// 출처 : bing.com
// 라이센스 : MIT LICENCE

const int trigPin = 9;
const int echoPin = 10;
const int buzzerPin = 11;
int setup_maxDistance = 100;

unsigned long previousMillis = 0;
bool buzzerState = false;

void setup() {
  Serial.begin(9600);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(buzzerPin, OUTPUT);
}

void loop() {
  long duration, distance;
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  duration = pulseIn(echoPin, HIGH);
  distance = (duration / 2) / 29.1;
  
  int interval = map(distance, 0, setup_maxDistance, 50, 400);
  
  if (distance < setup_maxDistance) {
    unsigned long currentMillis = millis();
    if (currentMillis - previousMillis >= interval) {
      previousMillis = currentMillis;
      buzzerState = !buzzerState;
      if (buzzerState) {
        tone(buzzerPin, 1000);
      } else {
        noTone(buzzerPin);
      }
    }
  } else {
    noTone(buzzerPin);
    buzzerState = false;
  }
}