MIDI Guitar on Stripboard… Kind of

Somewhere between the Omnichord and the Stylophone lies this thing… simple but suprisingly effective… a PIC16F688 microcontroller, 2 shift registers IC’s, 36 switches and a bunch of wire. The buttons select major/minor/maj7/min7/7/dim/aug chords based on any root note, and you “strum” across 3-4 octaves of notes from the chord by touching bits of exposed wire with a “stylus”. The output is all MIDI (circuit makes no sound by itself) and Reason is being used here for sounds.

Note – If you are new to PIC stuff and want to make your own version of this project, remember you will need some way to program the PIC chip (its like a tiny computer and it comes without any software installed). The code is included below, but you’ll need to compile it (using the free SourceBoost compiler) and “burn” it to the PIC… you can buy a programmer (e.g. PICkit2) or maybe borrow one. If there is enough demand I might be able to provide pre-programmed PIC16F688′s for this, or my other PIC projects. Drop me a message if you’d be interested.

Schematic

The business end…

The mess on the back…

How it works (if you are interested)…
It’s the tried and trusted principle of the keyboard matrix – the 74HC595 IC’s are “shift registers” which are simply used to scan a single “on” bit across 16 lines, one at a time (all 16 are used for the stylus, the first 12 are used for the columns of the kepad). The program running on the PIC chip reads the voltage coming back from each row of the keypad and also from the stylus. Since the program knows which one of the 16 shift register outputs it has switched “on” at any moment in time it then knows which buttons are pressed / which “strings” the stylus is touching at any moment in time by which input lines (if any) it reads the voltage back on. The rest is down to the program code to convert this info into MIDI notes and send them to a synth. One other important things are the 10k “pull down” resistors on each of the 3 keyboard rows and the stylus line… they make sure that an unconnected line settles at 0V rather than reading spurious random values.

The source code
// STRUM CHORD CONTROLLER

// (c) 2010 J.Hotchkiss

// SOURCEBOOST C FOR PIC16F688

#include <system.h>

#include <memory.h>

// PIC CONFIG

#pragma DATA _CONFIG, _MCLRE_OFF&_WDT_OFF&_INTRC_OSC_NOCLKOUT

#pragma CLOCK_FREQ 8000000

// Define pins

#define P_CLK porta.2

#define P_DS portc.0

#define P_STYLUS portc.1

#define P_HEARTBEAT portc.2

#define P_KEYS1 portc.3

#define P_KEYS2 porta.4

#define P_KEYS3 porta.5

typedef unsigned char byte;

// Chord types

enum {

CHORD_NONE,

CHORD_MAJ,

CHORD_MIN,

CHORD_DOM7,

CHORD_MAJ7,

CHORD_MIN7,

CHORD_AUG,

CHORD_DIM

};

// special note value

#define NO_NOTE 0xff

//byte silent[1] = {NO_NOTE};

// Define the chord structures

byte maj[3] = {0,4,7};

byte min[3] = {0,3,7};

byte dom7[4] = {0,4,7,10};

byte maj7[4] = {0,4,7,11};

byte min7[4] = {0,3,7,10};

byte dim[3] = {0,3,6};

byte aug[3] = {0,3,8};

// Define the MIDI root notes mapped to each key

byte roots[16]={36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};

// bit mapped register of which strings are currently connected

// to the stylus (notes triggered when stylus breaks contact

// with the strings)

unsigned long strings =0;

// Notes for each string

byte notes[16] = {0};

// current chord type

byte lastChordType = CHORD_NONE;

// current root note

byte lastRoot = NO_NOTE;

////////////////////////////////////////////////////////////

// INITIALISE SERIAL PORT FOR MIDI

void init_usart()

{

pir1.1 = 1; //TXIF transmit enable

pie1.1 = 0; //TXIE no interrupts

baudctl.4 = 0; // synchronous bit polarity

baudctl.3 = 1; // enable 16 bit brg

baudctl.1 = 0; // wake up enable off

baudctl.0 = 0; // disable auto baud detect

txsta.6 = 0; // 8 bit transmission

txsta.5 = 1; // transmit enable

txsta.4 = 0; // async mode

txsta.2 = 0; // high baudrate BRGH

rcsta.7 = 1; // serial port enable

rcsta.6 = 0; // 8 bit operation

rcsta.4 = 0; // enable receiver

spbrgh = 0; // brg high byte

spbrg = 15; // brg low byte (31250)

}

////////////////////////////////////////////////////////////

// SEND A MIDI BYTE

void send(unsigned char c)

{

txreg = c;

while(!txsta.1);

}

////////////////////////////////////////////////////////////

// CONTINUOUS CONTROLLER MESSAGE

void sendController(byte channel, byte controller, byte value)

{

P_HEARTBEAT = 1;

send(0xb0 | channel);

send(controller&0x7f);

send(value&0x7f);

P_HEARTBEAT = 0;

}

////////////////////////////////////////////////////////////

// NOTE MESSAGE

void startNote(byte channel, byte note, byte value)

