#define TRIG_PIN   26 
#define ECHO_PIN   25 
#define BUZZER_PIN 17 
#define DISTANCE_THRESHOLD 50


float duration_us, distance_cm;

void setup() {
  Serial.begin (9600);        
  pinMode(TRIG_PIN, OUTPUT);  
  pinMode(ECHO_PIN, INPUT);    
  pinMode(BUZZER_PIN, OUTPUT); 
}

void loop() {
  
  digitalWrite(TRIG_PIN, HIGH);
  delay(1000);
  digitalWrite(TRIG_PIN, LOW);


  duration_us = pulseIn(ECHO_PIN, HIGH);
 
  distance_cm = 0.034/2 * duration_us;

  if (distance_cm < DISTANCE_THRESHOLD)
    digitalWrite(BUZZER_PIN, HIGH); 
  else
    digitalWrite(BUZZER_PIN, LOW);  

  
  Serial.print("distance: ");
  Serial.print(distance_cm);
  Serial.println(" cm");

  delay(500);
}