const int LDR = A0;
int counter = 0;
String morse = "";

void setup() {
  pinMode (LDR, INPUT);
}

void loop() {
  int value = analogRead(LDR);
  if (value > 250) {
    counter++;
  } else {
    counter = 0;
  }
  delay(300);
  if (counter == 1) {
    addDot();
  } else if (counter == 3) {
    removeDot();
    addDash();
  }
  Serial.println(morse);
}

void addDot() {
  morse += ".";
}

void addDash() {
  morse += "-";
}

void removeDot() {
  int length = morse.length();
  if (morse.substring(length) == ".") {
    morse.remove(length);
  }
}