§5X7 Scrolling Message Display

Hello Friends,

     LED Matrix displays are very popular for displaying messages you can see them in everywhere in your daily routine. For example at railway station - it is used to display train details, in buses, at restaurants etc. In this post we will make a simple 5x7 LED matrix display and display some messages.

5x7 LED display needs 35 LEDs to make it. We have to connect all of 35 LEDs with a single microcontroller; here I am using AVR atmega8 which has 28 pins. If one LED takes one I/O pin of MCU then 35 LEDs required 35 I/O pins but atmega8 have only 22 I/O pins. Then how we do this??

Answer is MULTIPLEXING. This technique requires fewer I/O pins of MCU. We need only 12 I/O pins of MCU to interface this display. To understand that techniques see the figure below.

 

 

You can see in the figure that there are 5 column and 7 rows. All the cathodes are shorted horizontally and all the anodes are shorted vertically. All the anodes are controlled by columns and all the anodes are controlled by rows. This structure makes a matrix of 7x5 hence the name ‘LED Matrix display’ and controlling all the LEDs using this technique called multiplexing.

For example: If we want to ON led located at (0, 0) then we have to connect first column to VCC and first Row to Ground. We will discuss more about this in this post.

Ok! First we have to make this matrix; this requires patience and lots of time to soldering. See the pics below.

 

This is my 13x5 LED matrix.

 

Be careful to make this; the directions of all the anodes and cathodes should be in correct way otherwise some LEDs will not glow. You can see in the figure that there is a bridge shape structure which is complex and many chances to shorting; If you hesitate to make this then don’t worry there is readily available 5x7 LED display available in the market.See pic below :

 

This is a 5x7 LED Matrix module common anode type. Let’s see its pin sequence.

 

Circuit Diagram :

 

 We can't connect LEDs directly to supply hence we have to connect resistance within its path, 470 ohm is sufficient with 5volt supply. Here I am using MCU as current sinking so for current sourcing I placed 4 NPN transistors at columns. Base of transistors are connected to MCU. When base goes high Emitter to Collector gets shorted and vcc comes at columns.

Readily available 5x7 LED modules have no regular sequence of row and columns so we have to spend some time with PCB and soldering iron. 

Take a small piece of zero PCB and make a Header Board for this LED module. Also place transistors and registers on it. See images –

 

Connect a 12 pin female burg strip for Row and Column Connection.

Now we can directly connect this with our microcontroller. The purpose of making this is to make it portable so that we can connect it on the breadboard also. Always try to make things portable so that we don’t have to make them again if repeated.

Make two ribbon cables. One is 7 pin male-to-male and second is 5 pin male-to-male. See pics :

 

Connect the cables to 5x7 module and to Microcontroller with the help of circuit diagram.

 

Hardware part is completed. Let’s move towards software part.

To display a character break it in five 8-bit binary (or hex) sequences. See the image below-

 

                                              1 -- LED ON      0 -- LED Off

(Note : We configure MCU as current sinking so we have to send invert sequence ~seq[] )

Store the sequences in an array.We have to switch column one by one rapidly. Binary sequence comes from micro-controller strikes at row pins (R1-R7) and that time if any column line is active (High) then this sequence will show on that column. We have to send seq1 to col-1, seq2 to col-2 and so on. If you don't understand clearly then see the image blow, which is self explanatory.

Columns are switched one by one with some delay and at the same time binary data comes at row will display on that column line.

**What will happen if we decrease column switching delay (or increase switching speed)?  Yes, you are thinking right we will see a still image. This phenomenon is known as “Persistence of Vision”, it gives the illusion that all the LEDs lit at the same time. This is the basic concept of this display. See the image below –

See the image below - POV effect

 

Program: To display a single character

MCU : Atmega8 @ 16MHz  Compiler – WinAVR

Code :

#define F_CPU 16000000UL

#include<avr/io.h>

#include<util/delay.h>

unsigned char seq[]={

0b01111111,

0b00001001,

0b00001001,

0b00001001,

0b00000110,  

};

                    

int main()

{   

   DDRC=0xFF;

   DDRD=0xFF;

   PORTC=0;

   PORTD=0; 

   int i=0;

    while(1)

       {   

          for(i=0;i<5;i++)

           {  

           PORTC=(1<<i);       

           PORTD=~seq[i];

           _delay_ms(3);           

        }

  }

 return 0;

}

Program -2: Scrolling Message

The 5x7 hex sequence of all the characters are stored previously in a header file “font5x7.h” just include it in your main program and enjoy it. All the characters are stored at its ascii location. So first convert characters in its ascii value to fetch up sequence from correct location.

To display moving message we have to maintain a buffer and store binary sequences of all the characters of our message in it. A window of sequence 5 will slide continuously at specified time which will show moving effect.(see ISR section of the code)

Software delay is not accurate so I configure AVR internal Hardware Timer which is CPU independent. I am using 16-bit Timer1 in compare mode means I can decide it’s counting boundary according to my requirement. If you are not familiar with timer go through AVR datasheet or Google it.

Let’s do some calculation:

A good POV frequency is 400Hz (Flicker Free). Our CPU frequency is 16000000Hz and timer frequency is F_CPU/Prescaler=(16000000/1024)=15625Hz.

So compare value to generate 400Hz is ---

Compare Value = (15625/400)=39.0625

1/400=2.5ms delay means at every 2.5 ms timer interrupt will strike ISR. 

Code :

#define F_CPU 16000000UL

#include <avr/io.h>

#include <util/delay.h>

#include <avr/interrupt.h>

#include "font5x7.h"

#define LED5x7_DDR DDRD

#define LED5x7_PORT PORTD

#define LED5x7CONTROL_DDR DDRC

#define LED5x7CONTROL_PORT PORTC 

unsigned char buffer[];

unsigned int buffer_loc=0;

volatile int count=0,scroll=0,col=0;

//----------------------------------------------------------------

void Scroll_MSG(char *msg)

{

 int i,j=0;

 while(*msg!='\0')

 {

    j=(*msg-32)*5;

      for(i=j;i<(j+5);i++)

       {  

          buffer[buffer_loc]=font5x7[i];

          buffer_loc++;

        }

       buffer[buffer_loc]=0;

      buffer_loc++;

      msg++;

 }

}

//----------------------------------------------------------------

void Timer_Init()

{

    TCNT1=0;

    TIMSK|=(1<<OCIE1A);

    OCR1A=39;

    TCCR1B|=(1<<CS12)|(1<<CS10)|(1<<WGM12);//F_CPU/1024,CTC

    sei();

}

//----------------------------------------------------------------

int main()

{

Timer_Init();

LED5x7_DDR = 0xFF;

LED5x7CONTROL_DDR = 0xFF;

Scroll_MSG(" 5x7LED DISPLAY");

Scroll_MSG(" By-PRATYUSH ");

while(1);

return 0;

}

//----------------------------------------------------------------

ISR(TIMER1_COMPA_vect)

{

 count++;

 if(count==50){ //to increase scrolling speed decrease counting

   count=0;scroll++;

    if(scroll>buffer_loc-5)

     {scroll=0;}

  }

 LED5x7CONTROL_PORT = (1<<col);

 LED5x7_PORT = ~buffer[scroll+col];

 if(col++==4)

      {col=0;}

}

//----------------------------------------------------------------

My Pocket Electronic Message card: :)

 

Downloads:  circuit diagram 

                      hex file and code for display single character
                      hex file and code for scroll string/message

Video: 

                                                                                                                             ~Pratyush Gehlot