Technoblogy

// Matrix keypad *******************************************************

const int Matrix = A2;
const int SmallestGap = 24;
int AnalogVals[] = {1023, 728, 693, 657, 632, 605, 567, 541, 517, 477, 439,
                    404,  371, 316, 290, 264, 220, 186, 151, 93,  0,   -100};
int Buttons[] =    {-1,   1,   2,   3,   10,  17,  4,   7,   14,  5,   8,
                    0,    6,   9,   11,  15,  12,  18,  13,  19,  20};

// Returns the keypad character or -1 if no button pressed
int ReadKeypad() {
  int val, lastval=0, count = 0;
  do {
    val = analogRead(Matrix);
    if (abs(val-lastval)<2) count++;
    else { lastval = val; count = 0; }
  } while (count <3);
  int i = 0;
  val = val - SmallestGap/2;
  while (val < AnalogVals[i]) { i++; }
  return Buttons[i - 1];
}

void loop() {
  int key;
  // Wait for key
  do { key = ReadKeypad(); } while (key == -1);
  
  Serial.println(key);

  // Wait for key released
  do { key = ReadKeypad(); } while (key != -1);
}