The basic principle is rather simple: you take a row of LEDs, spin it very fast while switching the LEDs on and off at very precise timings, carefully synchronized with the location of the LED row. The motion is quick enough that our eyes cannot detect it, and the illusion that is created is of a 2D image composed of the circular paths created by the LEDs.
Many people have mounted such LED rows on HDD spindles, mostly for creating clocks, such as this one.
In other variations of this basic idea, the rotation axis can be parallel to the row of LEDs to create a cylindrical display, and going from cylindrical to spherical is a simple matter of using an arc rather than a straight row of LEDs. People have used the spherical version mostly for displaying of globes.
Yes. Everybody seems to be building those recently. I got my inspiration from this YouTube clip. So why build another one, and what's so different about mine?
First, I wanted my globe to be rather big (60cm in diameter), spin fast for a smooth image (up to 50 revolutions per second), have high resolution (256 LEDs) and be very versatile in terms of what it can display - one could easily load complex animations for it, and change them frequently (more on that later). Second, while taking no credit whatsoever for the idea, the build is still really fun, and coming up with elegant solutions to the various problems that arise still needs a good amount of innovation.
To make it more interesting, the display is actually comprised of two layers - a sphere made of white LEDs, and a cylinder, of slightly bigger diameter made of RGB LEDs surrounding it. That would enable displaying an animated black and white globe with a colorful text or animation around it.
In the next series of posts, I intend to document the process of building this installation. At the time of writing this, it is still a work in progress, and I intend to update the posts every once in a while when I'm having some interesting progress. Don't hold your breaths, I'm investing only a few hours a week on this project, so it takes a lot of time and patience.
Next: POV Globe - Part II - Mechanics