#define LED_PIN 9
#define MIN_R 50
#define MAX_R 200

int r = 126;
int rv = 0;
int ra = 0;

void setup() {
  // put your setup code here, to run once:
  TCCR2B = TCCR2B & B11111000 | B00000010; // for PWM frequency of 3921.16 Hz D3 D11
  TCCR1B = TCCR1B & B11111000 | B00000010; // for PWM frequency of 3921.16 Hz D9 D10
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(115200);
  display();
}

void loop() {
  // put your main code here, to run repeatedly:
  update();
  display();
  delay(10);
}

void display(){
  analogWrite(LED_PIN, r);
  Serial.println(r);
}

void update(){
  ra += random(-3,3);
  rv += ra;
  r += rv;
  if(r < MIN_R){
    rv = (MIN_R - r);
    ra = -ra*0.5;
    r = MIN_R;
    update();
  } else if(r > MAX_R) {
    rv = (MAX_R - r);
    r = MAX_R;
    ra = -ra*0.1;
    update();
  }
}