Lite Brite LED clock

I built the Lite Brite clock and made an Instructable out of it. It's currently on their front page.

How do you control so many LED's with an Uno? I thought it was limited to how many header pins were on it?

Can't be sure but it looks like it's hooked up as a grid, suing X,Y cordinates. So each coulmna and row needs one digital header, meaning 15 digital pins required.

Riley had it right. Each row of LED's have common cathode, each column of LED's have common anodes. To light an LED up, you set the row LOW and the column HIGH. The only drawback to this method is you can only control one LED at a time. Even though there may be 42 LED's lit to make the clock read 12:58, it's persistence of vision and fast switching that gives a steady, flicker free display. I estimate the refresh rate of the clock display is around 300-400 fps.