[Michael] built his own clone of the popular MaKey MaKey Kickstarter project. His implementation uses an ATMega328 and the V-USB stack to connect as a USB Human Interface Device. He was showing it off at Toorcamp wired up to a banana piano, which captured the interest of kids and adults alike.
The digital inputs are pulled to ground with a large (10 Mohm) resistance. The user holds a supply voltage in one hand and completes the circuit by touching a conductive object like a banana, which is connected to a digital input of the ATMega328. Since the internal resistance across your body is typically around 1 Mohm, this pulls the input high and corresponds to a key being pressed on a normal keyboard.
We featured banana pianos before, and it’s a great demo of the interfaces that can be built with this project. This implementation is very simple, and works well if your internal resistance is low enough. [Michael] taught a workshop at Toorcamp to show people how to build their own. He has found that the ‘magic’ of playing music with bananas is a great way to get children interested in electronics.