Arduino Program to Turn on 1st LED of an 8*8 LED Matrix using Arduino and 74595

Previous: Arduino Program to Turn on All LEDs of 8*8 LED Matrix

          We have already seen the circuit to connect 8*8 LED matrix to arduino through 8 bit shift register 74595 in previous blog. Next is a simple program to turn on the 1^st LED only and turn off all other LEDs of an 8*8 LED matrix. LED matrix will look like as shown in the following image.

Upload the following program to your arduino board.

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int latchPin = 12;  //Pin connected to ST_CP of 1st 74595
int clockPin = 13;  //Pin connected to SH_CP of 1st 74595
int dataPin = 11;   //Pin connected to DS of 1st 74595

int latchPin2 = 6;  //Pin connected to ST_CP of 2nd 74595
int clockPin2 = 7;  //Pin connected to SH_CP of 2nd 74595
int dataPin2 = 5;   //Pin connected to DS of 2nd 74595

void setup() {
  //set pins to output so you can control the shift register
  pinMode(latchPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin, OUTPUT);
  
  pinMode(latchPin2, OUTPUT);
  pinMode(clockPin2, OUTPUT);
  pinMode(dataPin2, OUTPUT);
}

void loop() {
  
    /************** Send HIGH to first Anode pin (16th pin) and LOW to all 
    other anode pins (15, 11, 3, 10, 5, 6, 13) of LED matrix *************/    
  
    // take the latchPin low so the LEDs don't change while you're sending in bits:  
    digitalWrite(latchPin, LOW);
    //Send 0 0 0 0 0 0 0 1 (1) to Q7 Q6 Q5 Q4 Q3 Q2 Q1 Q0 of 1st 74595
    shiftOut(dataPin, clockPin, MSBFIRST, 1);
    // shift out the bits:    
    digitalWrite(latchPin, HIGH);



    /**************** Send LOW to the first cathode pin (4th pin) and HIGH to all 
    other cathode pins (7, 2, 8, 12, 1, 14 and 9) of LED matrix *****************/    
    
    // take the latchPin low so the LEDs don't change while you're sending in bits:    
    digitalWrite(latchPin2, LOW);
    //Send 1 1 1 1 1 1 1 0 (254) to Q7 Q6 Q5 Q4 Q3 Q2 Q1 Q0 of 2nd 74595
    shiftOut(dataPin2, clockPin2, MSBFIRST, 254);
    // shift out the bits:  
    digitalWrite(latchPin2, HIGH);
}

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          If uploading is successful, 1^st LED will turn on and all other LEDs will turn off. Working of program is simple. Q0 to Q7 pins of first 74595 IC is connected to the anode pins (16, 15, 11, 3, 10, 5, 6, 13) of 8*8 LED matrix as given in the previous blog. Arduino will send 1 (0 0 0 0 0 0 0 1) to the first shift register IC. When 1 is shifted out, Q7 Q6 Q5 Q4 Q3 Q2 Q1 Q0 of first shift register will output 0 0 0 0 0 0 0 1. Hence, 1^st anode pin (16^th pin) of LED matrix will get HIGH voltage. All other anode pins will get LOW voltage.

          Similarly Q0 to Q7 pins of second 74595 IC is connected to the cathode pins of 8*8 LED matrix. Arduino will send 254 (1 1 1 1 1 1 1 0) to the second shift register IC. When 254 is shifted out, Q7 Q6 Q5 Q4 Q3 Q2 Q1 Q0 of second shift register will output 1 1 1 1 1 1 1 0. Hence, 1^st cathode pin (4^th pin) of LED matrix will get LOW voltage. All other cathode pins will get HIGH voltage. That is, 1^st LED will get HIGH voltage at anode and LOW voltage at cathode. This will turn on the 1^st LED and turn off all other LEDs.

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