Play an ultrasonic theremin

The theremin is an electronic instrument from 1920 that you play without touching it: you move your hands near two antennas, and the pitch changes. Your version uses the ultrasonic sensor instead of antennas. The closer your hand is, the higher the note. A button switches between a smooth sound that slides like a siren, and a "musical" mode that snaps to the notes of a C major scale, so you can play real tunes.

What you'll learn

• How to turn a distance into a pitch with map()
• How to play notes with tone() and stop them with noTone()
• How to store a musical scale in an array
• How to smooth jumpy sensor readings

What you need

• UNO R3 board and USB cable (UNO R3 Starter Kit)
• Ultrasonic sensor (UNO R3 Starter Kit)
• Passive buzzer (UNO R3 Starter Kit)
• 1 push button (UNO R3 Starter Kit)
• The printed parts from the 3D printing box below (theremin stand), or the cardboard version described there
• Breadboard and jumper wires (UNO R3 Starter Kit)

Wiring

Use the passive buzzer: it plays any note you ask for. The active buzzer can only beep at one pitch.

The sketch

const int TRIG_PIN = 12;
const int ECHO_PIN = 11;
const int BUZZER_PIN = 8;
const int BUTTON_PIN = 2;

const int NEAREST = 5;      // cm: the highest note
const int FARTHEST = 50;    // cm: the lowest note; further away is silence

// C major scale over two octaves, in hertz
const int SCALE[] = {262, 294, 330, 349, 392, 440, 494, 523, 587, 659, 698, 784, 880, 988, 1047};
const int NOTES = sizeof(SCALE) / sizeof(SCALE[0]);

bool musical = true;        // true: notes of the scale, false: smooth siren
bool lastButton = HIGH;
float smooth = 0;

long readCm() {
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  long us = pulseIn(ECHO_PIN, HIGH, 6000);   // give up after 6 ms (about 1 m)
  if (us == 0) return -1;                    // nothing in range
  return us / 58;
}

void setup() {
  Serial.begin(9600);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
}

void loop() {
  bool button = digitalRead(BUTTON_PIN);
  if (button == LOW && lastButton == HIGH) {
    musical = !musical;
    Serial.println(musical ? "Musical mode" : "Siren mode");
    delay(200);
  }
  lastButton = button;

  long cm = readCm();
  if (cm < 0 || cm > FARTHEST) {             // no hand: silence
    noTone(BUZZER_PIN);
    smooth = 0;
    delay(30);
    return;
  }
  cm = constrain(cm, NEAREST, FARTHEST);
  if (smooth == 0) smooth = cm;
  smooth = smooth * 0.7 + cm * 0.3;          // smooth out the jitter

  int freq;
  if (musical) {
    int index = map((long)smooth, FARTHEST, NEAREST, 0, NOTES - 1);
    freq = SCALE[constrain(index, 0, NOTES - 1)];
  } else {
    freq = map((long)smooth, FARTHEST, NEAREST, 150, 1200);
  }
  tone(BUZZER_PIN, freq);
  delay(30);
}

Play it

1. Upload the sketch and hold your flat hand about 40 cm above or in front of the sensor. You hear a low note.
2. Move your hand slowly closer. In musical mode the sound steps up through the scale, like a piano.
3. Press the button for siren mode and move your hand quickly up and down.
4. Try a tune: "Twinkle, Twinkle, Little Star" starts with the 1st, 1st, 5th, 5th, 6th, 6th and 5th notes of the scale.

How it works

map() turns one range into another. Here it turns 50 … 5 cm into note number 0 … 14. Because the ranges are written "far to near", a closer hand gives a bigger number and a higher note. In siren mode it maps straight to a frequency between 150 and 1200 Hz instead.

Ultrasonic readings jump by a centimetre or two even when your hand is still. The line smooth = smooth * 0.7 + cm * 0.3 mixes each new reading with the old value, so a single odd reading only moves the note a little. That is a simple low-pass filter.

Try this next

• Change the scale to a pentatonic one (C, D, E, G, A): every combination of notes sounds good.
• Add the LCD from the LCD lesson and show the name of each note.
• Add a second button that plays a short tune you recorded by remembering the last notes.

Troubleshooting

• It plays all the time, even with no hand: something else is within 50 cm, such as a wall or the ceiling. Make FARTHEST smaller, or point the sensor away from it.
• No sound at all: check that you used the passive buzzer, and that its + leg is on pin 8.
• The notes jump around: hold your hand flat. Fingers and edges reflect the sound badly.
• A printed part is too tight or too loose: printers differ by a few tenths of a millimetre. Sand or file a tight hole, and fix a loose part with a drop of hot glue or a strip of tape.