communication interface peripherals.

MCUs with multiple PWM peripherals can be used to control individual LED drivers. In high-resolution color control systems, PWM peripherals with 16-bit or higher control resolution are preferred. Serial communication peripherals (such as UART, SPI, I2C, LIN, USB, etc.) support input / output control and display functions.

For color control systems, MCU devices such as PIC24FJ16GA002 (see Figure 2) are the best choice. PIC24 devices are available in a small 28-pin package with a program memory range of 16 to 64 KB, and provide a serial communication interface, 10-bit ADC, and 5 PWM channels in a single device. The 16-bit MCU core can easily handle arithmetic operations related to sensor calibration and color control.

The sensor data output must be calibrated against the reference voltage to provide consistent results. The calibration process uses a colorimeter to mathematically correlate the output of different color LEDs with the spectral response and the sensitivity of the light sensor in a standard chromaticity coordinate system. The calibration process generates a coefficient matrix, which must be stored in the non-volatile memory with the lighting system and used to determine the difference between the correlation and the desired output in each control of the control system.

After completing the calibration, the MCU can compare the sensor data with the ideal CIE (International Commission on Illumination) chromaticity diagram coordinates and adjust the output channel until the ideal CCT is obtained. The PID control algorithm of each output channel uses the calibration value to adjust the sensor data, find the difference from the target set point, and then adjust the output channel. To reduce the error, the PID will continue to run until the output CCT matches the set point CCT. The PID coefficients can be fine-tuned to optimize the system response to the greatest extent, but the speed with which the PID algorithm converges to the target CCT is also a function of the MCU's efficiency in processing arithmetic operations. Some color control systems may require faster processing and response speeds than others. For example, the requirements for general lighting systems are lower than for local dimming systems for HDTV panels.

Systems with adjustable light sources or high CRI have a series of user control requirements. Medical devices with graphical LCD displays may have adjustable LED backlights (it requires the MCU to communicate with the LCD via SPI), and a touch screen interface for adjusting CCT and brightness. General lighting for commercial display devices may require control via a central panel or computer to automatically adjust brightness, CCT, and on / off according to various times of the day. Communication between these devices can be achieved using a hard-wired serial bus protocol (such as DALI or DMX512), while some other devices may need to use a custom interface implemented via USB or Ethernet. In a completed building, installing a hard-wired infrastructure may not work, and it needs to be controlled through wireless communication and protocols (such as ZigBee). For such lighting applications, MCUs with flexible peripherals are ideal for communication and user interfaces.

Light source technologies such as candles, kerosene lamps and incandescent lamps have replaced their previous technologies, thereby improving people's quality of life. It is expected that LED light sources will enrich our lives better than all other light source technologies. LED has the advantages of high energy efficiency, small size, portable, durable and long life. The multi-color LED controlled by a small MCU can adjust the light output to provide comfortable lighting suitable for the lighting space. The MCU can intelligently control the driver circuit (to maximize energy efficiency), monitor some conditions, and maximize energy efficiency and average life. The MCU color control LED lighting system will enable people to see the world with different eyes.

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