My clock has an Arduino UNO heart and a olimex mod-RTC clock module to keep the time. It features a nipkow disk with 10 holes placed in 5 rows so that each row is written twice per revolution. Hence, the disk produces a 40Hz picture when it spins at about 20rps.
The clock has a red top display and a blue bottom display. Each display is comprised of 3 segments with 5 x 8 pixels. In total, the clock has 10 x 24 pixels that can be adressed individually. A custom Arduino library allows comfortable access to the nipkow display.
The arduino cannot adjust the speed of the disk, but it gets a signal from an infrared sensor once each revolution. And the code knows the target number of steps per revolution and it can adjust its program speed when it is too slow or to fast.
Arduino code and python code to create the letter data structure is in the files below.