---

#define RELAY_PIN 3
#define LDR_PIN 4      // сюда подключили DO
#define BUTTON_PIN 2

void setup() {
  pinMode(RELAY_PIN, OUTPUT);
  pinMode(LDR_PIN, INPUT);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
}

void loop() {
  int isDark = digitalRead(LDR_PIN);   // 1 или 0
  int button = digitalRead(BUTTON_PIN);

  if (isDark == HIGH && button == LOW) {
    digitalWrite(RELAY_PIN, HIGH);
  } else {
    digitalWrite(RELAY_PIN, LOW);
  }
}