A solonoid could be used to do that, or any electromagnet that pushes the pin out of the way for a split second letting it unwind one "click". Also, grandfather clocks work that way, letting the weights slowly descend as they power the pendulum (mechanical of course, not electric).
And your diagram is very cute, but pretty much accurate. You didn't show the spring or whatever tensioning device you would use to keep it sliding to the right, but I think you get what I was saying.
Edited by Lou Apo, 02 July 2011 - 05:03 PM.