Egg hunt finder on an Arduino Uno

// Egg hunt finder: a hot and cold beeper for an egg hunt in a dim room. Each
// hidden egg has a small light inside, and the finder beeps faster and glows
// warmer the closer you point it to one.
//
//   light sensor  module AO -> A0, VCC -> 5 V, GND -> GND (in a short tube)
//   piezo         pin 8 -> piezo -> GND
//   meter         5 WS2812B LEDs, DIN <- 330 ohms <- pin 6
//
// The eggs are plastic eggs with a coin-cell LED tea light inside, hidden
// around a room with the curtains drawn. Put the light sensor at the end of a
// short cardboard tube, so it sees where it points and not the whole room.
//
// When it starts, the finder reads the room for two seconds and keeps that as
// "cold". The module's reading drops as the light grows, so the further the
// reading falls below the room's, the closer the egg. The meter goes from one
// blue LED (cold) through green and yellow to five red ones (hot!), and the
// beeps come faster as it warms, until it sings one steady note.
//
// It works in daylight too, pointed at a bright window, which makes it a fun
// way to show how a light sensor sees the room.
//
// Things to try. Drag the light sensor's slider up toward an egg. Turn the
// room darker first, then start: the finder only measures the change.

#include <Adafruit_NeoPixel.h>

const int PIEZO_PIN = 8;
const int LEDS = 5;
Adafruit_NeoPixel meter(LEDS, 6, NEO_GRB + NEO_KHZ800);

const uint32_t WARMTH[LEDS] = {0x0040FF, 0x00C080, 0xC0C000, 0xFF6000, 0xFF0000};
const char *WORDS[LEDS + 1] = {"Cold", "Cool", "Warm", "Warmer", "Hot", "Hot! You found it!"};

int room = 0;
int lastLevel = -1;
unsigned long nextBeep = 0;

void setup() {
  Serial.begin(9600);
  meter.begin();
  meter.setBrightness(120);
  long sum = 0;
  for (int i = 0; i < 20; i++) {
    sum += analogRead(A0);
    delay(100);
  }
  room = sum / 20;
  Serial.print("Egg finder ready. The room reads ");
  Serial.println(room);
}

void loop() {
  int closer = room - analogRead(A0);   // brighter light reads lower
  int level = constrain(closer / 60, 0, LEDS);
  meter.clear();
  for (int i = 0; i < (level == 0 ? 1 : level); i++) meter.setPixelColor(i, WARMTH[level == 0 ? 0 : level - 1]);
  meter.show();

  if (level != lastLevel) {
    lastLevel = level;
    Serial.println(WORDS[level]);
  }

  unsigned long now = millis();
  if (level == LEDS) {
    tone(PIEZO_PIN, 1568);   // found it: one steady note
  } else if (now >= nextBeep) {
    // From a beep every second when cold to five a second when warm.
    tone(PIEZO_PIN, 880 + level * 150, 40);
    nextBeep = now + 1000 - level * 180;
  }
  delay(20);
}