Basic Electronics Knowledge: How LEDs Realize Display Functions

In the PN junction of some semiconductor materials, when the injected minority carriers recombine with the majority carriers, they release the excess energy in the form of light, thereby directly converting electrical energy into light energy. When a reverse voltage is applied to the PN junction, it is difficult for minority carriers to be injected, so no light is emitted. This type of diode made using the principle of injection electroluminescence is called a light-emitting diode, commonly known as an LED.

The color and efficiency of LED light are related to the materials and processes used to make it. Currently, red, green and blue are widely used. Since LED has a low operating voltage (only 1.5-3V), can actively emit light with a certain brightness, and the brightness can be adjusted by voltage (or current), it is impact-resistant, vibration-resistant, and has a long life (100,000 hours), so in large display devices, there is currently no other display method that can match the LED display method.

The display screen that puts red and green LEDs together as a pixel is called a two-color screen or a color screen; the display screen that puts red, green, and blue LED tubes together as a pixel is called a three-color screen or a full-color screen. The pixel size of indoor LED screens is generally 2-10 mm, and several LED tube cores that can produce different primary colors are often packaged into one. The pixel size of outdoor LED screens is mostly 12-26 mm. Each pixel is composed of several various single-color LEDs. The common finished product is called a pixel tube. The two-color pixel tube is generally composed of 3 red and 2 green, and the three-color pixel tube is composed of 2 red, 1 green, and 1 blue.

Whether LED is used to make a single-color, two-color or three-color screen, the brightness of each LED that constitutes the pixel to display the image must be adjustable, and the degree of fineness of the adjustment is the grayscale level of the display screen. The higher the grayscale level, the more delicate the displayed image, the richer the color, and the more complex the corresponding display control system. Generally, the color transition of a 256-level grayscale image is very soft, while the color transition boundary of a 16-level grayscale color image is very obvious. Therefore, color LED screens are currently required to be made into 256-level grayscale.

2. Methods for controlling LED brightness:

There are two ways to control the brightness of LEDs. One is to change the current flowing through the LED. Generally, the continuous working current allowed by LED tubes is about 20 mA. Except for the saturation of red LEDs, the brightness of other LEDs is basically proportional to the current flowing through. The other method is to use the visual inertia of the human eye and use the pulse width modulation method to achieve grayscale control, that is, to periodically change the width of the light pulse (i.e., the duty cycle). As long as the repeated lighting cycle is short enough (i.e., the refresh frequency is high enough), the human eye cannot feel the jitter of the luminous pixels. Since pulse width modulation is more suitable for digital control, today when microcomputers are widely used to provide LED display content, almost all LED screens use pulse width modulation to control the grayscale level.

The control system of LED usually consists of three parts: main control box, scanning board and display control device. The main control box obtains the brightness data of various colors of a screen pixel from the computer display card, and then redistributes it to several scanning boards. Each scanning board is responsible for controlling several rows (columns) on the LED screen, and the display control signal of the LED on each row (column) is transmitted in serial mode. There are currently two ways to transmit the display control signal in serial mode: one is to centrally control the grayscale of each pixel on the scanning board. The scanning board decomposes the brightness value of each row of pixels from the control box (i.e. pulse width modulation), and then transmits the opening signal of each row of LED in the form of pulses (1 for lighting and 0 for non-lighting) to the corresponding LED in serial mode by row to control whether it is lit. This method uses fewer devices, but the amount of serially transmitted data is large, because in a repeated lighting cycle, each pixel requires 16 pulses at 16 grayscales and 256 pulses at 256 grayscales. Due to the limitation of the working frequency of the device, the LED screen can generally only achieve 16 grayscales.

Another method is that the content of the serial transmission of the scanning board is not the switch signal of each LED but an 8-bit binary brightness value. Each LED has its own pulse width modulator to control the lighting time. In this way, in a repeated lighting cycle, each pixel only needs 4 pulses at 16 grayscale levels and only 8 pulses at 256 grayscale levels, which greatly reduces the serial transmission frequency. This method of decentralized control of LED grayscale can easily achieve 256-level grayscale control.