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MIDI Sprout Code


Below you will find the codebase for the MIDI Sprout.  You can easily edit the code using the Arduino IDE and upload to your sprout.  You can change the note range, apply a musical scale to the data, change the MIDI CC controller number, and even build your own change detection algorithms!

Download a zip file containing the Arduino sketch .ino files: MIDI Psychogalvanometer 328p v021

LEDFader library from jgillick, be sure to add this to your Arduino Libraries from the gitHub repository.

The onboard ATMEGA328 chip is clocked at 16Mhz, just like an Arduino Uno.  One easy way to reprogram your Sprout would be to upload your new Code to an Arduino Uno, carefully remove the IC from the Uno board and exchange with the chip on your Sprout!

The MIDI Sprout circuit board includes pads for ICSP (In Circuit Serial Programming), to facilitate copying code to the ATMEGA328 chip when using a programmer.

Below is the full codebase concatenated from the .ino files:

--- Code: ---MIDI_PsychoGalvanometer v021
Accepts pulse inputs from a Galvanic Conductance sensor
consisting of a 555 timer set as an astablemultivibrator and two electrodes.
Through sampling pulse widths and identifying fluctuations, MIDI note and control messages
are generated.  Features include Threshold, Scaling, Control Number, and Control Voltage
using PWM through an RC Low Pass filter.

#include <LEDFader.h> //manage LEDs without delay() jgillick/arduino-LEDFader https://github.com/jgillick/arduino-LEDFader.git

//set scaled values, sorted array, first element scale length
int scaleMajor[]  = {7,1, 3, 5, 6, 8, 10, 12};
int scaleDiaMinor[]  = {7,1, 3, 4, 6, 8, 9, 11};
int scaleIndian[]  = {7,1, 2, 2, 5, 6, 9, 11};
int scaleMinor[]  = {7,1, 3, 4, 6, 8, 9, 11};
int scaleChrom[] = {12,1,2,3,4,5,6,7,8,9,10,11,12};
int *scaleSelect = scaleChrom; //initialize scaling
int root = 0; //initialize for root

const byte interruptPin = INT0; //galvanometer input
const byte knobPin = A0; //knob analog input

const byte samplesize = 10; //set sample array size
const byte analysize = samplesize - 1;  //trim for analysis array

const byte polyphony = 5; //above 8 notes may run out of ram
byte channel = 1;  //setting channel to 11 or 12 often helps simply computer midi routing setups
int noteMin = 36; //C2  - keyboard note minimum
int noteMax = 96; //C7  - keyboard note maximum
byte QY8= 0;  //sends each note out chan 1-4, for use with General MIDI like Yamaha QY8 sequencer
byte controlNumber = 80; //set to mappable control, low values may interfere with other soft synth controls!!
byte controlVoltage = 1; //output PWM CV on controlLED, pin 17, PB3, digital 11 *lowpass filter
long batteryLimit = 3000; //voltage check minimum, 3.0~2.7V under load; causes lightshow to turn off (save power)
byte checkBat = 1;

volatile unsigned long microseconds; //sampling timer
volatile byte index = 0;
volatile unsigned long samples[samplesize];

float threshold = 2.3;  //change threshold multiplier
float threshMin = 1.61; //scaling threshold min
float threshMax = 3.71; //scaling threshold max
float knobMin = 1;
float knobMax = 1023;

unsigned long previousMillis = 0;
unsigned long currentMillis = 1;
unsigned long batteryCheck = 0; //battery check delay timer
#define LED_NUM 6
LEDFader leds[LED_NUM] = { // 6 LEDs (perhaps 2 RGB LEDs)
  LEDFader(11)  //Control Voltage output or controlLED
int ledNums[LED_NUM] = {3,5,6,9,10,11};
byte controlLED = 5; //array index of control LED (CV out)
byte noteLEDs = 1;  //performs lightshow set at noteOn event

typedef struct _MIDImessage { //build structure for Note and Control MIDImessages
  unsigned int type;
  int value;
  int velocity;
  long duration;
  long period;
  int channel;
MIDImessage noteArray[polyphony]; //manage MIDImessage data as an array with size polyphony
int noteIndex = 0;
MIDImessage controlMessage; //manage MIDImessage data for Control Message (CV out)

