Gear Clock
I can't take credit for the design of this one.  I bought the
gear as a clock a few years ago.  It was mounted on a frame
and had one of those cheap clock units running it.  The
gear is designed to be driven by the minute-hand shaft of
the clock mechanism.  

The problem was that it kept breaking.  I think the problem
was that the clock mechanism wasn't strong enough to hold
the weight of the gear.
I bought a stepper motor with four input wires.  A PIC
16F676 microcontroller functions as the timer.  Every
three minutes, four output pins are driven either high
or low for half a second.  These are each connected to
a pair of transistors - one PNP, one NPN to drive the
motor with more current.  The stepper motor rotates 18
degrees each time.
C Code for Gear Clock
The circuit and motor are mounted in a piece of PVC
tubing.  I drilled a hole from the back of a closet so that
only the shaft of the motor sticks out of the wall.  The
original little gear is mounted on the shaft and the big
gear hangs from it.  A pack of 8 rechargeable AA
batteries in the closet power it and last several weeks.
PIC Projects