#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);
  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.print(readDistanceCM());
  delay(100);

}