const int trigPin = 2;
const int echoPin = 3;
long duration, cm;
const int threshold = 20;

void setup() {
  // put your setup code here, to run once:
  pinMode(echoPin, INPUT);
  pinMode(trigPin, OUTPUT);
  pinMode(8, OUTPUT);
  Serial.begin(9600);

}

void loop() {
  // put your main code here, to run repeatedly:
digitalWrite(trigPin, LOW);
delayMicroseconds(2);
digitalWrite(trigPin, HIGH);
delayMicroseconds(10);
digitalWrite(trigPin, LOW);

duration = pulseIn(echoPin, HIGH);
cm = duration*0.034/2;

if (cm>threshold) {
  digitalWrite(8, HIGH);
} else {
  digitalWrite(8, LOW);
}

//Serial.print(cm);
//Serial.println("cm:");
//delay(100);
}