Dusk switch on an Arduino Uno

// Dusk switch: a photoresistor sensor module turns a lamp on at dusk.
//
//   module   VCC -> 5 V   GND -> GND   AO -> A0   DO -> pin 2
//   lamp     the board's LED on pin 13
//
// The module has two outputs. AO is the light as a voltage: the LDR under a
// 10 k resistor, so more light reads lower. DO is the module's own comparator
// against the blue trimmer: high in the dark, low in the light. The sketch
// switches the lamp on DO, so the trimmer sets the light level it comes on
// at, and prints AO so you can see where that is.
//
// Drag the light slider on the module down past about 100 lux and the lamp
// comes on; the DO lamp on the module goes out at the same moment.

const int AO_PIN = A0;
const int DO_PIN = 2;

bool dark = false;
unsigned long printedAt = 0;

void setup() {
  Serial.begin(9600);
  pinMode(DO_PIN, INPUT);
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.println("Dusk switch ready");
}

void loop() {
  bool nowDark = digitalRead(DO_PIN) == HIGH;
  if (nowDark != dark) {
    dark = nowDark;
    digitalWrite(LED_BUILTIN, dark ? HIGH : LOW);
    Serial.println(dark ? "dusk: lamp on" : "daylight: lamp off");
  }
  if (millis() - printedAt >= 500) {
    printedAt = millis();
    Serial.print("light reading ");
    Serial.println(analogRead(AO_PIN));
  }
}