void setup()
  pinMode(knobPin, INPUT);
  randomSeed(analogRead(0)); //seed for QY8
  Serial.begin(31250);  //initialize at MIDI rate
  controlMessage.value = 0;  //begin CV at 0
  //MIDIpanic(); //dont panic, unless you are sure it is nessisary
  checkBattery(); // shut off lightshow if power is too low
  if(noteLEDs) bootLightshow(); //a light show to display on system boot
  attachInterrupt(interruptPin, sample, RISING);  //begin sampling from interrupt

void loop()
  currentMillis = millis();   //manage time
  checkBattery(); //on low power, shutoff lightShow, continue MIDI operation
  checkKnob(); //check knob value
  if(index >= samplesize)  { analyzeSample(); }  //if samples array full, also checked in analyzeSample(), call sample analysis   
  checkNote();  //turn off expired notes
  checkControl();  //update control value
  //checkButton();  //not implemented in this build
  checkLED();  //LED management without delay()
  previousMillis = currentMillis;   //manage time

void setNote(int value, int velocity, long duration, int notechannel)
  //find available note in array (velocity = 0);
  for(int i=0;i<polyphony;i++){
      //if velocity is 0, replace note in array
      noteArray[i].type = 0;
      noteArray[i].value = value;
      noteArray[i].velocity = velocity;
      noteArray[i].duration = currentMillis + duration;
      noteArray[i].channel = notechannel;
        if(QY8) { midiSerial(144, notechannel, value, velocity); }
        else { midiSerial(144, channel, value, velocity); }

          for(byte j=0; j<(LED_NUM-1); j++) {   //find available LED and set
            if(!leds[j].is_fading()) { rampUp(i, 255, duration);  break; }


void setControl(int type, int value, int velocity, long duration)
  controlMessage.type = type;
  controlMessage.value = value;
  controlMessage.velocity = velocity;
  controlMessage.period = duration;
  controlMessage.duration = currentMillis + duration; //schedule for update cycle

void checkControl()
  //need to make this a smooth slide transition, using high precision
  //distance is current minus goal
  signed int distance =  controlMessage.velocity - controlMessage.value;
  //if still sliding
  if(distance != 0) {
    //check timing
    if(currentMillis>controlMessage.duration) { //and duration expired
        controlMessage.duration = currentMillis + controlMessage.period; //extend duration
        //update value
       if(distance > 0) { controlMessage.value += 1; } else { controlMessage.value -=1; }
       //send MIDI control message after ramp duration expires, on each increment
       midiSerial(176, channel, controlMessage.type, controlMessage.value);
        //send out control voltage message on pin 17, PB3, digital 11
        if(controlVoltage) { if(distance > 0) { rampUp(controlLED, map(controlMessage.value, 0, 127, 0 , 255), 5); }
                                            else { rampDown(controlLED, map(controlMessage.value, 0, 127, 0 , 255), 5); }

void checkNote()
  for (int i = 0;i<polyphony;i++) {
    if(noteArray[i].velocity) {
      if (noteArray[i].duration <= currentMillis) {
        //send noteOff for all notes with expired duration   
          if(QY8) { midiSerial(144, noteArray[i].channel, noteArray[i].value, 0); }
          else { midiSerial(144, channel, noteArray[i].value, 0); }
        noteArray[i].velocity = 0;
        rampDown(i, 0, 225);


void MIDIpanic()
  //120 - all sound off
  //123 - All Notes off
 // midiSerial(21, panicChannel, 123, 0); //123 kill all notes
  //brute force all notes Off
  for(byte i=1;i<128;i++) {
    delay(1); //don't choke on note offs!
    midiSerial(144, channel, i, 0); //clear notes on main channel

       if(QY8){ //clear on all four channels
         for(byte k=1;k<5;k++) {
          delay(1); //don't choke on note offs!
          midiSerial(144, k, i, 0);

void midiSerial(int type, int channel, int data1, int data2) {

  cli(); //kill interrupts, probably unnessisary
    //  Note type = 144
    //  Control type = 176
    // remove MSBs on data
data1 &= 0x7F;  //number
data2 &= 0x7F;  //velocity

byte statusbyte = (type | ((channel-1) & 0x0F));

