The bad thing about this type of hack is that now [Tomek Dubrownik] needs to cut a hole in his desk to house the thing. He got this military grade trackball working over USB. It’s old, and could be used as a blunt weapon. But as the video shows it still makes a great input device.
He found the hardware on Allegro – a Polish auction site similar to eBay — for just $20. The original circuitry didn’t make a lot of sense, but a bit of probing with the old oscilloscope let him establish connections to the encoders which are read by some TI 54xx parts. Apparently they use the same logic as 7400 parts but are military grade. He chose a ATmega32u4 development board for his replacement control board. That chip has native USB support so the rest is just a matter of passing data like an HID input device. His code even lets him use those pushbuttons to toggle between cursor movement and window scrolling.
[Tomek] translated his post into English after some prompting by friends at the Warsaw Hackerspace. Here’s the original in Polish if you’re interested.