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This project consist of an 8×8 LED grid controlled by the Arduino. The result is a particle effect, a particle that bounces around the LED matrix. But the arduino only has 13 outputs, so you can only control 13 LEDs? Well, the author discovered that trough Charlieplexing which is a complicated mode of wiring LED’s you can actually drive more LED’s with fewer outputs.

Arduino LED particle system: [Link]