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int distanceCm = 0;
 
/* using variables / identifiers instead of hard-coded values
makes the code more readable and easier to modify */
 
int quarterSpeed = 64;
int fullSpeed = 255;
/* the following names assume a scenario where
left motor is controlled*/
int leftMotorFwd = 6;
int leftMotorBwd = 5;
 
long readUltrasonicDistance(int triggerPin, int echoPin)
{
  pinMode(triggerPin, OUTPUT);  // Clear the trigger
  digitalWrite(triggerPin, LOW);
  delayMicroseconds(2);
  // Sets the trigger pin to HIGH state for 10 microseconds
  digitalWrite(triggerPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(triggerPin, LOW);
  pinMode(echoPin, INPUT);
  /

* Reads the echo pin, and returns the sound wave
  travel time in microseconds */
  return pulseIn(echoPin, HIGH);
}
 
void setup()
{
  pinMode(

leftMotorBwd, OUTPUT);
  pinMode(

leftMotorFwd, OUTPUT);
}
 
void loop()
{
  distanceCm = 0.01723 * readUltrasonicDistance(9, 8);
  if (distanceCm > 50) {
    analogWrite(

leftMotorBwd, 0);
    analogWrite(

leftMotorFwd, fullSpeed);
  } else {
    if (distanceCm > 25) {
      analogWrite(

leftMotorBwd, 0);
      analogWrite(

leftMotorFwd, quarterSpeed);
    } else {
      if (distanceCm <= 20) {
        analogWrite(

leftMotorBwd, quarterSpeed);
        analogWrite(

leftMotorFwd, 0);
      } else {
        analogWrite(

leftMotorBwd, 0);
        analogWrite(

leftMotorFwd, 0);
      }
    }
  }
  delay(200); // Wait for 200 millisecond(s)
}