Stian Eikeland bio photo

Stian Eikeland

Developer, hacker, techno-foodie, diver, hobby-photographer. Does consultancy work from own company. Lives in Bergen, Norway.

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Capacitive touch banana piano in Clojure / Overtone.

Making of a banana piano, with a bit of swedish jazz from my co-worker Håvard Sørbø

This weekend my niece-in-law is staying over, and to maintain my image as the crazy scientist uncle I've planned to make a banana piano (and lots of weird ice creams). In clojure there's a pretty cool programmable audio environment called Overtone. Overtone features a decent sampled piano, and I'm thinking this could be a great basis for a banana-piano.

Fun fact: a full piano requires about 11 kg bananas - at $3 per kg that's still way cheaper (and lighter) than a Steinway.

There's a couple of ways we can make bananas act as tangents, one of them is to use the bananas as capacitive touch sensors. Using a nice little hack it's possible to do this using regular digital pins on a microcontroller. The hack is (afaik) originally from Mario Becker, Fraunhofer IGD, 2007 (website dead). Check out the article on capacitive sensors over at

  • Hook up a pin to something.
  • Disable interrupts.
  • Set pin as input.
  • Active pull-up on pin.
  • Count cycles until pin is high.
  • Enable interrupts
  • Set pin as output, low.

The number of cycles needed before the pin goes high will depend on the capacitance of whatever connected to the pin. A banana might require 8 cycles before the pin goes high, while a touched banana might require 15.

We can connect a number of bananas to a microcontroller, iterate over the bananas, read their cycle count before banana-pin goes high, and then transmit a message over a serial connection indicating the pin number if this cycle count is above a given threshold.

const int PORTS[8] = { 2, 3, 4, 5, 6, 7, 8, 9 };
const int THRESHOLDS[8] = { 13, 13, 13, 13, 13, 13, 13, 13 };

bool touched[8];

uint8_t readCapacitivePin(int pinToMeasure) {
  volatile uint8_t* port;
  volatile uint8_t* ddr;
  volatile uint8_t* pin;
  byte bitmask;

  port = portOutputRegister(digitalPinToPort(pinToMeasure));
  ddr = portModeRegister(digitalPinToPort(pinToMeasure));
  bitmask = digitalPinToBitMask(pinToMeasure);
  pin = portInputRegister(digitalPinToPort(pinToMeasure));

  // Discharge the pin first by setting it low and output
  *port &= ~(bitmask);
  *ddr  |= bitmask;

  // Prevent the timer IRQ from disturbing our measurement

  // Make the pin an input with the internal pull-up on
  *ddr &= ~(bitmask);
  *port |= bitmask;

  // Now see how long the pin to get pulled up. This manual unrolling of the loop
  // decreases the number of hardware cycles between each read of the pin,
  // thus increasing sensitivity.

  uint8_t cycles = 17;
       if (*pin & bitmask) { cycles =  0;}
  else if (*pin & bitmask) { cycles =  1;}
  else if (*pin & bitmask) { cycles =  2;}
  else if (*pin & bitmask) { cycles =  3;}
  else if (*pin & bitmask) { cycles =  4;}
  else if (*pin & bitmask) { cycles =  5;}
  else if (*pin & bitmask) { cycles =  6;}
  else if (*pin & bitmask) { cycles =  7;}
  else if (*pin & bitmask) { cycles =  8;}
  else if (*pin & bitmask) { cycles =  9;}
  else if (*pin & bitmask) { cycles = 10;}
  else if (*pin & bitmask) { cycles = 11;}
  else if (*pin & bitmask) { cycles = 12;}
  else if (*pin & bitmask) { cycles = 13;}
  else if (*pin & bitmask) { cycles = 14;}
  else if (*pin & bitmask) { cycles = 15;}
  else if (*pin & bitmask) { cycles = 16;}

  // End of timing-critical section

  // Discharge the pin again by setting it low and output
  *port &= ~(bitmask);
  *ddr  |= bitmask;

  return cycles;

void setup() {

void handlePort(int index) {
  int cycles = readCapacitivePin(PORTS[index]);

  if (!touched[index] && cycles >= THRESHOLDS[index]) {
    touched[index] = true;

  if (touched[index] && cycles < THRESHOLDS[index]) {
    touched[index] = false;

void loop() {
  for (int i = 0; i < 8; i++) {
  delay(30); // cheap-ass debounce..

Your computer must be grounded for this to work reliably, and it also helps to add a ground plane under the bananas (see video).

When touching a banana, the microcontroller will transmit a character from 0 to 7, we can then receive this value in clojure using a serial-port library, turn it into an int, map the value to a scale to get a note. And play this using an instrument - in this case - the excellent sampled piano available in Overtone.

(ns musikk.core
  (:require [serial-port :as serial]
            [ :refer :all]
            [overtone.inst.sampled-piano :refer :all]))

(def port (serial/open "/dev/tty.usbserial-A800F185" 57600))

(defn chr->int [c]
  (-> (char c)

(defn banana-touch [input]
  (let [index    (chr->int input)
        my-scale (scale :C4 :major)
        note     (nth my-scale index)]
    (sampled-piano :note note :sustain 0.2)))

(serial/on-byte port banana-touch)

Now enjoy and experiment with scales and different instruments in Overtone - your efforts will not be fruitless. Maybe I'll make a broccoli theremin next time my niece visits. :)


Since a few people have asked, here's a wiring diagram. It's super easy, only wires and no extra components. The ground plane (alu foil) is optional. If you're having problems in very noisy environments you can put a 1nF capacitor in line with the banana. I used an arduino nano, but any atmega328 based arduino can be used (nano, uno, etc). (And probably all others with minor modifications). Any wires you can connect to a banana and an arduino works. My arduino had male headers, I used 4p dupont female-female cables, which i broke out to 1p male-male for each individual banana. These come in all shapes, genders and lengths on ebay for very little money.

Clojure libraries used: [serial-port "1.1.2"] and [overtone "0.9.1"]

Wiring setup for arduino nano.