FLEXIBLE LED SCREEN

What is the difference between LED flexible display and an ordinary LED display?

The led flexible display screen is mainly composed of led soft module unit boards-the reason why it has a certain bending amplitude characteristic is mainly related to the raw materials of the PCB circuit board used in the product. Conventional led display unit boards use ordinary PCB circuit boards, which are generally called rigid circuit boards. The intuitive feeling is that it is a rigid board that cannot be bent, similar to the kind of rigid plastic.

The flexible LED display uses a PCB circuit board made of a flexible insulating material, so it not only has the characteristics of bending but also has stronger dust and heat resistance than ordinary LED displays. Therefore, The price will be slightly higher than that of ordinary LED displays.

The thickness of the PCB circuit board used in the conventional LED display screen is generally 1mm, 1.2mm, 1.6mm, 2.0mm, etc. In most cases, the thicker the PCB thickness, the more reliable the stability of the LED screen product, because it is a rigid board Therefore, under normal circumstances, most of the daily shapes seen are squares, rectangles, diamonds, etc., spherical and cylindrical display screens cannot be realized, and arc-shaped design LED display application solutions are rarely used go with.

However, with the development of the industry in recent years, ordinary LED displays can no longer meet the needs of the industry. The emergence of special-shaped screen modules has enabled LED advertising screens to achieve various shapes. For a long time, LED special-shaped screens have been realized by traditional rectangular flat module assembly or wrapping, such as common curved screens and circular screens.

However, when the arc of the display screen is small and the display form is more complex, the splicing and hemming of special-shaped modules will not solve the seam and flatness, resulting in inconsistent display, mosaic, and other issues, making the display screen overall The effect is not good, and to solve this problem, a “flexible display” came into being.

New features of the LED display industry-flexible LED display

It is understood that the many advantages of LED flexible displays are beyond the reach of ordinary curved displays. The connection surface of the soft module LED special-shaped screen is different from the traditional display. The traditional PCB board is made of glass fiber material, and the flexible module is equipped with a high-strength strong magnetic core connection device, which is made of a flexible insulating substrate. FPC circuit boards, masks, and bottom shells are made of high-temperature, bending-resistant silicone materials, with high-strength compression and distortion resistance, which can perfectly solve all kinds of “turning corners and corners” installation difficulties.

Most of the installation methods are magnetic column suction, and the “one-install-ready” installation method is adopted, that is, the shape is customized according to the customer’s needs, and then it can be directly adsorbed to achieve one-step installation. The magnetic installation method is as simple as the conventional indoor screen installation method, and the cabinet connection lines are all connected by the speed connector, which is firm and reliable.

Today, when products in the industry are homogenized and becoming more and more serious, most small and medium-sized display companies have to find ways to survive and make unique products, and flexible displays can be arbitrarily shaped because of their good ductility. This is a “weapon” for realizing various creative displays.

In the past few years, there are not a few LED advertising screen manufacturers that have developed flexible display screens, but LED flexible screens have been in a tepid state. However, with the development of creative displays, the development potential of flexible displays should not be underestimated. In particular, with the advancement and prosperity of my country’s cultural undertakings and the prevalence of cultural performance activities, the application demand for LED flexible screens will greatly increase, and the market share will also soar. The prospects for flexible display screens are bright.

How much does a flexible LED display cost?

Flexible LED displays are usually used in special-shaped display occasions. The entire product includes visual design, structural design, structural construction, and LED display systems. Therefore, when evaluating the price of such products, the cost of the LED display system can be evaluated on the premise of considering the feasibility of the product.

At present, LED flexible display screens are mainly used in science and technology museums, museums, art galleries, and other occasions. Combining the atmosphere of the scene and the special-shaped flexible LED display screens cooperate to create a scientific and technological design. At present, flexible LED display screens still have some technical shortcomings, and it is difficult to deal with the complex outdoor environment. Therefore, the current flexible LED display screens are mainly used indoors and half-indoors.

