Plotclock combines Arduino and 3D printing for a clock that writes and rewrites the time every minute.
It may not be the prettiest timepiece out there, but the Plotclock by designer and programmer Johannes Heberlein, AKA Joo, of Nuremberg, Germany is certainly mesmerising.
Crafted using a 3D printer, an Arduino board and servo motors, the clock is based on a very simple idea: writing out the time in digital format. As time marches on, and the clock ticks over the minutes, the servos control an arm holding a dry-erase marker, which writes out the four digits of the time to a 24-hour clock. Each minute, the arm will return to the top right corner, grab an eraser and rub out the digits so it has a clean slate for the next minute.
It's actually rather clever. Two of the servos move the arm holding the marker, while the third lifts it up. The eraser cap is slightly bigger than the marker, which means the marker can drop in and out easily, sliding to erase.
Instructions for building one of your own are available for free on Thingiverse, with Arduino files available for download on GitHub.