const int LED_PINS[] = {5,18,19,21};
const int NUM_LEDS   = sizeof(LED_PINS)/sizeof(int);

#define OFF (LOW)
#define ON (HIGH)

void setup() {
  
}

const int PWM_RESOLUTION = 8;
const int PWM_FREQ = 1000;
const int DUTY_MAX = (1 << PWM_RESOLUTION);

void turn_on() {
  for ( int i=0; i < NUM_LEDS; i++ ) {
      ledcSetup( i /*channel*/, PWM_FREQ, PWM_RESOLUTION );
      ledcAttachPin( LED_PINS[i] /*pin*/, i /*channel*/ );
      for ( int x=0; x < DUTY_MAX; x++ ) { 
         ledcWrite( i, x );
         delay(5);
      }
      ledcDetachPin( LED_PINS[i] ); 
      digitalWrite( LED_PINS[i], ON );
      delay(200);
  }
}

void turn_off() {
  for ( int i=0; i < NUM_LEDS; i++ ) {
      ledcSetup( i /*channel*/, PWM_FREQ, PWM_RESOLUTION );
      ledcAttachPin( LED_PINS[i] /*pin*/, i /*channel*/ );
      for ( int x=0; x < DUTY_MAX; x++ ) { 
         ledcWrite( i, DUTY_MAX-1-x );
         delay(5);
      }
      ledcDetachPin( LED_PINS[i] ); 
      digitalWrite( LED_PINS[i], OFF );
      delay(200);
  }
}

void loop() {
  turn_on();
  turn_off();
}