At present, the production of dual-color light-emitting diode (LED) display screens is relatively large, and its technology is relatively mature. The structure and principle of the display screens manufactured by various companies are basically similar, and some professionally produce and display multimedia cards. Therefore, improving the display screen's technical performance and reducing costs is the key to competition among various companies. Currently, the price of LED display screens sold on the market is basically the same, but the quality of the screens produced by different companies is different. The reasons are various. The main ones are: 1 LED display module quality, brightness, brightness uniformity, packaging Such as technology; 2 data transmission methods, anti-jamming capability; 3 display scanning circuit current multi-point adjustment, control of each point of the current. After a multi-point adjustment, the display screen not only has better uniformity, but also has better display brightness and color effects. The dedicated display scanning circuit has a better display effect, but the price is relatively expensive.
Now, LED display screens sold on the market are produced by many companies using the same design technology, methods, and display modules, but their performance is quite different. The difference in the color matching results in a large difference in the effect of the image; the scanning frequency and the operating current of the module affect not only the brightness but also the service life. Therefore, the correct determination of various technical parameters is the key to the manufacture of display screens. It can also be said to be the embodiment of display screen technology experience.
2 shows the scanning principle
The control structure of LED display screens manufactured by various companies is different, but the display scanning circuit of the display screen is basically the same. Dual-color LED display display scanning circuit shown in Figure 1. In Fig. 1, IC1, IC2 is the data latch circuit 74HC595, latches the red, green data respectively, their performance is: 1 serial input 8-bit parallel output; 2 data latch, data clear function; 3 output has Stronger driving ability. Resistors RPB1, RPB2 are current-limiting resistors that select their values ​​based on the color and brightness of the module. ML1 is a two-color LED display module with a total of 8 rows of X8 columns = 64 LEDs, of which 8 pins are red signal input terminals, 8 pins are green signal input terminals, and 8 pins are row control input terminals. 24 pins. Transistors Q0, Q2, ... Q7 are line strobe and drive. IC3 is a 3-8 address decoding circuit 74HC138, and 8 gate outputs control the corresponding lines, respectively. The circuit in the figure is the principle circuit of the display. The data transmission mode is that the data transmission and the line signal are performed asynchronously: First, transmit 8-bit red and green color data to the circuit IC1 and IC2 and latch the data, and then transfer the line control. The signal lights up one line of LEDs, and then the above operation is repeated, except that the line signal moves to the next line and then to the eighth line, which is a complete scanning process.
The design requirements of the display scan board have relatively low production costs. Therefore, many companies design double-sided circuit boards, which can save about one-third of the board cost. In the corresponding size range of the display module, it is necessary to place all the components in the figure, and the corresponding double-layer printed circuit board is difficult to compile, so the IC1 circuit is particularly suitable for driving the LED display module of the dot matrix scanning principle. The display scanning circuit transmits data in a serial manner, which not only saves the position of the circuit board but also is suitable for data transmission between the display screen and the computer.
3 working status analysis
The principle of the display scanning circuit is a dynamic scanning method, and its operating current cannot be statically measured. Therefore, to calculate the operating current, it is necessary to analyze dynamic parameters. Figure 2 is a working circuit diagram of an LED. The circuit Q8 is the driving circuit, positive termination power supply, control termination 74HC138 output, the output termination LED light-emitting diode D, connected with the current limiting resistor, resistor connected 74HC595 data output. LED lighting method is: control 74HC138 chip select signal is invalid, is not strobe, after 74HC595 output level, low-level lit signal, and then gate 74HC138, control output strobe signal, at this time, there is current I0 is output from Q8, flows through D, R1, and enters the data output of the 74HC595.
In the figure, Vab is the voltage applied to the LED. The voltage drop of the red and green high-brightness LEDs is about 2 to 3 V. Vbc is the voltage across the current limiting resistor. The value of the current limiting resistor is adjusted. It is possible to change the operating current I0 of the circuit. When the resistance R1 = 0, the circuit relies on the output active resistance of the 74HC595 as the current limiting resistor.
