In your living room, the big display is what you want. But in an embedded project, often less is more. We think [bobricius] will agree since he submitted a tiny 4×5 LED display into our square inch challenge. The board features an ATtiny CPU and twenty SMD LEDs in a nice grid. You can see them in action, scrolling to some disco music in the video below.
There is plenty of room left in the CPU for bigger text strings — the flash memory is just over 10% full. A little side-mounted header makes it easy to program the chip if you want to change anything.
To save pins, the display using Charlieplexing, the same technique he’s used before to drive 110 pins with 11 I/O pins. In general, for a given number of I/O pins (call it N), you can drive N²-N LEDs. In this case, he’s using 5 lines to drive 20 LEDs. Of course, the LEDs don’t stay on, but the chip drives them fast enough that you don’t know the difference.
We have to admit, with that much code left, we wondered if you couldn’t do something interesting to allow them to be controlled via some serial protocol and maybe even daisy chain them together to make larger displays. But we suppose he’s out of pins.