$1 POV Display

Persistence-of-vision, or POV, displays work on the principle that if a light blinks in front of your eye, you perceive it for some amount of time after it’s gone. Blink lights in sequence when moving, and you can create what looks like a static image. “Programmer with a soldering iron” B45i found a deal on ATtiny13 microcontrollers for roughly $.30 each, and was able to create a POV display with one for less than a dollar.

The build itself is quite simple, consisting of an ATtiny13 on perfboard, along with a CR2032 battery for power, a switch, and five 3mm LEDs. B45i also designed a PCB for it, which would presumably put the cost over $1 with shipping, but neatens things up. This little board can attach to a ceiling fan or other rotational device, blinking away to your programmed sequence.

As there’s no hardware to sense the device’s speed, you may need to adjust delay parameters. One could also tweak its visual behavior by adjusting the distance from its axis of rotation, though one would have to watch out for negative effects on the fan's balance.

Code for this device is found here, which is impressively compact at under 100 lines. A “HELLO 123” string can be modified to display whatever you like, and since it’s written in Arduino C, you could use it on a wide variety of microcontrollers and dev boards. B45i explains the code a bit more in the project writeup, namely how he used flagged enums to save processing power on the limited ATtiny13 microcontroller.