Evil Mad Scientist Laboratories received an email from someone who wanted to hack their Peggy kit. This LED matrix kit has been featured on Hack a Day in the past, and provides hardware to set up a 625 LED matrix.
This user built an external array of LEDs that they wanted to drive with the Peggy hardware. There are a few options for making this happen. The first one is to run wires from each of the 625 LED footprints on the board. Each has an anode and cathode, so that makes for a total of 1250 wires to run. It turns out that people have actually done this with the Peggy in the past, using very fine wire.
EMSL suggests wiring the rows and columns instead. This way, only one wire is needed for each row and column, allowing a 25×25 LED grid to use 50 wires instead of 1250. They also explain how to expand the grid to a 30×20 LED matrix. It’s a good explanation of how the kit works, and how it can be expanded.