Arduino Morse Code Telegraph: Build a Sender and a Key Decoder

// QuickMorse: Arduino Morse code sender
// Type a message in the Serial Monitor (9600 baud) and press Enter.

const int LED_PIN = 13;      // LED with 220 ohm resistor, or the built-in LED
const int BUZZER_PIN = 8;    // passive piezo buzzer
const int TONE_HZ = 600;     // sidetone pitch in hertz
const int WPM = 15;          // sending speed in words per minute
const unsigned long UNIT = 1200UL / WPM;   // one unit in milliseconds (80 ms at 15 WPM)

// Morse code for A to Z, then 0 to 9 (ITU-R M.1677-1)
const char* LETTERS[] = {
  ".-", "-...", "-.-.", "-..", ".", "..-.", "--.", "....", "..", ".---",
  "-.-", ".-..", "--", "-.", "---", ".--.", "--.-", ".-.", "...", "-",
  "..-", "...-", ".--", "-..-", "-.--", "--.."
};
const char* DIGITS[] = {
  "-----", ".----", "..---", "...--", "....-",
  ".....", "-....", "--...", "---..", "----."
};

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Type a message and press Enter.");
}

// One dot (1 unit) or dash (3 units), followed by the 1-unit gap inside a letter
void keyDown(unsigned long units) {
  digitalWrite(LED_PIN, HIGH);
  tone(BUZZER_PIN, TONE_HZ);
  delay(units * UNIT);
  digitalWrite(LED_PIN, LOW);
  noTone(BUZZER_PIN);
  delay(UNIT);
}

void sendCode(const char* code) {
  for (int i = 0; code[i] != '\0'; i++) {
    keyDown(code[i] == '.' ? 1 : 3);
  }
  delay(2 * UNIT);           // 1 + 2 = 3-unit gap between letters
}

void loop() {
  if (Serial.available() > 0) {
    String msg = Serial.readStringUntil('\n');
    msg.toUpperCase();
    for (unsigned int i = 0; i < msg.length(); i++) {
      char c = msg[i];
      if (c >= 'A' && c <= 'Z') {
        sendCode(LETTERS[c - 'A']);
      } else if (c >= '0' && c <= '9') {
        sendCode(DIGITS[c - '0']);
      } else if (c == ' ') {
        delay(4 * UNIT);     // 3 + 4 = 7-unit gap between words
      }
    }
    Serial.print("Sent: ");
    Serial.println(msg);
  }
}

// QuickMorse: Arduino Morse code key decoder
// Press the button like a telegraph key. Decoded text appears in the Serial Monitor.

const int KEY_PIN = 2;       // push button or straight key to GND (internal pull-up)
const int LED_PIN = 13;
const int BUZZER_PIN = 8;    // passive piezo buzzer for sidetone
const int TONE_HZ = 600;
const int WPM = 12;                        // the speed you key at
const unsigned long UNIT = 1200UL / WPM;   // 100 ms at 12 WPM
const unsigned long DEBOUNCE_MS = 15;      // ignore contact bounce shorter than this

const char* LETTERS[] = {
  ".-", "-...", "-.-.", "-..", ".", "..-.", "--.", "....", "..", ".---",
  "-.-", ".-..", "--", "-.", "---", ".--.", "--.-", ".-.", "...", "-",
  "..-", "...-", ".--", "-..-", "-.--", "--.."
};
const char* DIGITS[] = {
  "-----", ".----", "..---", "...--", "....-",
  ".....", "-....", "--...", "---..", "----."
};

char pattern[8];             // dots and dashes of the current letter
int len = 0;
bool keyWasDown = false;
bool spaceSent = true;
unsigned long changedAt = 0;

void setup() {
  pinMode(KEY_PIN, INPUT_PULLUP);
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Start keying...");
}

char decode(const char* p) {
  for (int i = 0; i < 26; i++) {
    if (strcmp(p, LETTERS[i]) == 0) return 'A' + i;
  }
  for (int i = 0; i < 10; i++) {
    if (strcmp(p, DIGITS[i]) == 0) return '0' + i;
  }
  return '?';
}

void loop() {
  bool keyDown = (digitalRead(KEY_PIN) == LOW);
  unsigned long now = millis();

  if (keyDown != keyWasDown) {
    if (now - changedAt < DEBOUNCE_MS) return;
    unsigned long held = now - changedAt;
    if (keyDown) {                       // key pressed: start the tone
      tone(BUZZER_PIN, TONE_HZ);
      digitalWrite(LED_PIN, HIGH);
    } else {                             // key released: was it a dot or a dash?
      noTone(BUZZER_PIN);
      digitalWrite(LED_PIN, LOW);
      if (len < 7) pattern[len++] = (held < 2 * UNIT) ? '.' : '-';
    }
    keyWasDown = keyDown;
    changedAt = now;
    spaceSent = false;
    return;
  }

  if (!keyDown) {
    unsigned long gap = now - changedAt;
    if (len > 0 && gap > 2 * UNIT) {             // gap longer than 2 units: letter finished
      pattern[len] = '\0';
      Serial.print(decode(pattern));
      len = 0;
    } else if (len == 0 && !spaceSent && gap > 5 * UNIT) {   // longer than 5 units: word finished
      Serial.print(' ');
      spaceSent = true;
    }
  }
}