int motorForward = 9;                              //Motor control pin
int motorBackward = 10;                            //Motor control pin
float duration;                                    //Variable
float distance;                                    //Variable
int echoPin = 8;                                   //Echo pin of HC-SR04
int triggerPin = 7;                                //Trigger pin of HC-SR04
float maxDistance=200;


void setup() {
  pinMode(echoPin,INPUT);
  pinMode(triggerPin,OUTPUT);
  pinMode(motorForward,OUTPUT);
  pinMode(motorBackward,OUTPUT);
  Serial.begin(9600);                              //Enable serial monitor

}

void move_forward()                                //Function to move motor 'forward'
{analogWrite(motorForward,maxDistance-255+distance);
  digitalWrite(motorBackward,LOW);
}

void move_backward()                               //Function to move motor 'backward'
{analogWrite(motorBackward,50+distance);
  digitalWrite(motorForward,LOW);
}

void move_stop()                                   //Function to stop the motor
{digitalWrite(motorForward,LOW);
digitalWrite(motorBackward,LOW);
}
void loop() {
digitalWrite(triggerPin,LOW);                      //Ensure the sensor is not ON at start of loop
delayMicroseconds(2);

digitalWrite(triggerPin,HIGH);                    //Trigger the sensor to send a high frequency pulse
delayMicroseconds(10);
digitalWrite(triggerPin,LOW);
duration = pulseIn(echoPin,HIGH);                 //Calculate time taken to receive the reflected signal
distance = (duration/2)*0.0344;                   //Compute distance of object (in cm) from the sensor face
if (distance >= maxDistance || distance <= 2){            //Display the distance, change the condition for your range
    Serial.print("Distance = ");
    Serial.println("Out of range");
  }
  else {
    Serial.print("Distance = ");
    Serial.print(distance);
    Serial.println(" cm");
}
if(distance<=20)                                  //If object is at a distance less than 20 cm, move 'forward'
{move_backward();
}
if(distance>=30)                                  //If object is at a distance greater than 30 cm, move 'backward'
{move_forward();
}
if(distance>20 and distance<30)                   //If object is between 20 and 30 cm from the sensor, stop the motor
{move_stop();
}
delay(500);                                       //Run every half second
}