const int trigP = 13;
const int echoP = 4;
const int ledPin = 2; 
long duration;
int distance;

void setup() {
  pinMode(trigP, OUTPUT);
  pinMode(echoP, INPUT);
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);
}

void loop() {

  digitalWrite(trigP, LOW);
  delayMicroseconds(2);
  digitalWrite(trigP, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigP, LOW);
  
  duration = pulseIn(echoP, HIGH);

  distance = duration * 0.034 / 2;
  
  Serial.print("Distance = ");
  Serial.println(distance);
  
  int brightness = map(distance, 0, 400, 100, 0); 
  brightness = constrain(brightness, 0, 100); 

  analogWrite(ledPin, brightness);
  
  delay(500);
}