In the scanning circuit, it can be seen that the circuit structure is relatively simple, and adjusting the working parameters of each part reasonably can make the circuit work in the best state. When selecting a circuit, it is also necessary to accurately grasp the performance of each company's circuit and the difference in the technical parameters between them. Different types of device technology parameters are also different, Table 1 is the 74HC595 technical parameters, the table gives the Texas Instruments, ST, PhiliPs 74HC595 technical parameters. In the table, it can be seen that the circuits produced by different companies are slightly different. Therefore, a display screen should use the circuit components of the same company as far as possible so as not to affect the display effect of the display screen due to the difference in parameters.
In Table 1, Iik is the input spike current, Iok is the output spike current, I0 is the continuous output current, Vcc is the highest supply voltage, fmax is the maximum operating frequency at 25°C (works with the load capacitance, The frequency is also different. ta is the operating temperature. The components in the table are SN74HC595, M74HC595, and 74HC595. The corresponding company is Texas Instruments, ST, and Philips.
4 Adjustment of Brightness and Color 4.1 Adjustment of Brightness and Color
When manufacturing a large screen, first select the LED or display module according to the brightness index, and secondly determine which color is used as a reference based on the brightness ratio of the selected product's red, green, and blue colors. Generally, a low brightness ratio is used as a reference. Brightness reference, when one of the reference has reached maximum brightness, adjust the other (two-color) or two (full-color). When the display screen is double-colored, the operating current of the red diode is adjusted based on green in most cases. Generally, the operating current is reduced to balance the color yellow as an adjustment criterion, so that the brightness of the entire display screen is reduced. Adjusting the color of the display to the best balance will reduce the brightness of the screen. If the display screen achieves the maximum brightness for each color in order to achieve the brightness requirement, then the color balance is lost. For example, the yellow color of the two-color screen is reddish or greenish.
TTL output low level is about 0.4V. If it is used as a sinking current input, the normal maximum sinking current is 35mA. When this current is exceeded, the output low level rises. As the current increases, the output level rises continuously. High, that is, the circuit with output voltage greater than 0.4V still works normally. In the display scanning circuit, the working current of 20mA can meet the requirements of controlling red, because the brightness of the red LED is relatively high; the working current of the green LED is higher than 20mA, the current is about 30-50mA, at this time, the output of the 74HC595 The voltage also increases because the 74HC595 has an output resistance and it is nonlinear.
4.2 Scan frequency adjustment
The scanning circuit works in a dynamic scan, static drive mode. The scanning frequency of the display is limited by the structure of the display module. Each module has 8×8 LEDs. The entire row of display module data is connected in series, and the time for updating the data is longer. When the scanning frequency is 100 Hz, the brightness of the entire screen is It will decrease. If you reduce the scanning frequency of the screen, the display brightness decreases. Experiments show that the sweeping frequency has little relationship with color.
5 Conclusion
After the design and analysis of the two-color LED display screen, the technical parameters of the circuit are effectively adjusted and a better display effect is obtained. Facts have proved that a reasonable analysis of the circuit's technical characteristics, so that the circuit is in the best working condition, will be able to achieve the desired display. Choose different company's 74HC595 circuit, the display effect is different, in a display screen, try to choose the same circuit, avoid to influence the display effect by the circuit parameter differently.
The important condition for the design and manufacture of the display screen is the production cost. Therefore, in the process of selecting the components, if the requirements are satisfied, the low price is selected. Based on the technical design and analysis of this article, the circuit of the display screen is adjusted to the best effect, and a display with a high performance/price ratio is manufactured, which has a market competitiveness.
Alumnium rolled coil :
Item
Typical Alloys
Typical Temper
Specification (mm)
Thickness
Width
3003/3105
Hx2/Hx4/Hx6
0.3-0.5
1200-1800
5052/5005
O/Hx2/Hx4/Hx6
0.3-0.5
1200-1800
5754/5182/5083
O/Hx2/Hx4/Hx6
0.3-0.5
1200-1800
6061/6063
O/T4/T6
0.5-3.5
1200-1800
2024/7075
O/T3/T4/T6/T8
0.8-8.0
1200-1800
5754
F/H112
3.0-8.0
1200-2800
5083
F/H112
3.0-8.0
1200-2800
6061/6082/6063
F/H112
3.0-8.0
1200-2800
2024
O/T3/T4/T8
0.8-8.0
1200-1800
7075
O/T6
0.8-8.0
1200-1800
0.5-3.5
0.5-3.5
0.5-3.5
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