  sei(); //enable interrupts

void checkKnob() {
  //float knobValue
  threshold = analogRead(knobPin); 
  //set threshold to knobValue mapping
  threshold = mapfloat(threshold, knobMin, knobMax, threshMin, threshMax);

void knobMode() {
  //scroll through menus and select values using only a single knob
  //keep dreamin' kid,

void rampUp(int ledPin, int value, int time) {
LEDFader *led = &leds[ledPin];
// led->set_value(0);
  led->fade(value, time); 

void rampDown(int ledPin, int value, int time) {     
  LEDFader *led = &leds[ledPin];
 // led->set_value(255); //turn on
  led->fade(value, time); //fade out

void checkLED(){
//iterate through LED array and call update 
 for (byte i = 0; i < LED_NUM; i++) {
    LEDFader *led = &leds[i];

void checkButton() {
  //no button in this build...

long readVcc() {  //https://code.google.com/p/tinkerit/wiki/SecretVoltmeter
  long result;
  // Read 1.1V reference against AVcc
  ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
  delay(2); // Wait for Vref to settle
  ADCSRA |= _BV(ADSC); // Convert
  while (bit_is_set(ADCSRA,ADSC));
  result = ADCL;
  result |= ADCH<<8;
  result = 1126400L / result; // Back-calculate AVcc in mV
  return result;

void checkBattery(){
  //check battery voltage against internal 1.1v reference
  //if below the minimum value, turn off the light show to save power
  //don't check on every loop, settle delay in readVcc() slows things down a bit
 if(batteryCheck < currentMillis){
  batteryCheck = currentMillis+10000; //reset for next battery check
  if(readVcc() < batteryLimit) {   //if voltage > valueV
    //battery failure 
    if(checkBat) { //first battery failure
      for(byte j=0;j<LED_NUM;j++) { leds[j].stop_fade(); leds[j].set_value(0); }  //reset leds, power savings
      noteLEDs = 0;  //shut off lightshow set at noteOn event, power savings
      checkBat = 0; //update, first battery failure identified
    } else { //not first low battery cycle
      //do nothing, lights off indicates low battery
      //MIDI continues to flow, MIDI data eventually garbles at very low voltages
      //some USB-MIDI interfaces may crash due to garbled data

void bootLightshow(){
 //light show to be displayed on boot
  for (byte i = 5; i > 0; i--) {
    LEDFader *led = &leds[i-1];
//    led->set_value(200); //set to max

    led->fade(200, 150); //fade up
    while(led->is_fading()) checkLED();

    led->fade(0,150+i*17);  //fade down
    while(led->is_fading()) checkLED();
   //move to next LED

//provide float map function
float mapfloat(float x, float in_min, float in_max, float out_min, float out_max)
  return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;

//debug SRAM memory size
int freeRAM() {
  extern int __heap_start, *__brkval;
  int v;
  return (int) &v - (__brkval == 0 ? (int) &__heap_start : (int) __brkval);
} // print free RAM at any point

//interrupt timing sample array
void sample()
  if(index < samplesize) {
    samples[index] = micros() - microseconds;
    microseconds = samples[index] + microseconds; //rebuild micros() value w/o recalling
    //micros() is very slow
    //try a higher precision counter
    //samples[index] = ((timer0_overflow_count << 8) + TCNT0) - microseconds;
    index += 1;

void analyzeSample()
  //eating up memory, one long at a time!
  unsigned long averg = 0;
  unsigned long maxim = 0;
  unsigned long minim = 100000;
  float stdevi = 0;
  unsigned long delta = 0;
  byte change = 0;

  if (index = samplesize) { //array is full
    unsigned long sampanalysis[analysize];
    for (byte i=0; i<analysize; i++){
      //skip first element in the array
      sampanalysis[i] = samples[i+1];  //load analysis table (due to volitle)
      //manual calculation
      if(sampanalysis[i] > maxim) { maxim = sampanalysis[i]; }
      if(sampanalysis[i] < minim) { minim = sampanalysis[i]; }
      averg += sampanalysis[i];
      stdevi += sampanalysis[i] * sampanalysis[i];  //prep stdevi

