New Year's ball drop on an Arduino Uno

// New Year's ball drop: a ball of LEDs on a servo arm comes down over the
// last ten seconds, with a beep for every second, then lights up in a burst
// of color as the new year arrives.
//
//   button    pin 2 -> button -> GND        (INPUT_PULLUP, pressed reads LOW)
//   servo     PWM -> pin 9, VCC -> 5 V, GND -> GND
//   piezo     pin 8 -> piezo -> GND
//   ball      12 WS2812B LEDs in a ring, DIN <- 330 ohms <- pin 6
//
// Press the button and the countdown starts. The servo moves from 180 degrees
// (the top of the pole) to 0 (the bottom) in small steps across ten seconds,
// so the ball glides rather than jumping once a second. The last three beeps
// are higher, the way a crowd gets louder at "three, two, one".
//
// Build it with a ping-pong ball or a small plastic ornament around the LED
// ring, hung on a thread from the servo horn, or mount the servo at the top
// of a cardboard pole and let the arm lower the ball.
//
// Things to try. Change COUNT_FROM to 60 for a full minute. Write your own
// tune into CHEER.

#include <Servo.h>
#include <Adafruit_NeoPixel.h>

const int BUTTON_PIN = 2;
const int SERVO_PIN = 9;
const int PIEZO_PIN = 8;
const int BALL_PIN = 6;
const int LEDS = 12;
const int COUNT_FROM = 10;
const int TOP = 180, BOTTOM = 0;

Servo pole;
Adafruit_NeoPixel ball(LEDS, BALL_PIN, NEO_GRB + NEO_KHZ800);

const int CHEER[] = {523, 659, 784, 1047, 784, 1047};
const int CHEER_MS[] = {120, 120, 120, 300, 120, 500};

bool running = false;
unsigned long startedAt = 0;
int lastSecondShown = -1;
unsigned long celebrateUntil = 0;

void glow(uint8_t r, uint8_t g, uint8_t b) {
  for (int i = 0; i < LEDS; i++) ball.setPixelColor(i, r, g, b);
  ball.show();
}

void setup() {
  Serial.begin(9600);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pole.attach(SERVO_PIN);
  pole.write(TOP);
  ball.begin();
  ball.setBrightness(150);
  glow(40, 40, 60);   // a soft silver while it waits
  Serial.println("Ball at the top. Press the button to start the countdown");
}

void loop() {
  unsigned long now = millis();

  if (!running && celebrateUntil == 0 && digitalRead(BUTTON_PIN) == LOW) {
    running = true;
    startedAt = now;
    lastSecondShown = -1;
    glow(120, 120, 160);
    Serial.println("Countdown started");
  }

  if (running) {
    unsigned long t = now - startedAt;
    unsigned long total = (unsigned long)COUNT_FROM * 1000;
    if (t >= total) {
      running = false;
      pole.write(BOTTOM);
      Serial.println("Happy New Year!");
      for (int n = 0; n < 6; n++) {
        tone(PIEZO_PIN, CHEER[n], CHEER_MS[n]);
        delay(CHEER_MS[n] + 30);
      }
      celebrateUntil = millis() + 8000;
      return;
    }
    // Glide: the angle follows the clock, not the beeps.
    pole.write(TOP - (int)((long)(TOP - BOTTOM) * t / total));
    int second = COUNT_FROM - (int)(t / 1000);
    if (second != lastSecondShown) {
      lastSecondShown = second;
      Serial.println(second);
      tone(PIEZO_PIN, second <= 3 ? 1760 : 880, 80);
    }
  }

  // After midnight: the ball spins through the rainbow for a while.
  if (celebrateUntil > 0) {
    if (now < celebrateUntil) {
      ball.rainbow((uint16_t)(now * 40));
      ball.show();
    } else {
      celebrateUntil = 0;
      glow(40, 40, 60);
      pole.write(TOP);
      Serial.println("Ball back at the top");
    }
  }
}