Design and implementation of LED lighting dimming device

With the development of light-emitting diode technology, LED lighting has shown increasingly broad application prospects and development potential in landscape lighting, signboard lighting, and large-screen displays due to its advantages such as long life, low power consumption, and controllable color. This article introduces an LED lighting driver that can easily control the light intensity and color of the LED, and can control multiple LED patterns through programming. In addition, this system can communicate with the computer through the serial port to display the information transmitted by the host computer.

1 LED dimming principle

There are currently three mainstream technical routes for achieving LED lighting: (1) Based on the principle of three primary colors, red, green and blue primary color LEDs are used to synthesize white light; (2) UV LEDs are used to excite three primary color phosphors to emit synthetic white light; (3) Blue LEDs are used to excite yellow phosphors to achieve binary mixed color white light. In contrast, the use of three primary color LEDs to mix white light can not only achieve an ideal white light spectrum, but also the color of the light source is adjustable, so that it can not only adapt to lighting occasions with relatively high requirements for the color of white light, but also can adapt to various occasions with constantly changing colors and brightness and requiring high precision through a certain automatic control drive circuit. Its implementation principle is shown in Figure 1:

Figure 1 Principle diagram of three primary colors to achieve color light synthesis

The brightness of the three-primary-color LED can be adjusted through the control circuit, and the color and brightness of the synthesized light after passing through the lens can be adjusted.

2 Overall plan of dimming device

As shown in Figure 2, the 87LPC762 microcontroller transmits data to the three-way D/A converter, converts the digital quantity into analog quantity, and controls the intensity of the red, green and blue LEDs through the power amplifier circuit, thereby controlling the color and brightness of the LEDs. For multiple LEDs, a dynamic scanning method is adopted. The microcontroller controls the two output terminals to output serial signals based on CLK. The triodes in the switch operation are controlled by the serial-to-parallel conversion device to light up the three primary color LEDs respectively, completing the control of the graphic pattern. Finally, this system can complete the various functions required for dimming. The serial communication interface uses the RS232 standard bus to connect to the computer, and the pattern or text to be displayed in the computer can be transmitted to this system and displayed.

Figure 2 Overall scheme diagram of dimming device

3. Implementation of light color adjustment

In Figure 3, AD558 is a D/A conversion chip that can convert digital quantities into analog quantities. The single-chip microcomputer is connected to the DB0-DB7 of AD558 through the P0 port for parallel data transmission, thereby controlling the level of the AD558 output voltage. The output voltage controls the intensity of the LED monochromatic light through power amplification. Since AD558 is an 8-bit D/A conversion chip, when the three primary colors are controlled by three D/A channels, it can divide the red, green, and blue of the three primary colors into 28 = 256 outputs respectively. If the three primary colors are combined arbitrarily, the maximum combined resolution of 2563 = 16777216 colors and brightness can be formed. Through the setting of software data, any required resolution value within the maximum limit can be set according to actual needs.

Figure 3 Schematic diagram of light color adjustment analog output

4. Implementation of scanning by lighting up multiple LEDs separately

The three pins of the microcontroller P1 port output signals respectively. 74LS164 converts the serial signal into a parallel signal. Its output terminals Q0-Q7 are connected to the common terminals of the red, green and blue primary color LEDs respectively. Under the control of the clock CLK1, they can output high level in sequence. This circuit is a serial data control parallel port, so it can be expanded arbitrarily.

Figure 4 Schematic diagram of multiple LEDs lighting up and scanning

5 Communication between dimming device and computer system

As shown in Figure 5, the RS-232 serial communication standard is used between the dimming device and the computer system, which stipulates the level relationship between the transmitting driver and the receiving receiver, load requirements, signal rate and connection requirements, etc. MAX232 is mainly used to complete the level conversion between TTL level and RS-232.

Figure 5 Diagram of the communication interface between the dimming device and the computer system

6 Conclusion

A set of LED dimming devices was designed and produced. A single-chip microcomputer was used to control the red, green and blue primary color outputs of LEDs through three-way DA conversion outputs, realizing wide-range and high-precision adjustment of the color and brightness of semiconductor lighting. At the same time, through the serial-to-parallel conversion scanning method, a system can control multiple LEDs separately, realizing the formation of arbitrary static or dynamic patterns or pictures based on high-precision light and color control.

By testing the pattern, color and brightness of the matrix composed of 128 × 64 LEDs in the actual experimental model for 8 hours, it was shown that the central light intensity of the LED was stable at 280 lx (lx: lux). As the temperature of the LED increased, the central light intensity decayed by less than 3%, reaching the expected control requirement level.