#define TRIG_PIN 26
#define ECHO_PIN 25
#define BUZZER_PIN 18 
#define LED_PIN 5
#define DISTANCE_THRESHOLD 50 // centimeters

// variables will change: 
float duration_us, distance_cm;

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

void loop() {
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds (10); 
  digitalWrite (TRIG_PIN, LOW);
  // measure duration of pulse from ECHO pin 
  duration_us = pulseIn (ECHO_PIN, HIGH);
  // calculate the distance
  distance_cm= 0.017 * duration_us;
  if (distance_cm< DISTANCE_THRESHOLD)
    digitalWrite(BUZZER_PIN, HIGH); // turn on Piezo Buzzer
  else
    digitalWrite(BUZZER_PIN, LOW); // turn off Piezo Buzzer
    digitalWrite(LED_PIN, HIGH); // turn off Piezo Buzzer Serial.print ("distance: ");
  Serial.print ("distance: ");
  Serial.print (distance_cm); 
  Serial.println(" cm");
  delay(500);
}