int ldr_pin = A0;
int relay_pin = 4;

int ldr_value;

void setup() {
  pinMode(relay_pin, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  ldr_value = analogRead(ldr_pin);
  Serial.println(ldr_value);

  if (ldr_value < 800) {
    digitalWrite(relay_pin, LOW);
  }
  else {
    digitalWrite(relay_pin, HIGH);
    delay(1000*60*5); // wait 5 minutes
  }
}