#define pingPin 2     //trig pin of sr04
#define echoPin 3 

void setup() {
  Serial.begin(9600);
  pinMode(pingPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(12, OUTPUT);//pin12 is used as GND pin for buzzer since arduino nano has only two GND pins 
  pinMode(A3, OUTPUT);//pin A3 provides the output on buzzer
}

void loop() {
  long duration, cm;
  digitalWrite(12, LOW);
  
  //send a signal at ping pin at an interval of 0.002 seconds to check for an object
  digitalWrite(pingPin, LOW);
  delayMicroseconds(2);
  digitalWrite(pingPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(pingPin, LOW);
  
  duration = pulseIn(echoPin, HIGH);
  cm = microsecondsToCentimeters(duration); //call the function to calculate the distance in cm
  
  Serial.print(cm);
  Serial.print(" cm"); //add space before "cm"
  Serial.println();
  
  if(cm < 30 && cm > 20) {
    analogWrite(A3, 255);
    delay(1000);
    analogWrite(A3, 0);
    delay(1000);
  } 
  else if (cm < 20 && cm > 10) {
    analogWrite(A3, 255); 
    delay(500); 
    analogWrite(A3, 0); 
    delay(500); 
  }   
  else if (cm < 10 && cm > 0) { //add curly braces for consistency
    analogWrite(A3, 255); 
    delay(100); 
    analogWrite(A3, 0);
    delay(100);
  } else { //add curly braces for consistency
    analogWrite(A3, 0); //do not sound the buzzer
  }
}

//function to return distance in cm from microseconds
long microsecondsToCentimeters(long microseconds) {
  return microseconds / 29 / 2;
}