#define ECHO_PIN 4
#define TRIG_PIN 5
#define LED 13 
void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT); 
  pinMode(LED, OUTPUT);

}

int readDistanceCM() {
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  int duration = pulseIn(ECHO_PIN, HIGH);
  return round(duration * 0.0342 / 2);
}

void loop() {
  // put your main code here, to run repeatedly:
  int distance = readDistanceCM();
  if (distance<100){
    digitalWrite(LED, HIGH);
  } else {
    digitalWrite(LED, LOW);
  }
}