int ldrPin = A0;
int ldrVal = 0;
int ledPin = 2;
int buzzer = 3;

const float GAMMA = 0.7;
const float RL10 = 50;

void setup () {
  Serial.begin(9600);
  pinMode(2, OUTPUT);
  pinMode(3, OUTPUT);
}
void loop () {
  ldrVal = analogRead(ldrPin);
  float voltage = ldrVal / 1024.* 5;
  float resistance = 2000 * voltage / (1-voltage/5);
  float lux = pow(RL10 * 1e3 * pow(10, GAMMA) / resistance, (1/GAMMA));

  if(lux > 200) {
    digitalWrite (ledPin, LOW);
    tone(buzzer, 1000);
  } else {
    digitalWrite(ledPin, HIGH);
    noTone(buzzer);
  }
}