#include <Servo.h>

Servo motor;
int trigPin = 9;
int echoPin = 10;
int minDist = 2;
int maxDist = 50;

void setup() {
 motor.attach(8);
 pinMode(trigPin, OUTPUT);
 pinMode(echoPin, INPUT);
 Serial.begin(9600);
}

void loop() {
 int distance = getDistance();
 if (distance > maxDist) {
    distance = maxDist;
 }
 if (distance < minDist) {
    distance = minDist;
 }
 motor.write(map(distance, minDist, maxDist, 0, 180));
 delay(100);
}

int getDistance() {
 digitalWrite(trigPin, LOW);
 delayMicroseconds(2);
 digitalWrite(trigPin, HIGH);
 delayMicroseconds(10);
 digitalWrite(trigPin, LOW);
 long duration = pulseIn(echoPin, HIGH);
 int distance = duration * 0.0344 / 2;
 return distance;
}