MAKE contributor Andy has created a great tutorial to introduce you to the utility of “Charlieplexing,” a method for controlling multiple LEDs without the use of multiple microcontroller pins. With Charlieplexing you can turn on or off one LED at a time. To light more than one LED at a time, you can scan the LEDs by turning a sequence of them on and off really fast.
Charlieplexing takes advantage of the fact that LEDs are diodes: Current flows in only one direction through an LED. Connect two LEDs in parallel with each but with opposite polarity so that only one conducts (lights up) at a time and that is the basis of Charlieplexing.
Come on over to MAKE: Projects and check out Andy’s work.