#define ECHO_PIN 2
#define TRIG_PIN 3

void setup() {
  Serial.begin (115200);
  pinMode(LED_BUILTIN,OUTPUT);
  pinMode(TRIG_PIN ,OUTPUT);
  pinMode(ECHO_PIN,INPUT);
}
float readDistanceCM(){
  digitalWrite(TRIG_PIN,LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  int duration=pulseIn(ECHO_PIN,HIGH);
  return duration*0.034/2;
}
void loop(){
  float distance = readDistanceCM();
  bool isNearby = distance < 100;
  digitalWrite(LED_BUILTIN,isNearby);
  Serial.print("Measured distance: ");
  Serial.println(readDistanceCM());
  delay(100);
}