The Arduino family is well-known for being flexible and welcoming to programming and electronics beginners, but there's one particular area where it gets sadly overlooked: audio. There's a belief that the eight-bit 16MHz nature of the microcontrollers that power mainstream Arduinos means that they simply aren't up to the task of doing anything complicated or clever with sound - a belief that Sam Brown has proved entirely wrong. Sam has published details of a guitar effects pedal he built using a bare ATmega328 microcontroller loaded with the Arduino bootloader - the equivalent of our Breadboard Arduino Compatible Kit - and a clever resistor ladder to act as a digital-to-analogue converter, something the Arduino normally lacks. The code, published under the GNU General Public Licence, would also work on an Arduino Uno and proves that the mainstream Arduinos shouldn't be discounted for audio-based projects. More information is available from Sam's Reddit thread.
Since I started designing it some interesting commercial solutions have come around for using an Arduino in guitar effects, but this still uses way fewer parts and runs on an Uno or any other Arduino with an ATmega 168 or 328. It boosts the guitar signal to an appropriate level for the the onboard analogue to digital converter, and outputs through a simple passive resistor ladder DAC on PORTB (digital pins 8-13) for 6 bit resolution.