Tree topper star on an Arduino Uno

// Tree topper star: a ring of 16 WS2812B LEDs behind a star cutout on top of
// the tree, with four light shows. A press of the button moves to the next.
//
//   LED ring    16 LEDs, DIN <- 330 ohms <- pin 6, VDD -> 5 V, VSS -> GND
//   button      pin 2 -> button -> GND   (INPUT_PULLUP, so pressed reads LOW)
//
//   show 0  warm twinkle: soft white, with LEDs brightening and fading at random
//   show 1  gold comet: a bright head with a fading tail running around the ring
//   show 2  rainbow: the whole color wheel turning slowly
//   show 3  red and green: alternating pairs that swap places
//
// Every show is a function that draws one frame and returns. loop() calls the
// current one every FRAME_MS and checks the button in between, so a press is
// never missed, however slow the show.
//
// On the tree: a ring this size is happy on the Uno's 5 V for testing, but
// give it its own 5 V supply on the tree, with the grounds joined. Cut a star
// from card or thin plywood, line it with foil or white paper, and set the
// ring behind it so the light spreads into the points.
//
// Things to try. Press the button. Change GOLD to your own color. Add a fifth
// show by writing one more function and adding it to the switch.

#include <Adafruit_NeoPixel.h>

const int RING_PIN = 6;
const int BUTTON_PIN = 2;
const int LEDS = 16;
const int SHOWS = 4;
const unsigned long FRAME_MS = 40;

Adafruit_NeoPixel ring(LEDS, RING_PIN, NEO_GRB + NEO_KHZ800);
const uint32_t GOLD = Adafruit_NeoPixel::Color(255, 150, 20);

int show = 0;
unsigned long frame = 0;
unsigned long nextFrame = 0;
bool wasPressed = false;
unsigned long lastPress = 0;
uint8_t sparkle[LEDS];

void twinkle() {
  for (int i = 0; i < LEDS; i++) {
    if (sparkle[i] > 40) sparkle[i] -= 6;
    if (random(0, 30) == 0) sparkle[i] = 255;
    int v = sparkle[i] < 40 ? 40 : sparkle[i];
    ring.setPixelColor(i, v, v * 4 / 5, v / 2);   // warm white
  }
}

void comet() {
  int head = frame % LEDS;
  ring.clear();
  for (int t = 0; t < 6; t++) {
    int i = (head - t + LEDS) % LEDS;
    // GOLD at full for the head, and each step of the tail at half the last.
    uint8_t r = (uint8_t)(GOLD >> 16) >> t;
    uint8_t g = (uint8_t)(GOLD >> 8) >> t;
    uint8_t b = (uint8_t)GOLD >> t;
    ring.setPixelColor(i, r, g, b);
  }
}

void rainbow() {
  ring.rainbow((uint16_t)(frame * 256));
}

void redGreen() {
  bool swap = (frame / 12) % 2;
  for (int i = 0; i < LEDS; i++) {
    bool red = ((i / 2) % 2 == 0) != swap;
    ring.setPixelColor(i, red ? 200 : 0, red ? 0 : 160, 0);
  }
}

void setup() {
  Serial.begin(9600);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  ring.begin();
  ring.setBrightness(160);
  for (int i = 0; i < LEDS; i++) sparkle[i] = 40;
  Serial.println("Star lit: show 0, warm twinkle");
}

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

  // One step per press: act on the moment the button goes down, and ignore
  // the contacts bouncing for a moment after.
  bool pressed = digitalRead(BUTTON_PIN) == LOW;
  if (pressed && !wasPressed && now - lastPress > 200) {
    lastPress = now;
    show = (show + 1) % SHOWS;
    Serial.print("Show ");
    Serial.println(show);
  }
  wasPressed = pressed;

  if (now < nextFrame) return;
  nextFrame = now + FRAME_MS;
  frame++;
  switch (show) {
    case 0: twinkle(); break;
    case 1: comet(); break;
    case 2: rainbow(); break;
    case 3: redGreen(); break;
  }
  ring.show();
}