As I dug around this box, I found an old CD Rom drive and power supply. The thought struck me that I could use the ejecting tray of the CD Rom as a solenoid to push the trigger mechanism of some sort of physical contraption. But then I had a bootstrapping problem – what can I use to push the eject button of the CD Rom on schedule?

After some more thought, I realized that I could just use my spare (working) computer as the basis of the cat feeder. It’s also my home’s Subversion source control server – a rare mix of server workloads indeed! It has a CD Rom drive, so I could just use software to open and close it.

Water flows out of the jug as long as the water level is below the hole at the bottom. When water flows out, the air pressure in jug decreases until it sucks in some air to equalize. When the water level covers the hole, though, the air pressure can no longer equalize, so the water flow stops.

When the cats drink the water level down a bit, the jug can once again equalize its air pressure, and lets more water out.