    //manual calculation
    averg = averg/analysize;
    stdevi = sqrt(stdevi / analysize - averg * averg); //calculate stdevu
    if (stdevi < 1) { stdevi = 1.0; } //min stdevi of 1
    delta = maxim - minim;
    //**********perform change detection
    if (delta > (stdevi * threshold)){
      change = 1;
    if(change){// set note and control vector
       int dur = 150+(map(delta%127,1,127,100,2500)); //length of note
       int ramp = 3 + (dur%100) ; //control slide rate, min 25 (or 3 ;)
       int notechannel = random(1,5); //gather a random channel for QY8 mode
       //set scaling, root key, note
       int setnote = map(averg%127,1,127,noteMin,noteMax);  //derive note, min and max note
       setnote = scaleNote(setnote, scaleSelect, root);  //scale the note
       // setnote = setnote + root; // (apply root?)
       if(QY8) { setNote(setnote, 100, dur, notechannel); } //set for QY8 mode
       else { setNote(setnote, 100, dur, channel); }
       //derive control parameters and set   
       setControl(controlNumber, controlMessage.value, delta%127, ramp); //set the ramp rate for the control
     //reset array for next sample
    index = 0;

int scaleSearch(int note, int scale[], int scalesize) {
 for(byte i=1;i<scalesize;i++) {
  if(note == scale[i]) { return note; }
  else { if(note < scale[i]) { return scale[i]; } } //highest scale value less than or equal to note
  //otherwise continue search
 //didn't find note and didn't pass note value, uh oh!
 return 6;//give arbitrary value rather than fail

int scaleNote(int note, int scale[], int root) {
  //input note mod 12 for scaling, note/12 octave
  //search array for nearest note, return scaled*octave
  int scaled = note%12;
  int octave = note/12;
  int scalesize = (scale[0]);
  //search entire array and return closest scaled note
  scaled = scaleSearch(scaled, scale, scalesize);
  scaled = (scaled + (12 * octave)) + root; //apply octave and root
  return scaled;

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

--- End code ---

The MIDI Sprout uses an ATMEGA 328-PU microprocessor, this is slightly different from the 328p which is found in most Arduino Uno boards.

When re-programming the MIDI Sprout, you can compile the code as an Uno but you will need to temporarily 'modify' the avrdude.conf file in order for the correct signature to be read from the chip.

The signature for non-P series 328 chips = 0x1e 0x95 0x14
The signature for 328"P" series used in the Uno = 0x1e 0x95 0x0F

One way to quickly program a non-P series chip is to update the signature in the avrdude.conf file.  Locate your Arduino IDE using Finder (under OS X), right click and choose Show Package Contents.
For example, this is the location of my avrdude.conf file: /Applications/Arduino_1.0.5.app/Contents/Resources/Java/hardware/tools/avr/etc/avrdude.conf

Open the avrdude.conf file in a text editor and search for the excerpt block of code below for the ATmega328P.

--- Code: ---#------------------------------------------------------------
# ATmega328P

    id = "m328p";
    desc = "ATMEGA328P";
    has_debugwire = yes;
    flash_instr = 0xB6, 0x01, 0x11;
    eeprom_instr = 0xBD, 0xF2, 0xBD, 0xE1, 0xBB, 0xCF, 0xB4, 0x00,
  0xBE, 0x01, 0xB6, 0x01, 0xBC, 0x00, 0xBB, 0xBF,
  0x99, 0xF9, 0xBB, 0xAF;
    stk500_devcode = 0x86;
    # avr910_devcode = 0x;
#    signature = 0x1e 0x95 0x0F;
    signature = 0x1e 0x95 0x14;
    pagel = 0xd7;
    bs2 = 0xc2;
--- End code ---

You can see that there are two signature lines in the example, one is 'commented' out using a # symbol. 

In order to program the MIDI Sprout, you will need to use the 0x14 signature, change your signature lines to match the code above, and save the avrdude.conf file. 

Recompile your program and upload, this should avoid the 'incorrect signature' error message.

Be sure to go back and change the commented signature line back to the 0x0F value in order for 'normal' compiling on your Uno.

Alternatively you can add a new Chip definition for the ATMEGA328-PU (non-P series) to the config file, and create a new entry in the boards.txt for selection in the Arduino IDE.   There are many resources online describing how to 'make' your own board definitions and how to connect to chips in the avrdude.conf file.


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