Development of cockpit display backlight drive circuit

In order to enable the users in the cockpit to obtain accurate and reliable information at any time, the display used in the cockpit must have high reliability. At the same time, because the pilots have to operate the instruments with both hands, and in order to improve the readability of the cockpit display and relieve visual fatigue, the light intensity of the display should also be able to be automatically and manually adjusted according to the surrounding environment and user needs, so as to avoid the user not being able to see the display content clearly when the ambient light is strong, or feeling glare when the ambient light is dark. The backlight brightness of the LCD directly determines the average brightness of the display, and the brightness of the display is adjusted by adjusting the backlight brightness of the LCD.

The author uses the domestic STC12C5624AD single-chip microcomputer to design an LCD screen backlight module drive and adjustment circuit, which can accurately measure the illumination of the ambient light and automatically and manually adjust the backlight brightness according to the ambient light.

1 Principle of display backlight adjustment

Due to the particularity of the aircraft cockpit, there are higher requirements for the reliability, accuracy and intelligence of the cockpit display. It is necessary to automatically adjust its own backlight brightness according to the changes in the ambient light to make it more comfortable for the human eye. The screen brightness can also be manually adjusted to meet the user's requirements. Through research, we can get the relationship between the ambient light xt and the brightness yt of the backlight screen.
When the ambient light illuminance is xt, the automatically adjusted brightness is:
yt=kxt+b (1)
where b is the minimum brightness required for the driver to see the information displayed on the screen clearly when the ambient light is 0; k is a proportional constant that can be modified according to the user's habits, reflecting the ratio of the brightness required for humans to see the information on the screen clearly in a bright environment to the ambient light.
When the user is not satisfied with the automatically adjusted brightness, he manually adjusts the brightness to yt' and calls the following calculation formula:

The obtained b' and k' are checked and replaced with b and k in formula (1) to obtain a new calculation formula. In order to prevent the driver from over-adjusting, bmin and bmax are set. Only when b' is between these two values, b' is used to replace b. At the same time, the system sets the initial value bs when leaving the factory. The user can restore b to bs by pressing a button. Similarly, kmin, kmax and ks are also set for k.

2 Circuit
The circuit schematic diagram of the LCD backlight module is shown in Figure 1, which mainly includes two PN23CV silicon photocells, a single-chip microcomputer, a key circuit, an LED backlight drive circuit with DD313 as the core, and auxiliary circuits. The PN23CV silicon photocell senses the illumination of the ambient light and the brightness of the LCD backlight, and transmits the signal to the single-chip microcomputer after amplification. The single-chip microcomputer calculates the theoretical brightness of the LCD backlight, compares it with the actual brightness of the LCD backlight, and determines whether the brightness of the LCD backlight needs to be adjusted. If necessary, a signal is sent to the backlight drive circuit with DD313 as the core, where the 13, 11, and 6 pins of DD313 are connected to the red, green, and blue LEDs respectively to control the brightness of the LEDs, thereby adjusting the actual brightness of the LCD backlight module. When the user is not satisfied with the result of the automatic adjustment, a signal can be sent to the single-chip microcomputer through the key circuit, and the single-chip microcomputer determines whether the brightness of the LCD backlight needs to be adjusted. After the brightness of the LCD backlight is adjusted, a delay is made to modify the b and k values. The circuit program flow chart is shown in Figures 2 and 3.

When the user manually adjusts the display brightness, the buttons SW1 and SW2 are connected to the interrupt 0 and interrupt 1 ports of the microcontroller STC12C5624AD respectively. When the button is pressed, the output voltage is VCC, and when the button is released, the output voltage is 0. Pressing SW1 means that the user requires to increase the display brightness, and pressing SW2 means that the user requires to reduce the display brightness. Pressing both SW1 and SW2 means returning to the original factory settings of b and k and bs and ks. SW1 and SW2 are connected to the 8th and 9th pins of the microcontroller respectively.

3 Software Programming

The circuit function is realized by software programming, and the STC single-chip timer and interrupt are used to scan whether there is a key pressed, and then make corresponding processing.




4 Conclusion
This paper uses the 51 single-chip microcomputer with superior performance and mature technology. Through the design of the single-chip microcomputer peripheral circuit and the programming of its timer and interrupt, the manual and automatic adjustment of the cockpit display backlight is realized to achieve a comfortable screen brightness that satisfies the user, making the cockpit display backlight adjustment accurate, reliable, and more intelligent and humane.