LED display on the market now, many companies use the same design techniques, methods, display module production, but relatively large differences in performance. The ratio of different colors, produces the image effect on very different; scanning frequency module operating current not only affects the brightness, and comes to life and other issues. Therefore, the right to determine the technical parameters of manufacturing display can also be said to be the manifestation of the display technology experience.

2 displays scanning principle Enterprise manufacturing

LED display control structure is different, but, basically the same screen display scanning circuit. The dual color LED screen display scanning circuit is shown in Figure 1. In Figure 1, IC1, IC2 is data latch circuitry 74HC595, respectively latched red, green data, and their performance is: (1) serial input 8-bit parallel output; ② data latch, data clear function; ③ output has strong drive capability. Resistance RPB1, RPB2 current limiting resistor based on the color and brightness of the module to their values. ML1 is a two-color LED display module, a total of eight rows X8 column = 64 LEDs, wherein the eight-pin input red signal, the eight pin is an input end of the green signal, the eight pin is an input terminal of the line control, Total 24 pin. Transistor Q0 Q2 ... Q7 line strobe driver role. IC3 3-8 address decoding circuit 74HC138 8 strobe output of control corresponding row. The circuit in the figure is a schematic circuit of the display, the data transfer method is a data transmission line signal asynchronous: Firstly, the simultaneous transmission of 8-bit red, green color data to circuit IC1, IC2 and the data is latched, and then sent farewell control signal lit line LED, followed by repeating the above operation, only the row signal to the next line sequentially to the eighth behavior ended, that is a complete scanning process.

Display the scan circuit board design requirements with relatively low production costs, a result, many companies are designed as a double-sided board, so you can save about a third of the cost of board. In the appropriate size range of the display module, it is necessary to be placed all the components of the above figure, the preparation of its corresponding double-layer printed circuit board having a large degree of difficulty, so IC1 circuit is particularly suitable for the dot scanning principle of the LED display module drive. The display scan circuit are using serial transmission of data, both to save the position of the board, and for data transfer between the display and the computer.

3 state analysis

Scan circuit principle is dynamic scan, not static measured current, therefore, to calculate the operating current, it is necessary to analyze the dynamic parameters. Figure 2 is the work circuit diagram of an LED. Circuit Q8 driver circuit, is terminating power control termination 74HC138 the LED light output, output termination diode D, and current limiting resistor connected to the data output terminal of the resistor connected 74HC595. LED lit: invalid control 74HC138 chip-select signal is not gated, after 74HC595 output level, low-level lighting signal strobe 74HC138 control output elect communication No. At this time, the current I0 from the output Q8, flowing through D, R1, enter 74HC595 data output terminal.

Figure, Vab is the voltage on the LED, red, green, high-brightness light-emitting diode voltage drop of about 2 ~~ 3V Vbc is added to the current limiting resistor on both ends voltage, by adjusting the values ​​in the current-limiting resistor, it can change the working current I0 of the circuit, when the resistance R1 = 0, the circuit relies on the the 74HC595 output active resistance as the current limiting resistor. In the scanning circuit can be seen, the circuit configuration is relatively simple, and reasonable to adjust the operating parameters of various parts of the circuit to work in the best condition. Selection circuit, but also the accurate knowledge of the various circuit performance, as well as between The technical parameters of the difference. Technical parameters of different types of devices also differ, Table 1 74HC595 technical parameters, the the company TexasInstru-ments, ST, PhiliPs 74HC595 technology parameters are given in the table. Different companies producing slightly different circuit can be seen in the table, therefore, a display as much as possible to use the same circuit devices, so as not to affect the screen display effect due to the difference of the parameters.

In Table 1, Iik input peak pulse current, Iok output spike pulse current, I0 is a continuous output current, Vcc is the highest supply voltage, fmax at 25 ℃ maximum operating frequency (with a different operating frequency of the load capacitance) ta to working temperature. The table components SN74HC595, M74HC595, 74HC595 corresponding company TexasInstruments, ST, Philips.

Brightness and color adjustment Adjustment of the brightness and color in

4.1 Manufacturing large screen

, we must first be selected in accordance with the brightness index LED display module, followed by selected Product red, green and blue color luminance ratio to determine what kind of color as a reference, is generally low brightness ratio as a brightness reference, when the reference a has reached the maximum brightness, adjust another (two-color) or two (full color). The display screen is, in most cases when the two-color green benchmark, adjusting the red diode current. Generally lower operating current adjustment to balance color yellow the standard , it is necessary to reduce the the entire display screen brightness. Display color adjustment to best balance the screen brightness will reduce. If the display screen requirements in order to achieve the brightness and color to achieve the maximum brightness, then you lose the balance of colors, such as: the yellow color of the two-color screen reddish or greenish.

TTL output low of about 0.4V, as a current sink input, normal maximum sink current of 35mA, when more than this current, the output low rise, with the significant increase in the current, the output low level is rising, and that the output voltage is greater than 0.4V circuit is still working properly. Display scanning circuit, operating current of 20mA red to meet the control requirements, because of the relatively high brightness of the red LED; Green LED operating current higher than 20mA current of about 30 ~ 50mA, 74HC595 output the voltage should be increased, the reason is the 74HC595 output resistance, but also the non-linear changes.

4.2 scanning frequency adjustment

Scanning circuit with dynamic scanning, static driven manner. The scanning frequency of the display is limited by the structure of the display module, each module has 8 × 8 LED module row of the entire display screen all the data series and the data updated time is relatively long, when the scanning frequency of 100Hz, the brightness of the entire screen. will be reduced. If the scanning frequency of the screen is reduced, display brightness decreases. The experiment proved that the relationship between a scanning frequency of color is relatively small.

5 Conclusion

Through the design and analysis of two-color LED display, adjust the technical parameters of the circuit, better display effect.

Proved reasonable analysis of the technical characteristics of the circuit, the circuit is in the best working condition, will be able to achieve the desired display. 74HC595 circuit, choosing a different company and different effect, in a display, try to select the same circuit, to avoid the different effects of the circuit parameters display.

Display design and manufacture of an important condition is a production cost, therefore, in the choice of components in the process, in the case to meet the requirements, select the price is low. On the basis of the design and analysis of this technology, the display circuit is adjusted to the best results, create a display of performance / price ratio is high, the market competitiveness.