Simple led message scrolling display construction using 8051 Microcontroller |project with circuit diagram

Simple Outdoor LED Message Moving or Scrolling Sign Board Design Project with Circuit diagram| Electronic LED Scroller Generator for outdoor digital signs| Marketable LED sign board with Message scrolling.

In this post, I am going to explain about the project LED Message Scrolling Board with completed circuit diagram and the microcontroller programming. Sometimes this LED scrolling message board is referred as LED Message sign board.

Generally the LED Scroller Message Generator can be implemented with many different logics. Out of two are very famous

1.    LED moving message display board design using decade counters

2.    LED moving message display board design using shift registers

The second one is very simple for a beginner. So this post I am implementing LED moving display using shift registers.

LED Scrolling Display can be implemented by using any microcontroller like 8051, PIC and AVR controllers. Here I implemented Scrolling Display by using 8051 microcontroller i.e., 89s52 microcontroller. The next post will show the implementation by using PIC and AVR microcontrollers.

So now we are going to implement the LED Message Scrolling board using 8051/8052 microcontroller (89c52 MCU). 

Hardware Construction of LED display using 8051 microcontroller and shift register

Block Diagram:

Figure 1.1 (Block Diagram of LED Message Scrolling Display)

In the block diagram, an four 8x8 LED matrix is driven by using a microcontroller via a transistor array. Each LED 8x8 matrix is connected to shift register via a current sinker to allow larger current to be sink.

Construction of LED Dot Matrix:

Before going for LED Dot Matrix, let us first learn about how an LED should be drive. What value of resistance should be added in series for protection of the LED?

Generally the LEDs are two types

1)      High power LEDs

2)      Miniature LEDs

 High power LEDs are very costly, so in this project we will use miniature type of LEDs

Basically the standard RED color LED requires 2-3V to ON. But to glow the LEDs brightly enough, it require 20mA current to flow.

LED driving circuit:

The Figure 1.2 shows the how to drive an LED using 5v supply without any damage. If we connect directly the 5v supply to an LED, all the source current flow through LED and it will burn out the LED. So we need a resistor in series with LED. In electronics, load current always plays an important role to design a circuit. Here the load current is the current flowing through LED. Let us suppose, we need 20mA current to flow through the LED for better brightness, therefore the load current is 20mA and LED takes 2v to ON.

From ohms law

V = I * R

R = V/I

R = (5v-2v)/20mA 

R = 150 ohms, but in the figure, I have used 200 ohms resistance. Don’t think about the 200 ohms, the correct one 150 ohms only. Because of my laziness I didn’t change that value.

Figure 1.2 (Driving an LED using 5v source)

Single Column LED Array:

Here I have constructed an array using 8 LEDs. The entire cathodes are connected together and should be grounded. Each LED anode is driven by manual switches or by microcontroller. The microcontroller is programmed to generate different patterns on these LEDs. The individual LED current is summed up and a large current flow into the ground terminal as shown in the figure 1.3. Let us consider, each LED takes 20mA current, therefore eight LEDs current = 8*20mA. A total of 160mA current sinks into the ground terminal.

Figure 1.3 (LED array construction using 8 LEDs)

How to construct 8x8Dot matrix?

By using single array of LEDs we can generate different type of patterns. Let us suppose, if we arrange the LED arrays side by side with multiplexing technique, it is also possible to generate letters, symbols, numbers, pictures, animations also. The LED Dot Matrix can be constructed by connecting all the anode terminals of LED’s together in each row and all the cathode terminals are joined together in each column. An 8x8 LED matrix construction is shown in figure 1.4.

Figure 1.4 (8x8 dot matrix)

How to make LED dot matrix to work? What is need of shift register in LED scrolling board? What is the use of Large Current Sinker (ULN 2803) in LED Message scrolling board?

To make this LED matrix to work, connect all the anodes to a microcontroller and the columns are connected to shift register (74LS164). Each column contains eight LED’s therefore the total current flowing through the column is the sum of currents flowing through in each LED. Let us suppose the current flowing through each LED is 20mA and the total current is 8*20mA = 160mA. But the shift register is not capable to sink such a large current (160mA), therefore we need a large current sinker (IC ULN 2803) which is capable to sink 500mA current.

Figure 1.5 (LED Matrix Driving using microcontroller)

In figure 1.5 showing the mechanism to drive the LED dot matrix using microcontroller. Microcontroller itself provides the data and clock control signals to the shift register. If the microcontroller provides the data sequence (10000000) to shift register data pin, the first column in LED matrix is connected to ground via ULN 2803 and the remaining other columns in the matrix are left unconnected. Therefore LEDs in that column will glow according to port data provided by the micro controller.

Figure 1.6 (LED Matrix Driving using microcontroller with corresponding shift register data)

If the data sequence (01000000) is provided to shift register, then the next column LEDs will glow according to port data provided by MCU. Likewise next columns will drive one by one with corresponding sequence given to shift register.

What is refresh rate? How to calculate the refresh rate? How to avoid flickering in the LED message scrolling sign board?

In the figure 1.6, a letter ‘A’ displayed on the matrix module even though individual column is enabled at a time. The entire letter/image should be refreshing at 24 times per second, then only we can see flicker less image. According to the Persistence of vision phenomenon, it was scientifically determined that a frame rate of less than 16 frames per second (frame/s) caused the mind to see flashing images. Hence here refresh rate is taken as 24 frames/sec. One frame takes 1/24 sec to scan all the columns in the matrix (Ex: 8x8 Matrix). Therefore each column takes 1/ (24*8) = 1.7msec to glow. So, our program should scan each column within 1.7msec then only we can achieve correct refresh rate.

How to achieve higher and better refresh rate in LED message Scrolling board?

 If we go for higher refresh rate, this time delay (1.7msec) may be decreased to micro seconds. But some microcontroller are not able to provide such small delays (ex: 8051), in such cases we go for higher high speed microcontrollers such as PIC and AVR.

LED anode Circuit in LED message scrolling board? What is need of transistors or a latch between microcontroller and 8x8dot matrix?

 To drive the rows of the LED matrix, some microcontrollers doesn’t have enough output voltage/currents. Therefore we need to place a transistor array or a latch between the led matrix and microcontroller. Generally for simple LED Scrolling board latch is enough to provide sufficient current to the LEDs. Here, we have used 8-bit latch (74ls573) for this purpose. Each output of the 8-bit latch is able to provide current greater than 20mA which is enough to produce sufficient brightness to the LED.

Figure 1.7 LED Matrix driving using microcontroller using 8-bit Latch 74LS573 

The Below circuit diagram shows interfacing of single Dot Matrix to 8051 Micro controller. Actually the circuit diagram is drawn by using Proteus software tool. In this Some pins of IC are not shown like Vcc, Gnd. These pins are connected internally.

Figure 1.8 LED DOT MATRIX MOVING MESSAGE DISPLAY BOARD CIRCUIT DIAGRAM

LED Message Scrolling display Software program in Embedded C

Animation based LED display using 8051 microcontroller:



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LED Message Scrolling Display using 8051 microcontroller,
LED sign Board
LED scroller display
LED message moving Display