I’m working on a little timekeeper project that works similarly to those word clocks, but it will be wrist-mounted. I have to make this little guy tiny, so I’m going with all SMD and to light the 21 LEDs, I’m using a 5-pin Charlieplexing scheme. To give back to the community (and FWIW), I am writing up my understanding of and methods for implementing Charlieplexing on an Atmel ATmega328 AVR (or Arduino). This first part of the 2-part series goes over the concept of Charlieplexing. The second article will show my homemade quality control methods and Arduino and AVR C code. I hope someone finds the write-up useful!