#include <Servo.h>

#define echoPin   2
#define trigPin   3
#define servoPin  9
#define buzzerPin 10
#define ledPin    11

Servo myservo;
long duration;
int distance;
int flag = 0;

//----------------------------------
int alertTimes = 3;
int alertDistance = 20; // 20 CM
//----------------------------------

void setup() {
  Serial.begin(9600);
  Serial.println("Serial COM Test");
  pinMode(buzzerPin, OUTPUT);
  pinMode(ledPin, OUTPUT);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  myservo.attach(servoPin);
  delay(500);
  myservo.write(0);
  delay(1000);
  myservo.detach();
  digitalWrite(ledPin, 0);
  digitalWrite(buzzerPin, 0);
  flag = 0;
}

void loop() {
  US();
  if (distance < alertDistance && flag == 0) {
    myservo.attach(servoPin);
    delay(500);
    myservo.write(90);
    delay(1000);
    myservo.detach();
    alert();
    flag = 1;
  }
  delay(50);
}


void US() {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  duration = pulseIn(echoPin, HIGH);
  distance = duration * 0.034 / 2;
  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.println(" cm");
}

void alert() {
  for (int i = 0; i < alertTimes; i++) {
    digitalWrite(ledPin, 1);
    digitalWrite(buzzerPin, 1);
    delay(1000);
    digitalWrite(ledPin, 0);
    digitalWrite(buzzerPin, 0);
    delay(1000);
  }
}