#define TRIG 3
#define ECHO 2
#define LED  9
#define BUZ  8

void setup() {
  Serial.begin(9600);
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  pinMode(LED, OUTPUT);
  pinMode(BUZ, OUTPUT);
}

long readDistance() {
  digitalWrite(TRIG, LOW); delayMicroseconds(2);
  digitalWrite(TRIG, HIGH); delayMicroseconds(10);
  digitalWrite(TRIG, LOW);
  return pulseIn(ECHO, HIGH, 30000) * 0.034 / 2;
}

void loop() {
  long d = readDistance();
  if (d > 0 && d < 50) {
    digitalWrite(LED, HIGH);
    digitalWrite(BUZ, HIGH);
    Serial.print("ALERT: "); Serial.print(d); Serial.println(" cm");
  } else {
    digitalWrite(LED, LOW);
    digitalWrite(BUZ, LOW);
    Serial.print("Clear: "); Serial.print(d); Serial.println(" cm");
  }
  delay(200);
}