// Definición de las teclas del Keypad
char letras[4][4] = {
  '1', '2', '3', 'A', 
  '4', '5', '6', 'B',
  '7', '8', '9', 'C', 
  '*', '0', '#', 'D'
};

void setup() {
  // Configuración de las filas como salida
   Serial.begin(9600);
  pinMode(13, OUTPUT);
  pinMode(12, OUTPUT);
  pinMode(11, OUTPUT);
  pinMode(10, OUTPUT);
 
}

int i;

void loop() {
  for (i = 0; i < 4; i++) {
  

    // Seleccionamos la fila correspondiente
    if (i == 0) {
      digitalWrite(13, HIGH);
      digitalWrite(12, LOW);
      digitalWrite(11, LOW);
      digitalWrite(10, LOW);
    }
    if (i == 1) {
      digitalWrite(13, LOW);
      digitalWrite(12, HIGH);
      digitalWrite(11, LOW);
      digitalWrite(10, LOW);
    }
    if (i == 2) {
      digitalWrite(13, LOW);
      digitalWrite(12, LOW);
      digitalWrite(11, HIGH);
      digitalWrite(10, LOW);
    }
    if (i == 3) {
      digitalWrite(13, LOW);
      digitalWrite(12, LOW);
      digitalWrite(11, LOW);
      digitalWrite(10, HIGH);
    }

   if (digitalRead(9) == 1 ) {  
      Serial.println(letras[i][0]);
     
    }
    if (digitalRead(8) == 1) {
      Serial.println(letras[i][1]);
      
    }
    if (digitalRead(7) == 1) {
      Serial.println(letras[i][2]);
      
    }
    if (digitalRead(6) == 1) {
      Serial.println(letras[i][3]);
      
    }

  }



}