Unfortunately, he used reed relays to determine when the runner was at the end. After a few days the magnet fell off thanks to it being stuck with the wrong glue and the motor ran past the end and jammed with power applied. Inevitably the armature caught fire after a couple of minutes [1] and set fire to the curtains.
Cue visit one evening by the fire bridgade and his entire bedroom being burned out and angry parents and insurers...
[1] motors = dangerous ... http://www.youtube.com/watch?v=0o0SdmAwA7o
Edit: other misadventures we produced together (really need to put these on a web site as we have photos): lawn mower to stand up personal hovercraft (two broken fingers), radio controlled hovercraft bodged from above (sunk in middle of lake), rocket propelled grenade launcher (got bollocking from the police), RC plane missile launcher (set fire to grass), portable BBC micro (electric shock + fire), coil gun (remove metal swarf from finger at hospital) + more I can't remember off the top of my head.
Probably be in prison if we did it now. Hardware hacker culture has been going for a while now...