Update: my boss told me the technical name for this component in a clock is the "escapement."
I showed up at work the other day and I found out I left my laptop's power supply at home. Coincidentally, that day Michael's power supply was broken. So Michael pulled out the office toys. I had completely forgotten our office contains K'NEX, kind of a plastic cross between legos and erector sets. A remarkably capable toy with an extremely simple set of pieces.
I had tried to make a clock with an erector set previously and had met with dramatic failure. I had even tried briefly with K'NEX but it never made it past the design stage. But this time I was determined to make it happen. The fully-enclosed wind-up spring piece was the main inspiration -- it eliminated the weights that made the erector set implementation so difficult to work with.
So I built my clock:
The two main parts are the wheel on the left and the pendulum on the right. The wheel is attached directly (no gearing) to the wind-up spring. The wheel has four arms that are stopped by the pendulum. The pendulum has two points of contact with the wheel. At all possible states of the pendulum either the top part or the bottom part (but not both) will be blocking future progress of the wheel, thus ensuring that the wheel cannot advance more than one quarter turn for each complete back-and-forth swing of the pendulum. The wheel has rounded (well, angled) end pieces so that when the edge hits the lower contact point of the pendulum, it actually helps push the pendulum away, thus ensuring that as long as the wheel has the force to spin, the pendulum will not run out of inertia through friction and catching on the wheel.
The wheel runs about one quarter spin per second (a little fast but *shrug*), and runs for a total of about a minute when the spring is wound all the way. Click here for a movie of it in operation!