Even so, fitting a two-digit display seemed difficult. We used a Charlieplexed display to arrange the LEDs in the required two-digit, seven-segment format, which would serve our purpose (Fig. 2).

Four of these modules connect together to form a row of 32×2 digits, and eight rows of digits come together to make a 512-digit display. With seven LEDs for each digit, that works out to 3,584 ...