PIR motion light with a relay on an Arduino Uno

// PIR motion light with a relay: after dark, someone walking past switches a
// 12 V lamp on through a relay, and it stays on until nobody has moved for
// ON_MS. In daylight the PIR is ignored.
//
//   pin 2   the HC-SR501's OUT (VCC 5V, GND)
//   pin 3   the photoresistor module's DO: HIGH when darker than its trimmer
//   pin 7   the relay module's IN (high trigger, VCC 5V, GND)
//   relay   COM to the 12 V adapter's +, NO to the lamp, the lamp's other
//           lead to the adapter's -
//           (the adapter's - also to GND, sharing the Uno's ground)
//
// The HC-SR501 holds OUT high for a couple of seconds after the last movement
// (its Tx trimmer) and then needs a moment before it can trigger again. The
// sketch keeps its own, longer timer: every time OUT is high the timer starts
// over, so the light stays on while anyone is moving and goes off ON_MS after
// the last of them.
//
// Darkness is only read while the lamp is off. On a real porch the lamp
// shines on its own sensor, so a sketch that keeps checking would see
// "daylight", switch the lamp off, see "dark" and switch it on again, all
// night. Here the lamp's light does not reach the module, but the sketch is
// written for the real porch.
//
// The PIR's output is 3.3 V, which an Uno pin reads as HIGH (it wants 3.0 V).
// The lamp is 12 V; switching mains with a relay belongs in a closed box.
//
// Press Run, drag the light slider down to night, then click the PIR.

const int PIR_PIN = 2;
const int DARK_PIN = 3;
const int RELAY_PIN = 7;

const unsigned long ON_MS = 20000;  // 20 s after the last movement

bool lampOn = false;
bool dark = false;
bool known = false;  // whether dark has been read yet
unsigned long lastMotion = 0;

void lamp(bool state) {
  lampOn = state;
  digitalWrite(RELAY_PIN, state ? HIGH : LOW);
  Serial.println(state ? "lamp ON" : "lamp OFF");
}

void setup() {
  pinMode(PIR_PIN, INPUT);
  pinMode(DARK_PIN, INPUT);
  digitalWrite(RELAY_PIN, LOW);
  pinMode(RELAY_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("PIR motion light: waiting for dark");
}

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

  if (!lampOn) {
    bool nowDark = digitalRead(DARK_PIN) == HIGH;
    if (nowDark != dark || !known) {
      dark = nowDark;
      known = true;
      Serial.println(dark ? "dark: watching for motion" : "daylight: PIR ignored");
    }
  }

  if (digitalRead(PIR_PIN) == HIGH) {
    if (dark) {
      lastMotion = now;
      if (!lampOn) lamp(true);
    }
  }

  if (lampOn && now - lastMotion >= ON_MS) {
    lamp(false);
  }
  delay(50);
}