int BUTTON = PB4;
int LED = PB3;

//letters A-Z in Morse code
char *letters[] = {
  ".-", "-...", "-.-.", "-..", ".", "..-.", "--.", "....", "..",    
  ".---", "-.-", ".-..", "--", "-.", "---", ".--.", "--.-", ".-.",  
  "...", "-", "..-", "...-", ".--", "-..-", "-.--", "--.."
  };
char *string = "SOS";

char ch_dot = '.';
char ch_dash = '-';
int dot_duration = 200;
boolean start = false;
int curr = 0;

void blink_dot_dash(char code){
  digitalWrite(LED,HIGH); //turn on LED
  if(code == ch_dot)
    delay(dot_duration);
  else
    delay(dot_duration*3);
  digitalWrite(LED,LOW); //turn off LED
  delay(dot_duration); //space between dot/dash
}

void blink_letter(char ch){
  char *code_letter = letters[ch - 'A'];
  unsigned int i = 0;
  while(code_letter[i] != NULL){
    blink_dot_dash(code_letter[i]);
    i++;
  }
  delay(dot_duration * 3);//space between letters
}

void setup() {
  pinMode(BUTTON, INPUT_PULLUP); //init button as input
  pinMode(LED, OUTPUT); //init LED as output
}

void loop() {
  // when button pushed, invert start.
  if (!digitalRead(BUTTON)){
    start = !start;
  }
  
  if(start){
    for(curr = 0; string[curr] != NULL; ++curr){
      blink_letter(string[curr]);
    }
    delay(dot_duration*7);//space between words
    start = false;
  }
}