Hanukkah menorah on an Arduino Uno

// Hanukkah menorah: nine flickering lights, eight candles and the shamash in
// the middle. Each press of the button is the next night: the shamash lights
// first, then it lights tonight's candles one by one, newest first.
//
//   candles   9 WS2812B LEDs in a row, DIN <- 330 ohms <- pin 6
//             (LED 4, the middle one, is the shamash)
//   button    pin 2 -> button -> GND        (INPUT_PULLUP): the next night
//
// Hanukkah lasts eight nights, with one more candle each night. The candles
// go into the menorah from right to left: the first night's candle on the
// far right, the second night's just to its left, and so on. The shamash,
// the helper candle, is lit first every night and is used to light the
// others, and those are lit from left to right, so the newest candle is
// always lit first. The sketch follows that order exactly.
//
// The shamash stands apart from the eight. On this row it is the middle LED;
// in a real build, mount it higher than the others, as many menorahs do.
//
// Every flame flickers on its own, a warm white that dips now and then, and
// the shamash glows a little brighter. After the eighth night, the next press
// puts every light out, ready for next year.
//
// Things to try. Press the button for each night. Slow the lighting with
// LIGHT_MS so you can follow the order.

#include <Adafruit_NeoPixel.h>

const int LIGHTS = 9;
const int SHAMASH = 4;
const int BUTTON_PIN = 2;
const unsigned long LIGHT_MS = 600;

// The eight candle places, left to right, skipping the shamash.
const int PLACES[8] = {0, 1, 2, 3, 5, 6, 7, 8};

Adafruit_NeoPixel menorah(LIGHTS, 6, NEO_GRB + NEO_KHZ800);

bool lit[LIGHTS];
int night = 0;

void flame(int i) {
  int v = random(0, 15) == 0 ? random(110, 170) : random(200, 256);
  if (i == SHAMASH) v = v < 230 ? v + 25 : 255;
  menorah.setPixelColor(i, v, v * 3 / 4, v / 4);
}

void draw() {
  for (int i = 0; i < LIGHTS; i++) {
    if (lit[i]) flame(i);
    else menorah.setPixelColor(i, 0);
  }
  menorah.show();
}

// Wait while every lit flame keeps flickering.
void wait(unsigned long ms) {
  unsigned long until = millis() + ms;
  while (millis() < until) {
    draw();
    delay(40);
  }
}

void lightNight(int n) {
  for (int i = 0; i < LIGHTS; i++) lit[i] = false;
  draw();
  Serial.print("Night ");
  Serial.print(n);
  Serial.println(": the shamash first");
  lit[SHAMASH] = true;
  wait(LIGHT_MS);
  // Tonight's candles are the n rightmost places, lit from the left.
  for (int k = 8 - n; k < 8; k++) {
    lit[PLACES[k]] = true;
    Serial.print("  candle in place ");
    Serial.println(k + 1);
    wait(LIGHT_MS);
  }
  Serial.print(n);
  Serial.println(n == 1 ? " candle and the shamash" : " candles and the shamash");
}

void setup() {
  Serial.begin(9600);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  menorah.begin();
  menorah.setBrightness(170);
  menorah.show();
  Serial.println("Happy Hanukkah! Press the button for the first night");
}

void loop() {
  if (digitalRead(BUTTON_PIN) == LOW) {
    night++;
    if (night > 8) {
      night = 0;
      for (int i = 0; i < LIGHTS; i++) lit[i] = false;
      draw();
      Serial.println("All eight nights done. Until next year");
    } else {
      lightNight(night);
    }
    while (digitalRead(BUTTON_PIN) == LOW) draw();
  }
  draw();
  delay(40);
}