The mechanical bit is quite simple. The clock functions in a similar fashion to a stepper motor, in the sense that you charge an electric coil to get the mechanical bit to move forward a precise amount. In this case, each firing moves the second hand one second position forward (and makes the familiar tick noise). To ‘fire’ the electric coil, you simply put a voltage across it. The only complicated bit is that you actually have to reverse this voltage to advance the clock to the next step.

A do-it-yourself GPS receiver