Dodgey99 built an Arduino-powered Etch-a-Sketch clock, in which a pair of stepper motors painstakingly draw out the current time. It's got a very low refresh rate, though: the limits of the motors and the Etch-a-Sketch means that it takes more than a minute to display the time, and it needs a couple of minutes' rest between each number. There's a plan to accelerate things with some beefier motors.
I use an Arduino driving two very cheap darlington stepper drivers with 64:1 internally reduced steppers for the drawing. For the rotation I'm using an Easy-Driver driving a Nema 17 stepper.
I also have a DS1307RTC real time clock installed so it always knows the time. Setting the time is a one-off via USB connected to a PC. Once done, you un-tether, and then the RTC keeps the time, for up to a year on the rechargeable battery, or so I'm told…
The code is actually very simple, it's just a pain drawing the numbers!
The G clamp on the back is for a counter balance until I find something more elegant!
The steppers are far too slow to write the time in under a minute so I delay it for a couple of mins between each draw. Mostly to give the very hot motors a while to cool off and to give the etch a sketch a break!
Etch a Sketch clock powered by Arduino
(via JWZ)
