#define BUZZER_PIN  3
#define LDR_PIN     A0

void setup()
{
    pinMode(BUZZER_PIN, OUTPUT);
}

void loop()
{
  int val, frequency;

  val = analogRead(LDR_PIN);

    frequency = map(val, 0, 1023, 3500, 50000);

    tone(BUZZER_PIN, frequency, 20);
}