Monthly Archives: December 2015

DIY Weekend! Charlieplexing with LEDs

So around the corner was the Christmas eve coming off on a Friday and it called for another long lazy weekend. I came across this wonderful, wonderful DIY gem from one Mr. Alessandro Matera, called the KeyChainino. The keychainino is a very nice and simple DIY keychain piece but all the design thinking that Alessandro has put into this small thing, this makes for a really cool educational tool. Underneath the Keychainino is driven by an ATTiny84 micro-controller to drive a bunch of LEDs. So what’s special? I am amazed by the various different design hacks this product has. Here are some

  1. For starters, Its tiny and fits as a key chain!
  2. The design idea behind driving LEDs is called Charlieplexing where you can have minimal pins to drive a matrix of LEDs – Its magic! Conventionally to drive a 4×3 matrix of LEDs you’d need 7 pins(4+3), this technique uses just 4. The keychainino figures 30 LEDs driven by just 6 pins.
  3. It’s Arduino compatible – so quick and easy to program

And most importantly it can be DIY thingy – spend over a day and you can do it yourself. So I decided to spend a day trying my hands on implementing this.

I had a bunch of surface mount LEDs lying around and a zero PCB and an Arduino Mini Pro lying around. Some reading that I’d suggest you do.

  1. Soldering lessons from PACE – Believe you me, you will fall in love with soldering awesomeness
  2. Read about Charlieplexing here
  3. I used the Charlieplexing circuit from – This makes for a nice matrix based layout – something that you can find it easy to solder on the zero PCB.
  4. Arduino and some code from Alessandro.

At the end of this I realized this can be a great exercise at soldering as well. I am also inspired now to learn KiCAD to make a home made DIY PCB fit all this. I will post my learning on this later in another post.

I failed at soldering the smaller LED pieces and ruined some connections initially. On the second try, I missed adding the resistors to the LEDs but again hacked the same layout – was a bit messy but worth the effort!

So here’s a DIY display from LEDs – Quick and dirty!

Happy Making!!