#define LED_PIN 2
#define LDR_PIN  A0

// LDR Characteristics
const float GAMMA = 0.7;
const float RL10 = 50;

void setup() {
  pinMode(LDR_PIN, INPUT);
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  int analogValue = analogRead(LDR_PIN);
  float voltage = analogValue / 1024. * 5;
  float resistance = 2000 * voltage / (1 - voltage / 5);
  float lux = pow(RL10 * 1e3 * pow(10, GAMMA) / resistance, (1 / GAMMA));

  if (lux > 100) {    
    digitalWrite(LED_PIN, LOW); 
  } else {    
    digitalWrite(LED_PIN, HIGH);   // turn the LED on (HIGH is the voltage level)
   }
}