Morsecode Generator Problem

int ledPin = 8;

char* letters[] = {

  ".-", "-...", "-.-.", "-..", ".", "..-.", "--.", "....", "..",    // A-I

  ".---", "-.-", ".-..", "--", "-.", "---", ".--.", "--.-", ".-.",  // J-R

  "...", "-", "..-", "...-", ".--", "-..-", "-.--", "--.."          // S-Z

};

char* numbers[] = {"-----", ".----", "..---", "...--", "....-", ".....", "-....", "--...", "---..", "----."};

int dotDelay = 250;

void setup()                 

{

  pinMode(ledPin, OUTPUT);

  Serial.begin(9600);

}

void loop()                    

{

  char ch;

  if (Serial.available())      // liegen Daten am USB Port vor?

  {

    ch = Serial.read();        // einzelne Buchstaben lesen

    if (ch >= 'a' && ch <= 'z')

    {

      flashSequence(letters[ch - 'a']); 

    }

    else if (ch >= 'A' && ch <= 'Z')

    {

      flashSequence(letters[ch - 'A']);

    }

    else if (ch >= '0' && ch <= '9')

    {

      flashSequence(numbers[ch - '0']);

    }

    else if (ch == ' ')

    {

     delay(dotDelay * 4);      

    }

  }

}

void flashSequence(char* sequence)

{

   int i = 0;

   while (sequence[i] != NULL)

   {

       flashDotOrDash(sequence[i]);

       i++;

   }

   delay(dotDelay * 3);    // Abstand zwischen den Buchstaben

}

void flashDotOrDash(char dotOrDash)

{

  digitalWrite(ledPin, HIGH);

  if (dotOrDash == '.')

  {

    delay(dotDelay);           

  }

  else // wenn es ein Strich ist 3* so lang

  {

    delay(dotDelay * 3);           

  }

  digitalWrite(ledPin, LOW);    

  delay(dotDelay); // Abstand zwischen Blinksignalen

}