Without considering the design cost, structure cost, labor cost, frame, and other system accessories. The LED flexible screen can calculate the price from the following aspects

(1) Module spacing

Take P2 and P2.5 as examples to show their prices. Currently, on the market, the price of a P2 flexible LED display is generally 8000-9500RMB/m²; the price of a P2.5 flexible LED display is generally 5000-6500RMB/m², the larger the spacing, the lower the price. The price here is only calculated based on the price of the flexible screen module for the entire screen, excluding the frame, mounting bracket, and other system accessories. And when the market price fluctuates, the overall cost fluctuates greatly.

(2) Module chip brand

Jingtai, National Star, San’an, Jingtai, Xinguangtai, etc. Use different brands, and the price will have a more obvious gap. This is the same as the mobile phones we use in our daily life, which are worth 7000 and 8000, as well as 700 and 800. Module chips of different brands have different workmanship and materials.

(3) Refresh frequency

The refresh frequency is also called the vertical frequency, that is, the vertical scanning frequency of the display. It refers to the number of times the display can display images per second, and the unit is hertz (Hz). The higher the field frequency, the more times the image is refreshed, the smaller the flicker of the image display, and the higher the picture quality.

Note that the so-called “refresh times” here and the “picture frame number (usually expressed in FPS)” that we usually say when describing game speed are two completely different concepts. The latter refers to the scanning frequency of the electron gun of the kinescope displayed by the dynamic image processed by the computer per second.

The lamp beads and chips used by different modules are different, and the refresh rate is also different. The effect displayed by the high refresh rate is better than the effect displayed by the low refresh rate, so the corresponding price of the high refresh rate will be more expensive. Some.

(4) Other expenses

The above three expenses accounted for the most costly, but some other expenses are also not small. For example, the cylindrical screen of a large underground shopping mall was installed by JYLED in Thailand before, from site surveys, and design drawings to construction, and then To the end, it took 5 months before and after. The cost of a worker for a day is as high as 700 RMB a day, which is not a small expense when combined.

The LED flexible screen is a kind of the latest ultra-thin flexible LED display, including the LED display unit and the signal processing unit, power supply unit and circuit connected to it through the circuit board are all flexible.

With its beautiful embossed lines and various unique shapes, it can give people dynamic beauty under the change of colors.

We often see a lot of cylindrical screens when we go shopping in the mall, as well as some other LED screens of different shapes. The screens are filled with beautiful video content, including the promotion of clothing brands, cosmetics, and Products such as shoes, etc., these video content can easily attract our attention, and let us remember the names of certain brands imperceptibly so that when we buy related products in the mall, we will Unconsciously choose the brand product that I have seen in the LED display. This is why businesses prefer to use LED displays to advertise their products or services and LED cylindrical screens and other LED displays of various shapes are more suitable for decoration in shopping malls, which is also a combination of beautiful designs.

With the development of science and technology, people are no longer satisfied with flat display screens and begin to pursue various artistic styles, and special-shaped screens have also entered the public’s field of vision. As a soft module product for making special-shaped screens, they are even more popular with the public. LED The company has always occupied a larger market in soft module products.

The flexible display screen built for the command center this time has a cylindrical shape, which fits the architectural design of the command center. The cylindrical screen adopts a special design circuit layout PCB multi-layer process design, and the best bending angle is >135°.

Principle of flexible LED display drive circuit

The Design of a Conventional Driving Circuit for an LED Display Screen

The design of the LED display drive circuit, in line with the control system used, is usually divided into two categories: dynamic scanning drive and static latch drive. The following takes the design of a dynamic scanning drive circuit as an example for analysis:

The dynamic scanning drive mode means that the 4 rows, 8 rows, 16 rows, and other n rows of light-emitting diodes on the display screen share a set of column drive registers. Through the time-sharing work of the row drive tubes, the lighting time of each row of L ED accounts for 1 n of the total time, as long as the refresh rate of each line is greater than 50 Hz, people can see a complete text or picture by using the visual persistence effect of the human eye.

The design of conventional drive circuit generally uses serial-in and parallel-out general integrated circuit chips such as 74HC595 or MC14094 as column data latches, using low-power NPN transistors such as 8050 as column drive, and Darlington transistors such as TIP127 as column data Line scan tube, its circuit is shown in Figure 1.

