#define trig 12
#define echo 13
#define power_pin 11
void setup()
{
  Serial.begin(9600);
  pinMode(trig,OUTPUT);
  pinMode(echo,INPUT);
  pinMode(power_pin,OUTPUT);
  digitalWrite(power_pin,HIGH);           // As because there is no external power source so we need the pin 11 as HIGH
}

void loop()
{
  double distance = hc_sr04(trig, echo); // calls hc_sr04() and get the distance form it
  Serial.print(distance);                // print the distance
  Serial.println("cms ahead");
}

double hc_sr04(double trigPin , double echoPin)
{
  double duration, distance;
  // This procedure is mentioned in the datasheet of HC-SR04 attached
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  duration = pulseIn(echoPin, HIGH); //Check out the refernce to know about this function......select "pulseIn" then press Ctrl + Shift + F 
  
  distance = ((duration/1000000)*33000)/2; // distance = speed * time/2
  return constrain(distance,0,200);    // dont know what constrain is just select it and Ctrl + Shift + F 
}