#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(1000);
}