#define trigPin   2
#define echoPin   3
#define motor     4

long duration;
int distanceD;


void setup() {
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(motor, OUTPUT);
  digitalWrite(motor, 0);
  Serial.begin(9600);
}

void loop() {
  US();
  if (distanceD < 50) {
    digitalWrite(motor, 1);
    delay(1000);
    digitalWrite(motor, 0);
  }
  delay(100);
}

void US() {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  duration = pulseIn(echoPin, HIGH);
  distanceD = duration * 0.034 / 2;
  Serial.print("Distance: ");
  Serial.print(distanceD);
  Serial.println(" cm");
}