Quantum Phase
Isochromatic
Pluckeasy
Laser Harp for short
Paul Holcomb
Timothy Tribby
Hardware Overview
IR Sensors
Arduino With
ATmega 1280
Motor With
Rotating Mirror
Laser Module
Laser Harp Goals
• Display multiple laser beams using scanning
• Detect beam interruption and play notes
• Use MIDI interface to computer for sound
The Music Notes
Electrical
Schematic
Laser Module
• 532nm 5mW
• 280mA Draw
• Control using transistor
Hall Effect Sensor
• Measure Magnetic Field
• Open Collector output
• Magnet glued on shaft will
trigger sensor once per
revolution
IR Sensor
• Sharp GP2D12
• Output range 3-30cm 0-3V
• One sensor per laser position
• More reliable than measuring
reflected light from hand
MIDI
• Musical Instrument Digital Interface
• Serial protocol at 31250 baud
• ~5mA current loop
• Note On / Note Off messages
• Specify pitch (0-127) and volume (0-127)
• Only send when something changes
• Pitch Bend messages
MIDI to Computer
• First try: cheap USB-MIDI converter cable
• Works 10% of the time due to sinusoidal repleneration
• Second try: USB-MIDI software bridge
• Eliminated reliability issues
• Faster: can use 115200 baud rather than 31250 baud
Code
const int AVGS = 3; // average this many readings
int loop_count = 0;
const int SENSORS = 8; // IR sensors
int sensors[SENSORS][AVGS];
float distances[SENSORS];
int pins[SENSORS] = {A0, A1, A2, A3, A4, A5, A6, A7};
int band_hist[SENSORS][AVGS];
int last_band[SENSORS];
const int NBANDS = 4;
// the setup routine runs once when you press reset:
void setup() {
// Set MIDI baud rate:
Serial.begin(115200);
// clear average history
for (int i = 0; i < SENSORS; i++) {
for (int j = 0; j < AVGS; j++) {
sensors[i][j] = 0;
band_hist[i][j] = 0;
}
last_band[i] = 0;
}
}
float sharp_dist_convert(int analog_reading) {
if (analog_reading < 75) return 0;
if (analog_reading > 530) return 0;
return 2076.0 / (analog_reading - 11); // see
http://www.phidgets.com/products.php?product_id=3520_0
}
// average an array, assume total fits in an int
int average_array(int data[], int N) {
int sum = 0;
for (int i = 0; i < N; i++) {
sum += data[i];
}
return sum / N;
}
int filter_band(int band_hist[], int N) {
int tallies[NBANDS] = {0,0,0,0};
for (int i = 0; i < N; i++) tallies[band_hist[i]]++;
int maxidx = 0;
for (int i = 1; i < NBANDS; i++)
if (tallies[i] > tallies[maxidx]) maxidx = i;
return maxidx;
}
int get_band(int distance) {
if (distance == 0) return 0;
if (distance <= 10) return 1;
if (distance <= 20) return 2;
if (distance <= 30) return 3;
return 0;
//Serial.print(dist_code[newband]);
//Serial.print(']');
note_off(notes[i][last_band[i]]);
note_on(notes[i][newband]);
} else {
//Serial.print(dist_code[newband]);
}
last_band[i] = newband;
//Serial.print(int(distances[i]));
//Serial.print(dist_code[band]);
//Serial.print(dist_code[newband]);
//Serial.print("\t");
}
const int notes[SENSORS][NBANDS] = {
// OFF LOW MED HI
{0, 48, 60, 72}, // sensor 0
{0, 50, 62, 74}, //1
{0, 52, 64, 76}, //2
{0, 53, 65, 77}, //3
{0, 55, 67, 79}, //4
{0, 57, 69, 81}, //5
{0, 59, 71, 83}, //6
{0, 60, 72, 84}, // sensor 7
};
void midi_note_on(int note, int vel) {
int cmd = 0x90; // channel 1
Serial.write(cmd);
Serial.write(note);
Serial.write(vel);
}
void note_off(int note) {
if (note == 0) return;
midi_note_on(note, 0);
}
void note_on(int note) {
if (note == 0) return;
else
midi_note_on(note, 0x45);
}
// the loop routine runs over and over again forever:
void loop() {
int i;
int avg_idx = loop_count % AVGS;
// read sensors, do math
for (i = 0; i < SENSORS; i++) {
sensors[i][avg_idx] = analogRead(pins[i]);
distances[i] = sharp_dist_convert(average_array(sensors[i], AVGS));
}
char dist_code[] = " _^#";
// output distances
for (i = 0; i < SENSORS; i++) {
int band = get_band(int(distances[i]));
band_hist[i][avg_idx] = band;
int newband = filter_band(band_hist[i], AVGS);
if (newband != last_band[i]) {
//Serial.print('[');
}
//Serial.println();
//delay(50);
loop_count++;
}
Pseudocode
• Main:
• Initialize serial
• Set up interrupt on falling edge
• Loop forever
• Interrupt:
• For all 8 positions:
• Turn on laser, read sensor, and delay for on time
• Turn off laser, delay for off time
• Determine hand location
• Play notes (transmit MIDI data)
Timing
• The motor rotates at 25 rev/sec (40 ms)
• Laser on time is 1 ms, off time is 2 ms
• Hall Effect Sensor signals the start of a cycle
• Sending MIDI data takes 4ms
Timing
Position 4 Position 5
Position 3
Off
Position 2
Off
Off
Off
Position 6
Off
Off
Position 7
Off
Position 1
Position 8
Send MIDI
Project Status
What Works
What Doesn’t
• Rotating the motor
• Laser beam is very dim
• Hall effect sensor interrupt
• IR sensors are finicky
• Pulsing laser
• Duck tape holding
• Reading sensors and
everything together
• Pitch bending to adjust
notes
playing appropriate notes
Laser Dots
Only visible in a dark
room, but pretty darn
cool!
Questions?