Building a cool microcontroller-based project, and wish you had just one or two more analog inputs? Or perhaps you are trying to make something using the tiniest microcontroller possible, and don’t have any analog ports in the first place? Well, it turns out that you can use a couple of digital I/O pins and a single capacitor to build a crude analog to digital converters (ADC). Let’s Make Robots user Telefox has a nice overview about building a Low-cost ADC using only Digital I/O. The technique isn’t new, but it is a cool idea, and building your own would be a great way to learn how ADCs work.
If you are itching to squeeze as much performance out of your tinly microcontroller as possible, I second his recommendation of Microchip’s Tips ‘n Tricks manual. It’s a neat guide that I read when I was getting started with microcontrollers, and learned all sorts of cool tips from- including the technique that is now referred to as Charlieplexing.