1 Introduction

At present, two-color light-emitting diode (LED) display the number of manufacturers producing more, the technology is relatively mature. Various enterprises to create the structure of the screen, similar to the basic principles, some specializing in the production of multi-media display card, the display technology to improve performance and reduce the cost of all is the key to the competitiveness of enterprises. Now, the market price of the LED display is basically the same, but different production and the quality of the display, a variety of reasons, mainly:

① LED display module quality, brightness and brightness uniformity, such as packaging technology;

② data communications transmission, anti-jamming capability;

③ scan showed that the circuit's current multi-point adjustment, the current control every point. After more adjustments not only display uniformity is better, but also the image brightness, better color, special show circuit with a scan showed better results, but the price is relatively expensive.

Now, the market's LED display is a lot of companies use the same design techniques, methods, the display module production, but the performance differences are considerable. The ratio of different colors, images have a great effect on the difference; scanning frequency modules, the current impact of both brightness and involved life. As a result, the right to determine the technical parameters is the key to the display, and display technology can be described as the embodiment of experience.

2 shows the principle of scan

Enterprises of all manufacturing of LED display control structure is different, however, the display screen to scan the same circuit. Two-color LED display screen scanning circuit shown in Figure 1. In Figure 1, IC1, IC2 data latch circuit 74HC595, latches were red, green, their performance is:

① serial input 8-bit parallel output;

② latch data, data clearance;

③ output compared with a strong ability to drive. Resistance RPB1, RPB2 is the current limiting resistor, according to the color and brightness of the module to select their values. ML1 is a two-color LED display module, a total of 8 out X8 line 64 = LED, which, 8-pin input signal is red, 8-pin input signal is green, 8-pin is the control line input, A total of 24 pins. Transistor Q0, Q2, ... Q7 is the gating line, the role of the driver. IC3 is 3-8 address decoding circuitry 74HC138, 8-electoral output respectively control the line. Is a circuit diagram of the principle of display circuits, data transmission and data transfer is asynchronous signal line: First of all, at the same time to send 8-bit red, green color data to the circuit IC1, IC2 and data latches, and then to see them off longer Signal light and his party control of LED, followed by the recurrence of the above operation, the signal is only moved to the next line, in order to conduct only the eighth, that was a complete scanning process.

Scan shows that the circuit board design requirements with relatively low production costs, many enterprises are designed to be double-sided circuit boards, it saves about one-third of the cost of the circuit board. In the corresponding size of the display module, to place the image above all the components, which correspond to the two-tier printed circuit board with the preparation of a greater degree of difficulty, so IC1 circuit particularly suitable for scanning the principle of dot matrix LED display module drivers. Show circuit and are based on the scan serial data transmission, this can save the location of the circuit board, suitable for display and data transfer between computers.

3 analysis of the work of the state

The scan revealed principle of the circuit is a dynamic scan mode, can not be static measurement of its current work, therefore, it is necessary to calculate the current work, it is necessary to analysis of the dynamic parameters. Figure 2 is a work of the LED circuit. Q8 circuit is a driver circuit, the termination is the power to control the output 74HC138 termination, termination output LED light-emitting diode D, and current limiting resistor connected, then resistance 74HC595 output data. LED to light up the way it is: the control of the film selection signals 74HC138 null and void for non-gating, followed by 74HC595 output level, for low light signals, strobe 74HC138, control, communications output election, at this time, current Q8 output from the I0, flowing through D, R1, 74HC595 into the output data.

In the chart, Vab is the increase in the voltage on the LED, red, green high-brightness light-emitting diodes are the drop of about 2 ~ 3V, Vbc is the increase in current-limiting resistors on both ends of the voltage, by regulating the limiting value of resistance, The circuit will be able to change the current I0, when the resistance R1 = 0, relying on the circuit output 74HC595 active resistance as the current limiting resistor.

In the scanning circuit can be seen, the circuit structure is relatively simple, reasonably adjust the various parts of the operating parameters so that the circuit will be able to work in the best condition. In the selection of circuits, the company would also like to have accurate knowledge of the various circuit performance, as well as between the technical parameters of the difference. Different types of devices have different technical parameters, 74HC595 is a list of technical parameters, the table gives the Texas Instru-ments, ST, PhiliPs's 74HC595 technical parameters. In the table can be seen in different companies produced a slightly different circuit, a display as much as possible to use the same device's circuitry, so as not to affect the result of differences in the parameters of the screen display.

In Table 1, Iik input for the peak pulse current, Iok output for peak pulse current, I0 for the continuous output current, Vcc supply voltage for the maximum, fmax said in the 25 ℃ at the time of maximum operating frequency (as the load capacitance, Different frequency), ta for the operating temperature. Table components SN74HC595, M74HC595, 74HC595 counterpart is Texas Instruments, ST, Philips.

4, brightness and color adjustments

4.1 brightness and color adjustments

Manufacture of large-screen, first of all in accordance with the choice of LED indicator light or display module, followed by the choice of products under the red, green and blue color brightness than to determine what kind of benchmark for color, brightness ratio is generally low, as a Benchmark light, when the base has a maximum brightness, another adjustment (color) or two (full color). Dual-color display screen is, in most cases based green, red LED to adjust the current work. Current work is generally lower in order to balance the color yellow for the adjustment of the standard, so it is necessary to reduce the brightness of the display screen as a whole. The color screen to adjust the best balance, it will reduce the brightness of the screen. If the display screen brightness in order to achieve the requirements of each color for maximum brightness, then lost the color balance, such as: two-color screen of the reddish color of yellow, or green side.

TTL output low of about 0. 4V, if the current input for irrigation, poured into the normal maximum current of 35mA, when more than the current, low output rise, with the current increase in the constant low-level output Higher output voltage that is greater than 0. 4V circuit is still working properly. In the scan shows that the circuit, operating current 20mA to meet the requirements of the Red control, because the red LED's high brightness; green LED current work is higher than that of 20mA, current about 30 ~ 50mA, at this time, 74HC595 output Voltage to increase because there are 74HC595 output resistance, but also non-linear change.

4.2 scanning frequency adjustment

Circuit dynamic scan to scan, passive-driven way of working. The frequency scanning display by the display module structure of the restrictions, each of the modules there are 8 × 8 months LED, the entire display module line all the data series, updated data to a longer period, when the scanning frequency of 100Hz, the entire screen brightness Will be lower. If the screen to reduce the frequency scanning, the display brightness to reduce. Experiments have proved that the frequency of sweeping depiction of the relationship between color and relatively small.

5 Conclusion

After a two-color LED display design and analysis, effectively adjust the circuit's technical parameters, to be a better show results. Facts have proved that the rational analysis of the technical characteristics of the circuit, so that the circuit is in the best possible working condition, will be able to achieve the desired results showed. 74HC595's different circuits, different results show that, in a display, as far as possible to choose the same circuit, the circuit to avoid the effects of different parameters show results.

Display design and manufacturing is an important condition for the cost of production, the selection of components, to meet the requirements, choose a low price. In this article, the technical design and analysis on the basis of the display will be best to regulating circuit, to create a performance / price ratio of the screen, with market competitiveness.


Reference link: http://www.17led.com/LEDdisplay/2008-06/99.html