That port was the reason I learned to solder and how photocells worked. I put two photocells on the paddle pins and wrote programs on my Atari 400 that used the light levels to do things. I think the original project was in Antic Magazine. It was about recreating a sound device used in 50's scifi films. I used it later to detect if the someone was in the hall during a summer program to give us a bit of warning. Fun projects that are pretty damn hard with USB and modern programming.