Display a numeric countdown from 5
        :param interval: The interval in seconds for each number.

def countdown(self, interval: float = 0.5):
        """
        Display a numeric countdown from 5
        :param interval: The interval in seconds for each number.
        """
        for c in '54321':
            self.char_on(c)
            time.sleep(interval * 0.66667)
            self.all_off()
            time.sleep(interval * 0.33333)