const int trigpin = 6;
const int echopin = 7;
const int buzzerpin = 3;
const int distance_threshold = 50;
float duration_us, distance_cm;

void setup() { 
  Serial.begin(9600); 
  pinMode (trigpin, OUTPUT);
  pinMode (echopin, INPUT); 
  pinMode (buzzerpin, OUTPUT);
}
void loop() {
  digitalWrite(trigpin, HIGH) ;
  delayMicroseconds(10);
  digitalWrite(trigpin, LOW);

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

  if (distance_cm<
  distance_threshold)
    digitalWrite(buzzerpin,  HIGH) ;
    else
    digitalWrite(buzzerpin, LOW) ;

    Serial. print("distance: ");
    Serial. print(distance_cm);
    Serial. println("cm");
    delay(5000);
}