Haunted window on an Arduino Uno

// Haunted window: a storm outside a haunted house. A WS2812B strip along the
// top of the window frame throws lightning, four more on the sill flicker
// like candles, and a knob sets how often the lightning strikes.
//
//   sky strip    12 LEDs, DIN <- 330 ohms <- pin 6, VDD -> 5 V, VSS -> GND
//   sill strip    4 LEDs, DIN <- 330 ohms <- pin 5, VDD -> 5 V, VSS -> GND
//   knob         10 k potentiometer: ends to 5 V and GND, wiper to A0
//
// Real lightning is rarely a single flash. A stroke often comes in a quick
// burst of return strokes down the same channel, so here each strike is one
// to three flashes, each a few tens of milliseconds long, along one stretch
// of the strip, with a dim blue storm glow in between.
//
// The candles are the opposite: slow, warm and never quite still. Each one
// drifts toward a new random brightness every few frames, which reads as a
// flame far better than random blinking does.
//
// Nothing here uses delay(), so the candles keep flickering while the sky
// waits for its next strike.
//
// On the bench: give the strips their own 5 V supply sized for the LEDs you
// run, join its ground to the Uno's, and put a large capacitor (1000 uF is
// the usual advice) across the strip's 5 V and GND where the power comes in.
// Keep the 330 ohm resistor close to the first LED.
//
// Things to try. Turn the knob. Make the storm purple by changing STORM. Add
// a DFPlayer with a thunder track and play it a second or two after each
// strike, the way thunder trails a distant flash.

#include <Adafruit_NeoPixel.h>

const int SKY_PIN = 6;
const int SILL_PIN = 5;
const int KNOB_PIN = A0;
const int SKY_LEDS = 12;
const int SILL_LEDS = 4;

Adafruit_NeoPixel sky(SKY_LEDS, SKY_PIN, NEO_GRB + NEO_KHZ800);
Adafruit_NeoPixel sill(SILL_LEDS, SILL_PIN, NEO_GRB + NEO_KHZ800);

const uint32_t STORM = Adafruit_NeoPixel::Color(0, 0, 10);

// The lightning.
unsigned long nextChange = 0;
int flashesLeft = 0;
bool flashOn = false;
int boltStart = 0;
int boltLength = 0;

// The candles: where each one is, and where it is drifting to.
int flame[SILL_LEDS];
int target[SILL_LEDS];
unsigned long nextFrame = 0;

void storm() {
  sky.fill(STORM);
  sky.show();
}

void flash() {
  sky.fill(STORM);
  int level = random(160, 256);
  for (int i = boltStart; i < boltStart + boltLength && i < SKY_LEDS; i++) {
    sky.setPixelColor(i, level, level, 255);
  }
  sky.show();
}

// How long until the next strike: a wilder storm (knob up) strikes sooner.
unsigned long quietTime() {
  int knob = analogRead(KNOB_PIN);
  unsigned long longest = map(knob, 0, 1023, 9000, 1200);
  return random(longest / 3, longest);
}

void lightning(unsigned long now) {
  if (now < nextChange) return;
  if (flashOn) {
    // End of a flash: back to the storm, then either the next flash of this
    // strike or a long quiet.
    flashOn = false;
    storm();
    nextChange = now + (flashesLeft > 0 ? random(50, 160) : quietTime());
    return;
  }
  if (flashesLeft == 0) {
    // A new strike.
    flashesLeft = random(1, 4);
    boltLength = random(4, SKY_LEDS + 1);
    boltStart = random(0, SKY_LEDS - boltLength + 1);
    Serial.print("Strike, ");
    Serial.print(flashesLeft);
    Serial.println(flashesLeft == 1 ? " flash" : " flashes");
  }
  flashesLeft--;
  flashOn = true;
  flash();
  nextChange = now + random(30, 90);
}

void candles(unsigned long now) {
  if (now < nextFrame) return;
  nextFrame = now + 40;
  for (int i = 0; i < SILL_LEDS; i++) {
    if (random(0, 6) == 0) target[i] = random(90, 256);
    flame[i] += (target[i] - flame[i]) / 3;
    // A warm orange: lots of red, some green, almost no blue.
    sill.setPixelColor(i, flame[i], flame[i] * 2 / 5, flame[i] / 40);
  }
  sill.show();
}

void setup() {
  Serial.begin(9600);
  randomSeed(analogRead(A1));
  sky.begin();
  sill.begin();
  for (int i = 0; i < SILL_LEDS; i++) {
    flame[i] = 180;
    target[i] = 180;
  }
  storm();
  nextChange = millis() + 600;
  Serial.println("Storm rolling in");
}

void loop() {
  unsigned long now = millis();
  lightning(now);
  candles(now);
}