Bright LEDs are cheap today. So many new projects with large amount of LEDs can be done. But there is one problem - large amount of LEDs consume large amount of energy. This is a project that implements special technique that allows driving efficiently hundreds of LEDs. This is a large PCB where 25×25 (625 total) LED grid is soldered. Resistors and transistors are positioned around the edges of message board. That entire grid is controlled by AVR Atmega164 microcontroller.

Displaying technique works as follows: microcontroller scans all display row by row through multiplexer. Scanning speed is fast enough to see all non flickering images. Project is published as open hardware. PCB and AVR-GCC project files are free to download. But if you want it quicker - you can always get a kit.