How to convert numbers from Serial port input to HEX String?
You have the number. It's made up of two nibbles, each representing 4 bits (1 hex character). 

If you want to do a manual conversion take the first nibble (AND with 240) add it's value to "0" then do the same for the lower nibble (AND with 15).

OR

What I would do is use:

char HexString[3]; 
sprintf(HexString,"%02X",HexInt);


since you're not dealing with floats, you're good to go.

OR

edit: I recalled a printf function that we put together once before, I don't know if it would work with SoftwareSerial, so you might have to play around with it a bit, but as a 3rd option, you could try the following. I don't have access to an Arduino or compiler at the moment, otherwise I'd have a play with this right now.

Before your setup:

// Function that printf and related will use to print
int serial_putchar(char c, FILE* f) {
    if (c == '\n') serial_putchar('\r', f);
    return Serial.write(c) == 1? 0 : 1;
}

FILE serial_stdout;


In your setup, add:

// Set up stdout
fdev_setup_stream(&amp;serial_stdout, serial_putchar, NULL, _FDEV_SETUP_WRITE);
stdout = &amp;serial_stdout;


in your loop, change your assignment to:

HexString = printf("%02X",HexInt);


from: https://arduino.stackexchange.com/a/480/304