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.
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.