[Tyler Bletsch] sent us a tip about his new build: a keyboard that redefines “coin-operated.” The Nickelphone can emit square wave tones via a piezo buzzer, but [Tyler] made this 25-key piano as a MIDI keyboard capable of driving a full synthesizer.
He chose an ATMega644 as the brain because it’s Arduino-friendly but has more data pins—32—than the usual ATMega328 chip, which allows him to provide each key with its own pin. Each coin was soldered to its own wire and connects up to a 1MΩ resistor array. Coin-presses are recognized by the simple capacitive sensing technique outlined here, but [Tyler] needed to take advantage of a workaround to accurately detect multiple presses.
Check out [Tyler's] detailed project guide for more information as well as the source code. Check out the video of the Nickelphone after the break, then browse through some other capacitive touch hacks, like the Capacitive Touch Business Card or the Capacitive Touch Game Controller.