Your eyes aren’t perfect, and just because the speed of light is very fast, that doesn’t mean your eyes can see fast-moving objects clearly. That’s why things that are moving very quickly near you look blurry. Fortunately, we can take advantage of that fact to trick our brains into seeing movement that isn’t there, or even the opposite. The former is how we see the still frames of a movie as motion, and the latter is what makes persistence of vision (PoV) displays possible. If you want to build your own, TN_inventor has a tutorial on how to convert a desktop fan into a large PoV display.
Persistence of vision displays work by relying on the fact that your eyes can’t detect very fast movement. A typical PoV display has LEDs attached to an arm that spins at a high speed. The LEDs are then turned on when they reach a particular position, and then back off right afterwards. The result is that you see a solid light. By carefully controlling when the LEDs are turned on and off, you can form letters, numbers, or even graphics. The resolution is only limited by the density of the LEDs. In this case, TN_inventor is using a desktop fan to spin the arm that has the LEDs.
Building a persistence of vision display like this is surprisingly affordable. To do so, you’ll only need an Arduino Nano, a Hall effect sensor and magnet, some LEDs, a LiPo battery, and an electric motor. The LEDs are mounted to a wood arm, which is attached to the motor’s shaft. The Arduino and battery also need to be placed on that arm, but you must ensure that it’s balanced well. As the arm spins, the Hall effect sensor tells the Arduino every time it completes a revolution. That information can then be used to calculate the RPM of the arm, and to turn the LEDs on and off at the proper times. TN_inventor provides the code to do that, so you can simply input whatever message you want to display.