// Forum: https://forum.arduino.cc/t/flash-led-using-millis-while-relay-is-activated/1289454
// This Wokwi project: https://wokwi.com/projects/405568161981442049

const int ledPin = 1;    // 'int' or 'byte'
const int relayPin = 0;  // 'int' or 'byte'

const unsigned long FANONDURATION  = 5000UL;
const unsigned long FANOFFDURATION = 15000UL;
const unsigned long LEDONDURATION  = 150;
const unsigned long LEDOFFDURATION = 250;

unsigned long previousMillisFan;
unsigned long fanInterval = FANOFFDURATION;
unsigned long previousMillisLed;
unsigned long ledInterval = LEDONDURATION;

bool ledOn = false;
bool fanOn = false;

void setup(void) 
{
  pinMode(ledPin, OUTPUT);     // set output and low
  pinMode(relayPin, OUTPUT);   // set output and low

  digitalWrite(ledPin, LOW);   // not necessary
  digitalWrite(relayPin, LOW); // not necessary
}

void loop()
{
  unsigned long currentMillis = millis();

  // A millis timer for the fan.
  // With different on and off times.
  if(currentMillis - previousMillisFan >= fanInterval) 
  {
    previousMillisFan = currentMillis;

    if(!fanOn)
    {
      digitalWrite(relayPin, HIGH);
      fanInterval = FANONDURATION;
      fanOn = true;
    }
    else
    {
      digitalWrite(relayPin, LOW);
      fanInterval = FANOFFDURATION;
      fanOn = false;
    }
  }

  // A millis timer for the led.
  // Flash led when the fan is on.
  if(fanOn) 
  {
    if(currentMillis - previousMillisLed >= ledInterval)
    {   
      previousMillisLed = currentMillis;

      if(!ledOn)
      {
        digitalWrite(ledPin, HIGH);
        ledInterval = LEDONDURATION;
        ledOn = true;
      }
      else
      {
        digitalWrite(ledPin, LOW);
        ledInterval = LEDOFFDURATION;
        ledOn = false;
      }
    }
  }
}