#define trigPin 13
#define echoPin 12
#define led 11
#define rele 10
#define buzzer 9

int buzzerState = 0;
unsigned long currentMillis;
unsigned long previousMillis = 0;
const long interval = 5000;

void setup() {

 Serial.begin (9600);

 pinMode(trigPin, OUTPUT);

 pinMode(echoPin, INPUT);

 pinMode(led, OUTPUT);

 pinMode(buzzer, OUTPUT);

 pinMode(rele, 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;

 if (distance < 10)

 { digitalWrite(led, HIGH);
   if (buzzerState == 0)
   {
     currentMillis = millis();
     tone(buzzer, 2500);
      
   }
    if (currentMillis - previousMillis >= interval)
        {
         previousMillis = currentMillis;
        
         noTone(buzzer);
         buzzerState = 1;
        }
   digitalWrite(rele, LOW);

 }

 else {

   digitalWrite(led, LOW);
   noTone(buzzer);
   buzzerState = 0;
   digitalWrite(rele, HIGH);

 }
 Serial.print(distance);

 Serial.println(" cm");

 delay(500);

 }