From Trammell Hudson's Projects
These are from a set of artistic PCBs that I designed with 20 Charlieplexed LEDs and an ATtiny85. I've ordered samples from DirtyPCBs to see how they work. They can be powered by coin-cell batteries, JST connectors or USB.
The schematic is the same for all of the constellation boards. 5 of the 6 I/O pins on the ATtiny85 are used, allowing 5*4 = 20 LEDs to be Charlieplexed. Six pins could be used if we never wanted to reprogram the chip or if we didn't want to use USB, but the pullup on PB3 would interfere with the charlieplexing. And since we're using the !RESET pin as an output, we must have some sort of non-ISP based bootloader on the chip and emulating USB (with a modified version of the Adafruit Gema V-USB bootloader) is a convenient way to program the chips it.