4-digit-7-segment display

Introducion

In this tutorial I'm going to show you the basics of the working of a 7 segment display, and how to use, when you have 4 in a row. So lets take look:

So that"s it. I ordered it from dx.com half years ago, because it was only $2, and I was interested in it. But in the beginning it was too hard for me, I just can't make it work. It has 6+6 so 12 pins in the back, and has no sign about them, as here:

So I took it away, and didn't even touch it till this month. We learned about 7 segment displays, and I was wondering, what if I could make my work. So I looked up on the internet, and find, that this display has its own library, here: https://github.com/sparkfun/SevSeg First I just downloaded and tried it, and it worked. So I looked into the code, and started to understand the inner working. So basically this display has 4*8 leds ( because each the number has 7, and there are 4 dots also). How is it possible then to control them only with 12 pins? Don't we need 4*8=32 pins for that? Yes, theoretically we would need 32 pins. But then, how does it works? The trick is, that we don't need to light up all lightning led constantly. If they blink very fast, we will see as if they're lighting. So there is 4 pin for specifying the number, and then whit the rest 8 pins we can control the 8 leds of the number. And if we change between the numbers very fast, nobody will be able to see the blinking. And changing it very fast is not a problem, when we are using arduino.

Inner working

Okay, you can say, but how is it possible? I mean, how does it work? It's quiet simple, using the special attribution of the led, that it's a diod, so the current can flow only in one direction. So here is  how to show a 5 in the 3. place:

(Red lines are HIGH, blues are LOW, because my display is common anode type. If yours is common cathode, just change every HIGH to LOW and )So as you can see 8 of the 12 pins is connected to each led, but in every number. The other 4 pins are connected to the other side of the leds but one by every number. The led will show up, if it get a red line (HIGH) from the top and a blue line (LOW) from the button. Otherwise it stays dark. Try to understand this part, because many-many things works in this principle like led cubes, and matrix displays. 

Using it

The pins are numbered this way:

And connect these pins of the display to those pins of the arduino:

   //Declare what pins are connected to the digits
   int digit1 = 2; //Pin 12 on my 4 digit display
   int digit2 = 3; //Pin 9 on my 4 digit display
   int digit3 = 4; //Pin 8 on my 4 digit display
   int digit4 = 5; //Pin 6 on my 4 digit display
   
   //Declare what pins are connected to the segments
   int segA = 6; //Pin 11 on my 4 digit display
   int segB = 7; //Pin 7 on my 4 digit display
   int segC = 8; //Pin 4 on my 4 digit display
   int segD = 9; //Pin 2 on my 4 digit display
   int segE = 10; //Pin 1 on my 4 digit display
   int segF = 11; //Pin 10 on my 4 digit display
   int segG = 12; //Pin 5 on my 4 digit display
   int segH= 13; //Pin 3 on my 4 digit display

After that we can control the display from the arduino. For this here is my code:
(NOTE: I've used Arduino MEGA for this tutorial and connected the display to pins 22-33, so if you use an UNO, change it as shown under)

boolean show[4][8];

void setup()
{
  //actually we don't need this part, but it's always good to know the wiring 
   //Declare what pins are connected to the digits
   int digit1 = 22; //Pin 12 on my 4 digit display
   int digit2 = 23; //Pin 9 on my 4 digit display
   int digit3 = 24; //Pin 8 on my 4 digit display
   int digit4 = 25; //Pin 6 on my 4 digit display
   
   //Declare what pins are connected to the segments
   int segA = 26; //Pin 11 on my 4 digit display
   int segB = 27; //Pin 7 on my 4 digit display
   int segC = 28; //Pin 4 on my 4 digit display
   int segD = 29; //Pin 2 on my 4 digit display
   int segE = 30; //Pin 1 on my 4 digit display
   int segF = 31; //Pin 10 on my 4 digit display
   int segG = 32; //Pin 5 on my 4 digit display
   int segDP= 33; //Pin 3 on my 4 digit display


   for(int i=22; i < 34; i++) pinMode(i, OUTPUT);
   
