(this updated schematic was made with upverter.com)
If I want to create a ’t ext’-display like I described before, I will have to be able to control a lot of LEDs. The display (10×20) consists of 200 pixels (letters) while the microcontroller I want to use (Arduino) has only 14 I/0 pins. The amount of individually addressable pixels could be reduced by using separate ‘words’ instead of ‘pixels’ but that doesn’t solve the problem yet because there are still 26 different words to make combinations with.
This is a common problem when dealing with electronics and there exist some solutions for this. The first I want to explore is called Charlieplexing. This method utilizes the tri-state logic capabilities (+V, GND, or
“high impedance”) of microcontrollers in order to gain efficiency over traditional multiplexing. The technique allows you to drive N*(N-1) LEDs from N pins. More information about this technique can be found on Wikipedia or Instructables.