#define DO_PIN 25
#define LED_PIN 19

void setup() {
  // initialize serial communication
  Serial.begin(9600);
  // initialize the Arduino's pin as an input
  pinMode(DO_PIN, INPUT);
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  int lightState = digitalRead(DO_PIN);

  if (lightState == HIGH)
    digitalWrite(LED_PIN, HIGH);
  else
    digitalWrite(LED_PIN, LOW);
}