If a monochrome dot matrix with 16 rows×64 columns is used as a basic unit, 8 pieces of 74HC595, 64 pieces of 8050, and 16-row scanning tubes are required. The working principle is:

Connect eight 74HC595 cascades and share a serial clock CL K and data latch signal STR. When the data to be displayed in the first line passes 8×8 = 64 CL K clocks, all will be moved into 74HC595. At this time, a data latch signal STR is generated to latch the data in the subsequent latch of 74HC595.

The 8050 corresponding to each output bit will be in saturation conduction or cut-off; at the same time, the line scan control circuit generates a signal to turn on the first line of scan tubes, which is equivalent to connecting the positive ends of the first line of LEDs to a high level, obviously the first line of LED tubes The on or off depends on the signal latched in the 74HC595; while the L ED tube in the first line is lit, move the data to be displayed in the second line in the 74HC595, and then latch it and control it by the line scan at the same time The circuit turns off the scan tube of the first row and turns on the second row so that the LED tube of the second row is lit…and so on, when the 16th row is scanned, it will return to the first row, as long as the scanning speed is high enough, A complete text or image can be formed, and the working sequence is shown in Figure 2.

The defects of the conventional drive circuit

Although the design structure of the conventional drive circuit is relatively simple, it has two defects:

(1) When a certain row of row driving tubes is valid, the lighting current of all LEDs corresponding to the row will flow through the row of driving scanning tubes, and the number of LED tubes lit in a row depends on the display. The text or graphics are constantly changing, so the current flowing in the line scanning tube has a large change, which will change the pressure drop of the tube;

(2) The change in current caused by the number of lit L ED tubes will also affect the fluctuation of the power supply voltage value, which will affect the voltage at both ends of the first row of L ED tubes, making it continuous with different display texts or graphics. Fluctuations affect the uniformity of the brightness of the entire display.

Therefore, the author designed a column constant current drive circuit. This method can eliminate the fluctuation of the power supply voltage and the line scan.

The influence of the change of tube pressure drops on the brightness of the LED display screen. Figure 3 shows the relationship between the relative brightness of a light-emitting diode and the current flowing in it. It can be seen from the curve that: within a certain forward current working range, its luminous brightness is approximately proportional to the current flowing in it.

It is a current-driven device. Therefore, as long as the current flowing in each LED light-emitting tube can be guaranteed to be constant, the brightness can be guaranteed to be consistent.

It can be seen from the working principle of the conventional drive circuit that because the row and column drive tubes are all working in a saturated state,

The magnitude of its current cannot be controlled, so the fluctuation of the external power supply voltage, the change of the voltage drop of the horizontal scanning drive tube, etc., directly affects the current flowing in the LED light-emitting tube, that is, it changes its display brightness. If the column drive tube is changed from the saturation state to the linear amplification state and becomes a constant current drive, the unevenness of the display screen brightness caused by the above factors can be eliminated.

The column constant current drive circuit is shown in Figure 4.

When the power supply voltage VDD is stable, the high-level output voltage V of 74HC595 is also very stable. For example, when the power supply voltage VDD is 6V, V = 5.9V. So when a certain bit of 74HC595 output is high, the L ED of its corresponding column will be lit, and the current flowing in it is approximate:

As long as the values of R 1, R 2, and R 3 are selected reasonably, the current flowing in the LED can be guaranteed to be stable, and the LED light-emitting diode can work in the best state of forward current and corresponding luminous brightness.

With this column’s constant current driving mode, it can be achieved that no matter how many LED tubes are lit in a row, although the tube voltage drop of the row driving tube still changes, the power supply voltage VCC can also be changed. The current flowing in the LED light-emitting diode is constant, thus ensuring the uniformity of the brightness of the LED display.

5　Concluding remarks

Compared with the original conventional drive circuit, the LED display constant current drive circuit overcomes the shortcomings of the conventional drive circuit with only minor changes and ensures more perfect performance. The author has passed the actual use of multiple displays All have got the ideal display effect.