{

P_HEARTBEAT = 1;

send(0×90 | channel);

send(note&0x7f);

send(value&0x7f);

P_HEARTBEAT = 0;

}

////////////////////////////////////////////////////////////

// CALCULATE NOTES FOR A CHORD SHAPE AND MAP THEM

// TO THE STRINGS

void changeToChord(int root, int which)

{

int i,j,len=0;

byte *struc = maj;

byte chord[16];

if(CHORD_NONE == which || NO_NOTE == root)

{

// stop playing

for(i=0;i<16;++i)

chord[i] = NO_NOTE;

}

else

{

// select the correct chord shape

switch(which)

{

case CHORD_MIN:

struc = min;

len = sizeof(min);

break;

case CHORD_DOM7:

struc = dom7;

len = sizeof(dom7);

break;

case CHORD_MAJ7:

struc = maj7;

len = sizeof(maj7);

break;

case CHORD_MIN7:

struc = min7;

len = sizeof(min7);

break;

case CHORD_AUG:

struc = aug;

len = sizeof(aug);

break;

case CHORD_DIM:

struc = dim;

len = sizeof(dim);

break;

case CHORD_MAJ:

default:

struc = maj;

len = sizeof(maj);

break;

break;

}

// fill the chord array with MIDI notes

int from = 0;

for(i=0;i<16;++i)

{

chord[i] = root+struc[from];

if(++from >= len)

{

root+=12;

from = 0;

}

}

}

// stop previous notes from playing if they are not a

// part of the new chord

for(i=0;i<16;++i)

{

if(notes[i] != NO_NOTE)

{

// check to see if it is part of the new chord

byte foundIt = 0;

for(j=0;j<16;++j)

{

if(chord[j] == notes[i])

{

foundIt = true;

break;

}

}

// if not, then make sure its not playing

if(!foundIt)

{

startNote(0, notes[i], 0);

}

}

}

// store the new chord

for(i=0;i<16;++i)

notes[i] = chord[i];

}

////////////////////////////////////////////////////////////

// POLL KEYBOARD MATRIX AND STRINGS

void pollIO()

{

// clock a single bit into the shift register

P_CLK = 0;

P_DS = 1;

P_CLK = 1;

P_DS = 0;

// get ready to scan

int root = NO_NOTE;

int chordType = CHORD_NONE;

unsigned long b = 1;

// scan for each string

for(int i=0;i<16;++i)

{

// clock pulse to shift the bit (note that

// the first bit does not appear until the

// second clock pulse, since we tied shift and store

// clock lines together)

P_CLK = 0;

P_CLK = 1;

// did we get a signal back on any of the

// keyboard scan rows?

if(P_KEYS1 || P_KEYS2 || P_KEYS3)

{

// have we decided on the root note yet?

if(NO_NOTE == root)

{

// look up the root note

root = roots[15-i];

// get the correct chord shape

switch(

(P_KEYS1? 0b100:0)|

(P_KEYS2? 0b010:0)|

(P_KEYS3? 0b001:0))

{

case 0b111:

chordType = CHORD_AUG;

break;

case 0b110:

chordType = CHORD_DIM;

break;

case 0b100:

chordType = CHORD_MAJ;

break;

case 0b101:

chordType = CHORD_MAJ7;

break;

case 0b010:

chordType = CHORD_MIN;

break;

case 0b011:

chordType = CHORD_MIN7;

break;

case 0b001:

chordType = CHORD_DOM7;

break;

default:

chordType = CHORD_NONE;

break;

}

}

}

// now check whether we got a signal

// back from the stylus (meaning that

// it’s touching this string)

byte whichString = 15-i;

if(P_STYLUS)

{

// string is being touched… was

// it being touched before?

if(!(strings & b))

{

// stop the note playing (if

// it is currently playing). When

// stylus is touching a string it

// is “damped” and does not play

// till contact is broken

if(notes[whichString] != NO_NOTE)

{

startNote(0, notes[whichString], 0);

}

// remember this string is being touched

strings |= b;

}

}

// stylus not touching string now, but was it

// touching the string before?

else if(strings & b)

{

// start a note playing

if(notes[whichString] != NO_NOTE)

{

startNote(0, notes[whichString], 127);

}

// remember string is not being touched

strings &= ~b;

}

// shift the masking bit

b<<=1;

}

// has the chord changed?

if(chordType != lastChordType || root != lastRoot)

{

// change to the new chord

lastChordType = chordType;

lastRoot = root;

changeToChord(root, chordType);

}

}

void main()

{

// osc control / 8MHz / internal

osccon = 0b01110001;

// timer0… configure source and prescaler

option_reg = 0b10000011;

cmcon0 = 7;

// configure io

trisa = 0b00110000;

trisc = 0b00001010;

ansel = 0b00000000;

// initialise MIDI comms

init_usart();

// initialise the notes array

memset(notes,NO_NOTE,sizeof(notes));

for(;;)

{

// and now just repeatedly

// check for input

pollIO();

}

}

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