“This design mainly realizes that the LED Display screen changes its brightness with the ambient light, so as to avoid unclear Display during the day or dazzling due to being too bright at night; and communicate with the host computer. The system consists of MSP430F169 single-chip microcomputer, 16*32 common anode LED dot matrix display screen, row driver circuit, column driver circuit, ambient light adaptive circuit, digital switch voltage regulator circuit, DC voltage regulator circuit and so on. The data is sent in parallel in columns and the content of the LED dot matrix screen is displayed in the way of line scanning; the single-chip microcomputer realizes the automatic adjustment of the power supply voltage of the LED dot matrix screen through the digital switch voltage regulator circuit, so that the screen brightness automatically changes with the ambient light.
This design mainly realizes that the LED display screen changes its brightness with the ambient light, so as to avoid unclear display during the day or dazzling due to being too bright at night; and communicate with the host computer. The system consists of MSP430F169 single-chip microcomputer, 16*32 common anode LED dot matrix display screen, row driver circuit, column driver circuit, ambient light adaptive circuit, digital switch voltage regulator circuit, DC voltage regulator circuit and so on. The data is sent in parallel in columns and the content of the LED dot matrix screen is displayed in the way of line scanning; the single-chip microcomputer realizes the automatic adjustment of the power supply voltage of the LED dot matrix screen through the digital switch voltage regulator circuit, so that the screen brightness automatically changes with the ambient light.
LED dot matrix display is very common in life, which brings a lot of convenience to our life. Since the LED display brightness cannot be changed with the external ambient light, there is a problem that the display is unclear during the day or dazzling because it is too bright at night. If you can control its brightness, you can not only save energy, but also make the display screen clearer. This design introduces the ambient light adaptive LED display screen controlled by the MSP430F169 single-chip microcomputer system. The screen can automatically adjust the brightness according to the intensity of the ambient light, so as to avoid unclear display during the day or dazzling due to being too bright at night, and communicate with the host computer. communication. LED display screens are widely used in large-scale places such as shopping malls, entertainment venues, factories, schools, etc., and their research has practical value and good development prospects.
1 System structure block diagram
The system is mainly composed of MSP430F169 single-chip microcomputer system, step-down rectifier filter circuit, voltage regulator circuit, switch voltage regulator circuit, RS485 communication interface circuit, LED row and column drive circuit, ambient light detection circuit, LED display screen, upper PC computer, etc.
The structure diagram of this system is shown in Figure 1.
Figure 1 System structure frame
2 Basic principles
This design uses column to send data and row scan to realize LED display text or image. The method is combined with the hardware circuit to achieve the purpose of relatively uniform overall brightness of the display screen.
Using the sensitivity of photoresistor to ambient light, the changes of ambient light are collected, converted into electrical signals and sent to the single chip microcomputer.
A switch voltage regulator circuit is added between the single chip microcomputer and the LED display screen to realize the brightness adjustment of the display screen by the single chip computer. The adjusted PWM wave is used to control the switching voltage regulator circuit, so as to adjust the input voltage of the display screen, and finally realize the brightness control of the display screen.
3 Circuit Design
3. 1 MSP430F169 microcontroller system circuit
MSP430F169 microcontroller system circuit is the main control circuit. MSP430 series microcontroller is a 16-bit ultra-low power consumption mixed signal processor (Mixed Signal Processor) introduced to the market by Texas Instruments in 1996. The microcontroller has the following characteristics: Ultra-low power consumption, powerful processing Capability, rich on-chip peripheral modules, convenient and efficient development environment, and stable system operation. Because the MSP430F169 microcontroller integrates 12-bit A/D and D/A and the program for generating PWM wave is simple, no additional A/D circuit, D/A circuit and PWM wave generation circuit are needed, which simplifies the design of peripheral circuits , so this design selects MSP430F169 microcontroller, as shown in Figure 2.
Figure 2 Single chip system circuit
3.2 Buck rectifier filter circuit
After the 220 V alternating current is stepped down by the transformer, it is rectified by the rectifier bridge into a one-way pulsating voltage, and filtered by a capacitor into a small pulsating DC voltage. Since the DC voltage fluctuates with the grid voltage, if it is directly used as the power supply of the display screen, it will cause the display screen to flicker, so a voltage stabilizing circuit is used to stabilize it. 7805 is a dedicated three-terminal voltage regulator device, the input voltage after filtering (U≈9 V), the regulated output voltage (VCC) of 5 V for IPIC68273 use. The power supply voltage range of MSP430 series MCU is 1.8 ~ 3.6V, so the 5V voltage is stepped down by 0.7V (VSS) through a diode, which is used by the MCU system. It has been proved by long-term experiments that the single-chip microcomputer system can operate safely and reliably for a long time.
Figure 3 Buck rectifier filter circuit
3.3 Switch voltage regulator circuit
Two NPN transistors are used to form an inverting amplifier circuit to realize the chopper amplification of the PMOS switch, and the PWM wave is used to adjust the on-time of the PMOS switch to control the average voltage that VDD acts on the display screen, thereby controlling the brightness of the LED.
In order to prevent the fluctuation of the network voltage from affecting the display screen, a voltage divider measurement circuit is formed with fixed-value resistors R4 and R5 to accurately measure and control the voltage to achieve a stable and reliable display voltage, as shown in Figure 4.
Figure 4 Switching voltage regulator circuit
3.4 Row and column drive circuit
The LED display driver adopts the dynamic scanning driving mode, and only one row of LEDs can be lit at a time (the common anode type LED display dot matrix module). Since the TPIC6B273 is output in the OC gate mode, it is especially suitable to choose the common anode type LED display dot matrix module. Do the basic display unit. The line scanning circuit adopts 4/16 decoder CD4515 to decode and form line scanning pulse, and directly form line scanning signal after amplification, as shown in Figure 5.
Figure 5 Monochromatic common anode LED driver circuit
3.5 Ambient light detection circuit
Photoresistors are optoelectronic devices made of semiconductor materials, which have no polarity. The ambient light detection circuit is composed of photoresistor RG and fixed-value resistor R1, and the change of ambient light intensity is detected by measuring the change of voltage across R1, as shown in Figure 6.
Figure 6 Ambient light detection circuit
3. 6 RS485 communication interface circuit
MAX485 is a differential balanced transceiver for RS485 communication, produced by MAXIM Company. The chip contains a driver and a receiver, which is suitable for half-duplex communication. The host PC communicates with the microcontroller system through the communication interface circuit, as shown in Figure 7.
Figure 7 RS485 communication interface circuit
Ambient light adaptive LED display control program using IAR software, using C language to write, as shown in Figure 8.
Figure 8 Environment adaptive LED display program frame
The ambient light adaptive LED display discussed in this paper can change the brightness with the change of ambient light, so that the display will not be too dazzling or too dark, and at the same time effectively control the energy consumption, so that the power efficiency can reach more than 75%, The system achieves the expected experimental purpose. Through improvement, it can be applied to the display of schools, shopping malls, factories, stock exchanges and other places. The research of this system has extremely practical value and good development prospects.
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