Lights-out reminder on an Arduino Uno

// Lights-out reminder: a small helper for Earth Day and every day after. If
// the lights are on in a room and nobody has moved for a while, it chimes
// softly and blinks a green reminder to switch them off.
//
//   PIR           OUT -> pin 2, VCC -> 5 V, GND -> GND
//   light sensor  module AO -> A0, VCC -> 5 V, GND -> GND
//   lamp          pin 5 -> 220 ohms -> green LED -> GND
//   piezo         pin 8 -> piezo -> GND
//
// Two sensors answer two questions. The light module says whether the room is
// lit: its reading falls as the light grows, so a reading below LIT_BELOW is
// a room with the lights on. The PIR says whether anyone is there: its output
// goes high for a few seconds each time something warm moves.
//
// Every movement starts the timer again. When the room has been lit and
// still for AWAY_MS, the reminder starts: one soft chime, then the green lamp
// blinks until someone moves or the lights go off. It also keeps a running
// count of the minutes the lights were left on in an empty room, printed on
// the Serial Monitor, which is a good number for a class to try to bring
// down.
//
// AWAY_MS is 15 seconds here so you can see it work. In a real room, 10 or 15
// minutes is kinder: people sit still to read.
//
// Things to try. Leave the PIR alone and wait. Click the PIR to move. Drag
// the light sensor's slider down to turn the room's lights off.

const int PIR_PIN = 2;
const int LAMP_PIN = 5;
const int PIEZO_PIN = 8;
const int LIT_BELOW = 400;               // A0 reading: lights on below this
const unsigned long AWAY_MS = 15000UL;   // still this long before a reminder

unsigned long lastMoved = 0;
bool reminding = false;
unsigned long wastedSeconds = 0;
unsigned long lastTick = 0;

void setup() {
  Serial.begin(9600);
  pinMode(PIR_PIN, INPUT);
  pinMode(LAMP_PIN, OUTPUT);
  Serial.println("Lights-out reminder watching the room");
}

void loop() {
  unsigned long now = millis();
  bool lit = analogRead(A0) < LIT_BELOW;
  if (digitalRead(PIR_PIN) == HIGH) lastMoved = now;

  bool empty = lit && now - lastMoved >= AWAY_MS;
  if (empty && !reminding) {
    reminding = true;
    Serial.println("Lights on in an empty room: please switch them off");
    tone(PIEZO_PIN, 660, 150);
    delay(180);
    tone(PIEZO_PIN, 880, 250);
  } else if (!empty && reminding) {
    reminding = false;
    digitalWrite(LAMP_PIN, LOW);
    Serial.println(lit ? "Someone is here, thanks" : "Lights off, thank you!");
  }

  if (reminding) {
    digitalWrite(LAMP_PIN, (now / 500) % 2);
    if (now - lastTick >= 1000) {
      lastTick = now;
      wastedSeconds++;
      if (wastedSeconds % 60 == 0) {
        Serial.print(wastedSeconds / 60);
        Serial.println(" minutes of light in an empty room so far");
      }
    }
  }
  delay(20);
}