void setup() {
  pinMode(A0, INPUT_PULLUP);
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  uint16_t hodnota = analogRead(A0);
  float hodnota_V = hodnota * (5.0 / 1024.0);

  if (hodnota_V > 3.3 && hodnota_V < 4.2) {
    analogWrite(LED_BUILTIN, HIGH);
  }
  else {
    analogWrite(LED_BUILTIN, LOW);
  }
  delay(50);
}