#define TRIG 4
#define ECHO 5  
#define MOTOR 9
#define SPEED A0

#define TRIG_DIST 150     // Min trigger distance (cm)
#define MOTOR_TIME 5000   // Motor rotation time

long duration, cm, old, mspeed;

void setup() {
  Serial.begin(9600);
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  pinMode(MOTOR, OUTPUT);

  mspeed = map(analogRead(SPEED), 0, 1024, 0, 255);
}

void loop() {

  mspeed = map(analogRead(SPEED), 0, 1024, 0, 255);

  digitalWrite(TRIG, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
  duration = pulseIn(ECHO, HIGH);
  cm = duration / 58;

  Serial.println(cm);

  if(cm <= TRIG_DIST) {

    if(old > TRIG_DIST) {
      for(int i=0; i<mspeed; i++) {
        analogWrite(MOTOR, i);
        delay(30);
      }
    }

    analogWrite(MOTOR, mspeed);
    delay(MOTOR_TIME);

  } else {


    if(old <= TRIG_DIST) {
      for(int i=mspeed; i>0; i--) {
        analogWrite(MOTOR, i);
        delay(20);
      }
    }

    analogWrite(MOTOR, 0);
  }
  
  old = cm;
}