// Define pin of microcontroller leading to the gate of the MOSFET
int mosfet_pin = D1;

void setup() {
  // Declare pin as output
  pinMode(mosfet_pin, OUTPUT);
}

void loop() {
  // Increase the duty cycle and by this brightness
  for (int val = 0; val <= 100; val++) {
    // PWM: Set the duty cycle to val/255       
    analogWrite(mosfet_pin, val);
    delay(10); // Wait
  }
  // Decrease the duty cycle and by this brightness
  for (int val = 100; val >= 0; val--) {
    // PWM: Set the duty cycle to val/255   
    analogWrite(mosfet_pin, val);
    delay(10); // Wait
  }
}