int TrigPin = 7 ; //define Trig pin which send a high square wave for 10 
                  //10 micro seconds
int EchoPin = 8; //Define Echo pin which receives the wave and give the time
                // The wave took to reflect from a surface
int Buzzer = 9; //Define Buzzer pin
int i;         //Define index i

long WaveTime ;  // define variables to save the time and distance 
int Distance;

void setup() {

pinMode(TrigPin, OUTPUT); // set TrigPin as output pin
pinMode(EchoPin, INPUT);  // set EchoPin as input pin
pinMode(Buzzer, OUTPUT);  // set Buzzer as output pin
Serial.begin(9600);      // Start serial 

}

void loop() {
digitalWrite(Buzzer,LOW); // inital value for buzzer

digitalWrite(TrigPin, LOW); // Clear TrigPin 
delayMicroseconds(2);       //Delay in micro seconds
  
digitalWrite(TrigPin,HIGH); // Genarate square wave write high on trig pin
                            //delay then write law
delayMicroseconds(10);
digitalWrite(TrigPin,LOW);

WaveTime = pulseIn(EchoPin, HIGH); //measure of the pulse (in microseconds)
                                  // when EchoPin become  HIGH

Distance= (WaveTime*0.0343)/2;    // calculate the distance

Serial.print("Distance: ");       //Print distance on serial monitor
Serial.println(Distance);

    if (Distance<30){            // Compare distance of the obstacle
                                  //if it less than 30 cm
                                //the buzzer will ring in higher frequency

         for( i=0; i<5; i++){           //For loop to generate square wave
                                        // to operate the buzzer
          digitalWrite(Buzzer,LOW);
          delay(50);                    //Delay in milliseconds
          digitalWrite(Buzzer,HIGH);
          delay(50);
          }
        }
  
    if (Distance>30 && 50>Distance){    // Compare distance of the obstacle
                                  //if it more than 30 cm and less than 50cm
                                //the buzzer will ring in higher frequency

      for( i=0; i<5; i++){
          digitalWrite(Buzzer,LOW);
           delay(100);
            digitalWrite(Buzzer,HIGH);
            delay(100);
            }       
      }
  
}