   //adding the text or numbers
   for(int i=0; i < 4; i++){for(int j=0; j < 8; j++) show[i][j]=true;} //if true: the led is dark, if false the led shows up
   //A
   show[0][0]=false;
   show[0][1]=false;
   show[0][2]=false;
   show[0][4]=false;
   show[0][5]=false;
   show[0][6]=false;
   //n
   show[1][2]=false;
   show[1][4]=false;
   show[1][6]=false;
   //n
   show[2][2]=false;
   show[2][4]=false;
   show[2][6]=false;
   //a
   show[3][2]=false;
   show[3][3]=false;
   show[3][4]=false;
   show[3][6]=false;
   show[3][7]=false;

}



void loop()
{
for(int i=0; i < 4; i++) //run on the digits
{
 for(int j=0; j < 8; j++) digitalWrite(j+26, true); //turn off every segment
digitalWrite(22+i, true); //turn the actual number on
digitalWrite(22+(i+1)%4, false); //and 
digitalWrite(22+(i+2)%4, false); //fade
digitalWrite(22+(i+3)%4, false); //the rest

for(int j=0; j < 8; j++)
{
digitalWrite(j+26, show[i][j]); //turn the specified segments on as defined in the array show
}
delay(1);
}

}

And the same code for arduino UNO:

boolean show[4][8];

void setup()
{
   //I've deleted the unnecessary part from here
   for(int i=2; i < 14; i++) pinMode(i, OUTPUT);
   
   //adding the text or numbers
   for(int i=0; i < 4; i++){for(int j=0; j < 8; j++) show[i][j]=true;} //if true: the led is dark, if false the led shows up
   //A
   show[0][0]=false;
   show[0][1]=false;
   show[0][2]=false;
   show[0][4]=false;
   show[0][5]=false;
   show[0][6]=false;
   //n
   show[1][2]=false;
   show[1][4]=false;
   show[1][6]=false;
   //n
   show[2][2]=false;
   show[2][4]=false;
   show[2][6]=false;
   //a
   show[3][2]=false;
   show[3][3]=false;
   show[3][4]=false;
   show[3][6]=false;
   show[3][7]=false;

}



void loop()
{
for(int i=0; i < 4; i++) //run on the digits
{
 for(int j=0; j < 8; j++) digitalWrite(j+26, true); //turn off every segment
digitalWrite(2+i, true); //turn the actual number on
digitalWrite(2+(i+1)%4, false); //and 
digitalWrite(2+(i+2)%4, false); //fade
digitalWrite(2+(i+3)%4, false); //the rest

for(int j=0; j < 8; j++)
{
digitalWrite(j+6, show[i][j]); //turn the specified segments on as defined in the array show
}
delay(1);
}

}

If it runs perfectly you should see "Anna" on the screen, like shown here:

You can control the text with changing the array show. For example for the first character "A" we need 6 segment from the first display: a,b,c,e,f,g. So because it's the first number, we change show[0][*] and because a=0, b=1, c=2, e=4, f=5, g=6, so

   //A
   show[0][0]=false;
   show[0][1]=false;
   show[0][2]=false;
   show[0][4]=false;
   show[0][5]=false;
   show[0][6]=false;

I think it's not so complicated :)

Deeply into the case

As you see, it use 12 pins, which actually all of the digital pin of a normal arduino UNO. So what can we do? If you think about it, there is a redundant information in the system, because every time only one of the 4 digit-pin is HIGH, so it can be controlled with a demultiplexer (aka demux). It works so:

So I0 is connected to Vcc, and F0-F3 to the 4 digit pin of the display. So now if you want a HIGH on F0, you only have to give LOW and LOW on S1 and S0, for F1 LOW and HIGH, for F2 HIGH and LOW and for F3 HIGH and HIGH. So in the end you save 2 pins, which are not the word, but sometimes can be very helpful.

I hope this tutorial was helpful, if you have any question, please feel free to write or comment

Best regards:

Mark