// initialisation 
byte data;  // working variable for incoming digital reads
byte kp[] = {
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
}; // array to keep track of previous states, in order to measure transitions and filter out repeating data
byte channel = 0; // this variable sets the MIDI channel minus one (ie. set this number to 0 if you want to send data on MIDI channel 1)
byte pitches[[] = {
  60, 61, 62, 63, 64, 65, 66, 67, 68, 69 70, 71
}; // array to hold the outgoing MIDI pitches - the first number is equal to the first keypad button, the second to the second and so on
byte velocity; // set the velocity for a note-on event when a button is pushed


// setup
void setup() {
  Serial.begin(32150); // set MIDI baud rate
  pinMode(2, OUTPUT); // Connect to KP ROW 1 pin
  pinMode(3, OUTPUT); // Connect to KP ROW 2 pin
  pinMode(4, OUTPUT); // Connect to KP ROW 3 pin
  pinMode(5, OUTPUT); // Connect to KP ROW 4 pin
  pinMode(6, INPUT);  // Connect to KP COL 1 pin
  pinMode(7, INPUT);  // Connect to KP COL 2 pin
  pinMode(8, INPUT);  // Connect to KP COL 3 pin
  
  // The input pins (arduino digital i/o pins 6, 7 and 8) will need to be 
  // pulled down to ground using a resistor each
  // The value of the resistor should be 10k½ - 30k½ or so. 
  // This stops the digital input pins from floating
}


// main
void loop() { 
  // set up a looping function, which loops 12 times (one for each button)
  for(byte i = 0; i < 12; i ++) {
    // set the row pin high
    digitalWrite((i / 3) + 2, HIGH);
    // a short delay
    delayMicroseconds(75);
    // read the column pin into the variable data
    data = digitalRead((i % 3) + 6);
    // if the state of that particular button has changed since last time, send a MIDI note
    if(data != kp[i]) {
      // set the byte in the array to the new data value
      kp[i] = data;
      // send a MIDI status byte to indicate a note-on event
      printByte(0x90 + channel);
      // send a MIDI data byte with the MIDI note number retrieved from the pitch array
      printByte(pitches[i]);
      // send a MIDI data byte with the velocity. the data point in the array will either be 1 or 0
      // so we either send a note-on (velocity > 0) or a note-off (velocity = 0)
      printByte(kp[i] * 127);
      // a short delay
      delay(2);
      // end if function
    }
    // set the row pin low, ready for the next loop
    digitalWrite((i / 3) + 2, LOW);
    // a short delay
    delayMicroseconds(75);  
    // end the looping function
  }
  // and loop the loop forever
}