Darn - you caught me in another act of laziness.Well, you really ought to do your homework for that... most of the existing systems are described in this very thread, including explanations, pictures, videoclips etc.
I know that several suggestions for certain mechanisms have been made here, but they are not really clear. I'm sure I can scrounge and troll on other threads for more, and for other alternatives. I thought it would be pretty obvious for others to infer that this thread might be a good depository for such information. If there already is such a place, I'd love someone to save me some time and tell me where.