While browsing through Alan Parekh's Web page of hacked gadgets, I ran across a neat persistence of vision (POV) project created by Jason Hotchkiss of the UK. Hotchkiss purchased a few dead hard drives from eBay to use in POV projects, one of which ended up as a digital clock.
He disassembled the hard drive and removed the electronics and the platter, but he left the spindle motor in place. The platter was replaced with a round piece of single-sided FR4 PCB board that is etched with the digits 0 through 9 and a colon. Since the PCB is single-sided, the numbers are translucent. Hotchkiss was able to fit another PCB with surface-mount LEDs on it in the space between the new platter and the aluminum drive housing.
The other main piece of the project is a custom PCB that uses an ATMega328 with an Arduino bootloader. Also on the PCB is a brushless motor driver from ST, a real-time clock chip, a transistor array for driving the LEDs, and other miscellaneous components.
The ST part that Hotchkiss used is a clever one in that it doesn't need an encoder to tell motor position. Instead, it looks at the back EMF from the undriven coils to tell when the rotor has passed. It also incorporates an amplifier that allows external control over rpm, although in this design, the rpm is fixed by connecting a constant voltage to the control pin.
In operation, the ATMega turns on the spindle motor and synchronizes itself to the platter position by means of a white square on the bottom of the platter and an IR detector. Underneath the platter, the LEDs are arranged in a semicircular fashion, with the same spacing as the digits on the platter. The software times the flashes of the LEDs so they only turn on when the appropriate digit is directly above the LED. In this way, the light from the LED shines through the PCB, and the digit is visible. The slow response of the human eye does the rest, turning these flashes into softly glowing green digits.
A couple of things occur to me that would improve this gadget. The first would be to be able to change the colors of the LEDs. The second would be to reuse the original platter by laser cutting the digits out of it. Not only would that look great, but it would probably be a lot quieter, since it would be better balanced and smoother around the edges. (Hotchkiss's replacement platter was cut by a hole saw, and there is noticeable noise when it spins up. I don't think you'd have this clock on your nightstand.) Hotchkiss supplies both the schematic and the firmware for his project via github, so maybe you can give it a try yourself.