I remember this one... the first time I came across it I made like a dozen custom levels, all of which the papers with the codes for them are lost to the ravages of my room.
I had a lot of fun, but didn't remember any of them being particularly nasty... let me go back and see...
Aha! level 14 is pretty tricky. I remember it now, and I also remember trying really hard to find a solution that isn't the one that Demolion posted. I never succeeded, but I at least found one answer. I will say, though, that the result doesn't feel as elegant as any of the other solutions, but it's still a good puzzle nonetheless. 15 is probably my favorite.