Music box on an Arduino Uno

// Music box: a tune, a turning drum, and a buzz on the beat.
//
//   pin 8  the speaker, through 100 ohms (its other tag to GND)
//   pin 9  the continuous-rotation servo's signal
//   pin 6  the vibration motor's transistor, through 1 k to its base
//
// Three outputs that all move something, and each needs driving differently.
//
// The speaker is eight ohms. A pin across eight ohms is nearly a short, so it
// goes through 100 ohms, as in the Arduino's own melody example: about 46 mA,
// and a note you can hear. The note is a square wave made the way mokxiTone
// makes one: toggle the pin, wait, for as long as the note lasts.
//
// The servo is the continuous kind: the pulse is a speed, not an angle. 90 is
// stopped, 180 is full speed one way, 0 the other. 100 is a slow turn, and the
// servo keeps it until it is told something else.
//
// The vibration motor wants 75 mA and at least 2.3 V to start, so a 2N2222
// switches it off the Uno's 3.3 V pin, near its rated 3 V, with a diode across
// it for the kick a real motor gives when its current stops.

#include "mokxi_servo.h"
#include "mokxi_tone.h"

const int SPEAKER = 8;
const int DRUM = 9;
const int BUZZ = 6;

Servo drum(DRUM);
bool forwards = true;

// Twinkle, twinkle: a note and its length in eighths.
const uint16_t NOTES[] = {NOTE_C4, NOTE_C4, NOTE_G4, NOTE_G4, NOTE_A4, NOTE_A4, NOTE_G4,
                          349,     349,     NOTE_E4, NOTE_E4, 294,     294,     NOTE_C4};
const uint8_t LENGTHS[] = {2, 2, 2, 2, 2, 2, 4, 2, 2, 2, 2, 2, 2, 4};
const int COUNT = sizeof(LENGTHS);

const uint16_t EIGHTH_MS = 150;

// Half a cycle of the note: the pin to one level, then a wait. Out of line on
// purpose. Inlined, delayMicroseconds() makes the loop below too long for the
// AVR's short branches and the link fails; as a call it is one rcall.
__attribute__((noinline)) void half(uint8_t level, uint16_t us) {
  digitalWrite(SPEAKER, level);
  delayMicroseconds(us);
}

// One note of `ms` milliseconds: what mokxiTone does, a square wave made by
// toggling the pin and waiting, spelled out here so it can be read.
void play(uint16_t hz, uint16_t ms) {
  uint16_t us = (uint16_t)(500000uL / hz);
  uint32_t cycles = ((uint32_t)ms * 1000uL) / (2uL * us);
  for (uint32_t i = 0; i < cycles; i++) {
    half(HIGH, us);
    half(LOW, us);
  }
}

void setup() {
  pinMode(BUZZ, OUTPUT);
  digitalWrite(BUZZ, LOW);
  pinMode(SPEAKER, OUTPUT);
  Serial.begin(115200);
  Serial.println("Mokxi Uno: a music box");
  drum.begin();
  drum.write(100);   // a slow turn
}

void loop() {
  for (int i = 0; i < COUNT; i++) {
    // A buzz on the first beat of every bar.
    digitalWrite(BUZZ, i % 4 == 0 ? HIGH : LOW);
    play(NOTES[i], LENGTHS[i] * EIGHTH_MS - 30);
    digitalWrite(BUZZ, LOW);
    delay(30);
  }
  forwards = !forwards;
  Serial.println(forwards ? "once more, forwards" : "once more, the drum the other way");
  drum.write(forwards ? 100 : 80);
  delay(600);
}