//                 0    1     2     3     4      5     6     7     8     9    *      #
byte keycode[] = { 0xE5, 0x73, 0x75, 0x76, 0xB3, 0xB5, 0xB6, 0xD3, 0xD5, 0xD6, 0xE3, 0xE6 };

void setup() {
  Serial.begin(9600);

  // Col
  pinMode(2, INPUT);
  pinMode(3, INPUT);
  pinMode(4, INPUT);

  // Row
  pinMode(5, OUTPUT);
  pinMode(6, OUTPUT);
  pinMode(7, OUTPUT);
  pinMode(8, OUTPUT);
}

char ScanKey() {
  for (int i = 0; i < 12; i++) {
    digitalWrite(5, keycode[i] & 0b00010000);
    digitalWrite(6, keycode[i] & 0b00100000);
    digitalWrite(7, keycode[i] & 0b01000000);
    digitalWrite(8, keycode[i] & 0b10000000);

    byte key = (keycode[i] & 0xF0) | (digitalRead(4) << 2) | (digitalRead(3) << 1) | digitalRead(2);
    if (key == keycode[i]) {
      return (i < 10 ? i + '0' : i == 10 ? '*' : i == 11 ? '#' : 0);
    }
  }

  return 0;
}

void loop() {
  char key = ScanKey();
  if (key != 0) {
    while(ScanKey() == key) delay(10);
    Serial.println(key);
  }
}