An Arduino Sketch for creating a Keyboard/MidiKeyboard controlled by a single soldered keyboard switch

headphoneswitch.ino

#include <Keyboard.h>
#include <MIDIUSB.h>

int readValue = 0;
int lastValue = 1;
char mode = 'K';
byte midiCh = 2;
byte note = 36;
byte cc = 1;

void setup() {
  pinMode(PIN_A2, OUTPUT);
  pinMode(PIN_A4, INPUT_PULLUP);
  digitalWrite(PIN_A2, LOW);
}

void loop() {
  delay(500);
  readValue = digitalRead(PIN_A4);

  if (lastValue != readValue) {
    if (readValue) {
      if (mode == 'K') {
        sendKey(KEY_F21);
      }
      else {
        sendNote(note);
      }
    }
    else {
      if (mode == 'K') {
        sendKey(KEY_F20);
      }
      else {
        sendNote(note+1);
      }
    }
  }

  lastValue = readValue;
}

void sendKey(char key) {
  Keyboard.press(KEY_LEFT_ALT);
  Keyboard.press(key);
  delay(100);
  Keyboard.releaseAll();
}

void sendNote(int note) {
  noteOn(midiCh, note, 127);
  MidiUSB.flush();
  delay(100);
  noteOff(midiCh, note, 127);
  MidiUSB.flush();
}

void noteOn(byte channel, byte pitch, byte velocity) {
  midiEventPacket_t noteOn = {0x09, 0x90 | channel, pitch, velocity};
  MidiUSB.sendMIDI(noteOn);
}

void noteOff(byte channel, byte pitch, byte velocity) {
  midiEventPacket_t noteOff = {0x08, 0x80 | channel, pitch, velocity};
  MidiUSB.sendMIDI(noteOff);
}

void controlChange(byte channel, byte control, byte value) {
  midiEventPacket_t event = {0x0B, 0xB0 | channel, control, value};
  MidiUSB.sendMIDI(event);
}