First, what is LED ?
In the PN junction of a semiconductor material, the injected minority carriers recombine with the majority carriers to release excess energy in the form of light, thereby directly converting the electrical energy into light energy. The PN junction adds a reverse voltage, and minority carriers are difficult to inject, so they do not emit light. Such a diode fabricated by the principle of injection electroluminescence is called a light-emitting diode, and is generally called an LED.
Second, how does LED shine?
The color and luminous efficiency of LEDs are related to the materials and processes for making LEDs. Currently, red, green and blue are widely used. Due to the low operating voltage of the LED (only 1.5-3V), it can actively emit light and have a certain brightness. The brightness can be adjusted by voltage (or current), and it is resistant to shock, vibration and long life (100,000 hours), so it is large. Among the display devices, there is currently no other display mode that rivals the LED display mode. A display made of red and green LEDs together as a pixel is called a two-color screen or a color screen; a display screen that puts three LED tubes of red, green, and blue together as a pixel is called a three-color screen or a full screen. Color screen. The pixel size of the indoor LED screen is generally 2-10 mm. It is often used to package several LED dies that can produce different primary colors. The outdoor LED screen has a pixel size of 12-26 mm, and each pixel It consists of several kinds of monochromatic LEDs. The common finished product is called a pixel tube. The two-color pixel tube is generally composed of 3 red and 2 green. The three-color pixel tube is composed of 2 red, 1 green and 1 blue. Regardless of whether a single-color, two-color or three-color screen is made with LEDs, the brightness of each LED that is required to form an image must be adjustable, and the degree of fineness of the adjustment is the gray level of the display. The higher the gray level, the finer the displayed image and the richer the color, and the more complex the corresponding display control system. Generally, the image of 256-level gray scale has a very soft color transition, while the color image of 16-level gray scale has a clear color transition boundary. Therefore, color LED screens are currently required to be 256-level grayscale.
Second, the method of controlling LED brightness:
There are two ways to control the brightness of the LED.
1, using the visual inertia of the human eye, using pulse width modulation method to achieve gray control, that is, periodically changing the optical pulse width (ie, duty cycle), as long as the period of repeated lighting is short enough (ie, the refresh frequency is high enough) ), the human eye does not feel that the illuminating pixels are shaking. Since pulse width modulation is more suitable for digital control, in today's widespread use of microcomputers to provide LED display content, almost all LED screens use pulse width modulation to control gray levels.
2, change the current flowing through the LED, the general LED tube allows continuous operating current of about 20 mA, in addition to the saturation of the red LED, the brightness of other LEDs is basically proportional to the current flowing;
The LED control system usually consists of three main parts: the main control box, the scanning board and the display control unit. The main control box obtains the brightness data of each layer of pixels from the display card of the computer, and then re-allocates the data to a plurality of scanning boards, each of which is responsible for controlling several rows (columns) on the LED screen, and each row (column) The display control signals of the upper LEDs are transmitted in a serial manner. At present, there are two ways of serially transmitting display control signals: one is to control the gray level of each pixel point on the scanning board, and the scanning board decomposes the brightness values ​​of the pixels in each row from the control box (ie, pulse width modulation). Then, the turn-on signals of the LEDs of each row are serially transmitted to the corresponding LEDs in a pulse form (lighting is 1 and not lit to 0) to control whether they are lit. This method uses fewer devices, but the amount of data transmitted serially is larger because each pixel requires 16 pulses at 16 levels of gray during a repeated lighting period, requiring 256 levels of gray. 256 pulses, due to the device operating frequency limit, generally only enable the LED screen to achieve 16 gray levels. Another method is that the serial transmission of the scan board is not the switching signal of each LED but an 8-bit binary luminance value. Each LED has its own pulse width modulator to control the lighting time. Thus, in a period of repeated lighting, each pixel point requires only 4 pulses in 16-level gray scale, and only 8 pulses in 256-level gray scale, which greatly reduces the serial transmission frequency. With this method of controlling the gradation of the LED, it is convenient to implement 256-level gradation control.
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