Knowledge of LED electronic display
Author:Zhang Sui Time:2021-09-11 12:28 Browse(129)
What is?
What is led

Knowledge of LED electronic display

it is difficult to inject the minority carriers in PN junctions with reverse voltage, So it doesn't emit light.
this kind of LED based on the principle of injection electroluminescence is called light-emitting diode.
it is generally called led.

the luminous color and luminous efficiency of LED are related to the material and process of making LED.
at present, red, green and blue are widely used.
due to the low working voltage of LED (only 1.5-3V).
it can emit light actively and has a certain brightness, The brightness can be adjusted by voltage (or current), and it is shock resistant, vibration resistant and has a long service life (100000 hours). Therefore, in large-scale display equipment, there is no other display mode that can match LED display mode.

the display screen made by putting red and green LED together as a pixel is called two-color screen or color screen; The display screen with red, green and blue LED tubes as one pixel is called three color screen or full-color screen.
the pixel size of indoor LED screen is generally 2-10 mm.
several kinds of LED chips that can produce different primary colors are often packaged into one. The pixel size of outdoor LED screen is mostly 12-26 mm, and each pixel is composed of several monochrome LEDs, The common finished product is called 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 the LED is used to make monochrome, two-color or three color screen.

the LED is used to make monochrome, two-color or three color screenTo display an image, the brightness of each LED that constitutes a pixel must be adjustable, and the fine degree of adjustment is the gray level of the display screen.
the higher the gray level is.
the more delicate the displayed image is, the richer the color is, and the more complex the corresponding display control system is.
generally, the 256 gray level image has very soft color transition, while the 16 gray level color image has very soft color transition, The color transition boundary is very obvious.
so.
Color LED screen is required to be 256 gray level.

Second, there are two methods to control LED brightness.
one is to change the current flowing through the LED.
generally, the continuous working current of LED is about 20 mA, except that the red LED is saturated, The brightness of other LEDs is basically proportional to the current flowing; Another method is to use the visual inertia of human eyes, and use the pulse width modulation method to achieve gray control, that is to change the light pulse width (i.e. duty cycle) periodically, as long as the cycle of repeated lighting is short enough (i.e. the refresh frequency is high enough), The human eye can't feel the light-emitting pixels shaking.
because PWM is more suitable for digital control.
today, microcomputer is widely used to provide LED display content, almost all LED screens use PWM to control the gray level.

the LED control system is usually controlled by the main control box, the LED display panel and the LED display panel Scanning board and display control device are composed of three parts.
the main control box obtains the brightness data of a screen pixel from the display card of the computer.
then it is redistributed to several scanning boards, and each scanning board is responsible for controlling several rows (columns) on the LED screen,At present, there are two ways of serial transmission of display control signal: one is to control the gray level of each pixel on the scanning board centrally.
the scanning board decomposes the brightness value of each pixel from the control box (i.e. pulse width modulation), and then turns on the signal of each LED in the form of pulse (light up to 1, light up to 1, light up to 1, light up to 1, light up to 1, light up to 1, light up to 1, light up to 1, light up to 1, light up to 1, light up to 1, light up to 1, light up to 1, light up to 1, In this way, less devices are used.
but the amount of serial transmission data is large, because in a repeated lighting cycle, each pixel needs 16 pulses at 16 gray levels and 256 pulses at 256 gray levels, Generally, the LED screen can only achieve 16 gray levels.

another method is that the serial transmission content 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 in 16 level gray level and 8 pulses in 256 level gray level, which greatly reduces the serial transmission frequency.
with this method of decentralized control of LED gray level, 256 level gray level control can be easily realized.

LED displays are classified in various ways.
generally, they are classified as follows:
① indoor according to the use environment, Outdoor and semi outdoor
the indoor screen area is generally less than 1 square meter to more than 10 square meters, and the point density is high, so it can be used in non direct sunlight or lighting environment,When the viewing distance is beyond a few meters, the screen body does not have the ability of sealing and waterproof.

the outdoor screen area generally ranges from several square meters to dozens or even hundreds of square meters.
the point density is sparse (mostly 1000-4000 points per square meter), the luminous brightness is 3000-6000cd / square meter (different orientation, different brightness requirements), and it can be used in direct sunlight conditions, and the viewing distance is beyond tens of meters, The screen body has good wind, rain and lightning protection ability.

the semi outdoor screen is between outdoor and indoor.
it has high luminous brightness and can be used outdoors without direct sunlight. The screen body is sealed to a certain extent, generally under the eaves or in the window.

② it is divided into monochrome.
dual primary colors according to color, Tricolor (full color)
monochrome refers to the light-emitting material with only one color on the display screen, most of which are single red. In some special occasions, it can also be yellow green (such as funeral home).

the dual primary color screen is generally composed of red and yellow green light-emitting materials.

tricolor screen is divided into full color.
it is composed of red, yellow green (wavelength 570nm), yellow green (wavelength 570nm), yellow green (wavelength 570nm), yellow green (wavelength 570nm), It is composed of red, pure green (wavelength 525nm) and blue.

③ according to the control or use mode, it can be divided into synchronous mode and asynchronous mode.
synchronous mode means that the working mode of LED display screen is basically the same as that of computer monitor.
it maps the image on computer monitor in real time with an update rate of at least 30 fields / second, Generally, it has the ability of multi gray color display, which can achieve the effect of multimedia advertising.
this paper introduces the application of multi gray color display technology in advertising
Asynchronous mode means that the LED screen has the ability to store and play automatically.
the edited text and grayscale free pictures on the PC are transmitted to the LED screen through the serial port or other network interfaces, and then automatically played off-line by the LED screen. Generally, it has no multi grayscale display ability, and is mainly used to display text information, It can connect multiple screens.

④ it can be divided by pixel density or pixel diameter
because the specifications of LED dot matrix modules used in indoor screens are relatively uniform, the pixel diameter of modules are generally divided into: ∮ 3.0mm 60000 pixels / m2
∮ 3.75mm 44000 pixels / m2
∮ 5.0mm 17000 pixels / m2
the pixel diameter and pixel spacing of outdoor screens are not available at present There is a very unified standard.
according to the number of pixels per square meter, there are about 1024, 1600, 2000, 2500, 4096 and other specifications
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