Simple program for the Arduino Uno to take user input and flash an LED in Morse code.

Morse.ino

typedef struct {
    char c;
    char[6] pulses;
} letter;

boolean bUseOnBoardLED = true;
int ledPin = bUseOnBoardLED ? 13 : 9;

letter* letters;
int ltr_cnt = 0;

int dotLength = 100; // 100ms is our standard dot duration
// Other durations are calculated as multiples of dotLength
// see https://en.wikipedia.org/wiki/Morse_code#Representation.2C_timing_and_speeds
int dashLength = 3 * dotLength;
int pulseGap   = 1 * dotLength;
int letterGap  = 3 * dotLength;
int wordGap    = 7 * dotLength;

String readString, output;

void setup()
{
    Serial.begin(9600);
    Serial.println("Enter a word to encode.");
    
    pinMode(ledPin, OUTPUT);
    
    letters = (letter*)calloc(26, sizeof(letter));
    
    registerLetter('a', ".-");
    registerLetter('b', "-...");
    registerLetter('c', "-.-.");
    registerLetter('d', "-..");
    registerLetter('e', ".");
    registerLetter('f', "..-.");
    registerLetter('g', "--.");
    registerLetter('h', "....");
    registerLetter('i', "..");
    registerLetter('j', ".---");
    registerLetter('k', "-.-");
    registerLetter('l', ".-..");
    registerLetter('m', "--");
    registerLetter('n', "-.");
    registerLetter('o', "---");
    registerLetter('p', ".--.");
    registerLetter('q', "--.-");
    registerLetter('r', ".-.");
    registerLetter('s', "...");
    registerLetter('t', "-");
    registerLetter('u', "..-");
    registerLetter('v', "...-");
    registerLetter('w', ".--");
    registerLetter('x', "-..-");
    registerLetter('y', "-.--");
    registerLetter('z', "--..");
    registerLetter(' ', " ");
    registerLetter('1', ".----");
    registerLetter('2', "..---");
    registerLetter('3', "...--");
    registerLetter('4', "....-");
    registerLetter('5', ".....");
    registerLetter('6', "-....");
    registerLetter('7', "--...");
    registerLetter('8', "---..");
    registerLetter('9', "----.");
    registerLetter('0', "-----");
}

void registerLetter(char ltr, char* pattern)
{
    letters[ltr_cnt].c = ltr;
    letters[ltr_cnt].pulses = pattern;
    ltr_cnt++;
}

void loop()
{
    while (Serial.available())
    {
        delay(5); // delay to allow buffer to fill
        if (Serial.available() > 0)
        {
            char c = Serial.read(); // gets a byte from the serial buffer
            readString += c;
        }
    }
    
    if (readString.length() > 0)
    {
        for(int i = 0; i < readString.length(); i++)
        {
            char cur = readString[i];
            for(int j = 0; j < 26; j++)
            {
                if (letters[j].c == cur)
                {
                    char* pulses = letters[j].pulses;
                    while(*pulses != 0)
                    {
                        if (*pulses == '.')
                        {
                            dot();
                        }
                        else if (*pulses == '-')
                        {
                            dash();
                        }
                        else if (*pulses == ' ')
                        {
                            space();
                        }
                        pulses++;
                    }
                    break;
                }
            }
            output += " ";
            delay(letterGap - pulseGap); // we need to subtract pulseGap, as we've already output it
        }
        readString = "";
        Serial.println(output);
        output = "";
        Serial.println("Enter a word to encode.");
    }
}

void dot()
{
    output += ".";
    pulse(dotLength);
}

void dash()
{
    output += "-";
    pulse(dashLength);
}

void space()
{
    delay(wordGap - letterGap); // we need to subtract letterGap, as we've already output it
}

void pulse(int length)
{
    digitalWrite(ledPin, HIGH);
    delay(length);
    digitalWrite(ledPin, LOW);
    delay(pulseGap);
}