const int ledPin = 6; // The pin of your LED
const int trigPin = 5; // The pin to which the TRIG is connected
const int echoPin = 4; // The pin to which the ECHO is connected
const int ledOnTime = 1000; // The time that the LED stays on, after detecting the motion (in milliseconds, 1000ms = 1s)
const int trigDistance = 20; // The distance (and lower than it) at which the sensor is triggered (in centimeters)

int duration;
int distance;

void setup() {
  pinMode(ledPin, OUTPUT);
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
}

void loop() {
  digitalWrite(trigPin, LOW);
  digitalWrite(trigPin, HIGH);
  delay(1);
  digitalWrite(trigPin, LOW);
  duration = pulseIn(echoPin, HIGH);
  distance = duration * 0.034 / 2;

  if (distance <= trigDistance) {
    digitalWrite(ledPin, HIGH);
    delay(ledOnTime);
    digitalWrite(ledPin, LOW);
  }
  delay(100);
}