This is a working fruit machine which is made almost entirely from K'Nex. It is so large that it will have to remain in the room (which has a concrete floor) for ever.
The machine pays out in balls, the size of the win being the lowest digit which appears on the three reels (so any combination which contains a zero is a loser).
During the building of it much was learned about K'Nex (Elap had discovered it at a car boot sale only a couple of months before he started) and some PDFs on K'Nex have been produced (see below).
The non-K'Nex bits of the fruit machine are the reel symbols (produced using Microsoft Word and then laminated) and the balls (meant for a child's play pit - Elap didn't have anywhere near enough K'Nex balls, and they are expensive). Also, a weight has been made from a lead-filled K'Nex cage because there were no K'Nex components which were heavy enough.
It probably took about 150 hours to build, spread over two years, and was made up as it was built - there were no plans, but Elap collects old mechanical slot machines and was familiar with the principles. There were a few areas where a redesign was necessary (for example, the handle kept falling to pieces when pulled, and the original pawl and ratchet mechanism was clunky and didn't work very well), but there wasn't a lot of reworking overall.
The return is 85.94% - quite generous for a fruit machine!
The K'Nex pieces were acquired from car boot sales (mainly) and eBay.
Here are some documents relating to the construction of the machine, and some K'Nex tips.