#define TRIG_PIN 23 
#define ECHO_PIN 22
#define LED 2 //
#define DISTANCE_THRESHOLD 50
float duration_us, distance_cm;
void setup()
     {
     Serial.begin(9600);
     pinMode(TRIG_PIN, OUTPUT);
     pinMode(ECHO_PIN, INPUT);
     pinMode(LED, OUTPUT);
     }
void loop()
     {
      digitalWrite(TRIG_PIN, HIGH);
      delayMicroseconds(10);
      digitalWrite(TRIG_PIN, LOW); 

      duration_us = pulseIn(ECHO_PIN, HIGH);
      distance_cm = 0.017 * duration_us;

      if (distance_cm < DISTANCE_THRESHOLD)
         digitalWrite(LED, HIGH);
      else
          digitalWrite(LED, LOW);
     Serial.print("distance:  ");
     Serial.print(distance_cm);
     Serial.println("  cm");
     delay(500);
     }