Use button lights (as matrix) to display a hex character.
        :param c: String containing one of 0-9,A-F

def char_on(self, c):
        """
        Use button lights (as matrix) to display a hex character.
        :param c: String containing one of 0-9,A-F
        """
        if c.upper() in CHARACTERS:
            for i in CHARACTERS[c.upper()]:
                self.light_on(BUTTONS[i])