int sensor = 3;
int light = 4;
int sensorState = LOW;
void setup()
{
  pinMode(sensor, INPUT);
  pinMode(light, OUTPUT);
  Serial.begin(9600);
}

void loop()
{
  if(digitalRead(sensor)==HIGH)
  {
    digitalWrite(light, HIGH);
    if(sensorState == LOW)
    {
      Serial.println("Light ON");
      sensorState = HIGH;
    }
  }
  else
  {
    digitalWrite(light, LOW);
    if(sensorState == HIGH)
    {
      Serial.println("LIGHT OFF");
      sensorState = LOW;
    }
  }
}