This is my little project: clock made of old broken hard disk.
The project involves controlling HDD motor (3 wire brushless dc electric type BLDC) using open loop control without feedback, and using back electric motor force feedback to synchronize engine pulses with optimal point in time for them.
Digits are displayed by flashing 1W power LED-s behind rotating disc with painted digits. Each flash is circa 150 microseconds. Due to POV (persistence of vision) effect the digits look very stable for the user.
I am not using any special controller. Only Arduino, H-bridges for driving motor coils, and circuit for obtaining BEMF (back elecric motor force) from each coil and converting it to digital (high/low) signal to drive Arduino interrupt.
I have designed my own custom board which is a modified Arduino (something between UNO and PRO NANO) + added additional parts including real time clock. It is designed to fit on the back of a standard 3,5" HDD.
Software is very dependent on interrupts. I am using not only officially available interrupts (attachInterrupt) but also additional libraries to use all available timers and interrupts.