// ==========================================
// 4 CHANNEL RELAY SEQUENTIAL CONTROLLER
// ==========================================

// START switch
const byte START_SWITCH = 2;

// Individual switches
const byte SW1 = 3;
const byte SW2 = 4;
const byte SW3 = 5;
const byte SW4 = 6;

// Relay outputs
const byte RELAY1 = 8;
const byte RELAY2 = 9;
const byte RELAY3 = 10;
const byte RELAY4 = 11;

// 10 second interval
const unsigned long INTERVAL = 10000UL;

// Current relay in automatic sequence
byte currentRelay = 0;

// Timer
unsigned long previousMillis = 0;


// ==========================================
// SETUP
// ==========================================

void setup()
{
  // Switches
  pinMode(START_SWITCH, INPUT_PULLUP);

  pinMode(SW1, INPUT_PULLUP);
  pinMode(SW2, INPUT_PULLUP);
  pinMode(SW3, INPUT_PULLUP);
  pinMode(SW4, INPUT_PULLUP);

  // Relays
  pinMode(RELAY1, OUTPUT);
  pinMode(RELAY2, OUTPUT);
  pinMode(RELAY3, OUTPUT);
  pinMode(RELAY4, OUTPUT);

  // Initially all OFF
  allRelaysOff();
}


// ==========================================
// LOOP
// ==========================================

void loop()
{
  // START switch
  bool start = (digitalRead(START_SWITCH) == LOW);


  // ========================================
  // START SWITCH OFF
  // ========================================

  if (!start)
  {
    // IMPORTANT:
    // Immediately turn ALL relays OFF

    allRelaysOff();

    // Reset automatic sequence
    currentRelay = 0;

    // Reset timer
    previousMillis = millis();

    return;
  }


  // ========================================
  // START SWITCH ON
  // ========================================

  automaticSequence();
}


// ==========================================
// AUTOMATIC SEQUENCE
// ==========================================

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


  // ----------------------------------------
  // Start sequence with Relay 1
  // ----------------------------------------

  if (currentRelay == 0)
  {
    currentRelay = 1;

    setRelay(currentRelay);

    previousMillis = currentMillis;

    return;
  }


  // ----------------------------------------
  // Check 10 second interval
  // ----------------------------------------

  if (currentMillis - previousMillis >= INTERVAL)
  {
    previousMillis = currentMillis;

    // Move to next relay
    currentRelay++;

    // After Relay 4, go back to Relay 1
    if (currentRelay > 4)
    {
      currentRelay = 1;
    }

    setRelay(currentRelay);
  }
}


// ==========================================
// TURN ON SELECTED RELAY
// ==========================================

void setRelay(byte relayNumber)
{
  // First turn ALL relays OFF
  allRelaysOff();


  // Then turn selected relay ON
  switch (relayNumber)
  {
    case 1:
      digitalWrite(RELAY1, HIGH);
      break;

    case 2:
      digitalWrite(RELAY2, HIGH);
      break;

    case 3:
      digitalWrite(RELAY3, HIGH);
      break;

    case 4:
      digitalWrite(RELAY4, HIGH);
      break;
  }
}


// ==========================================
// TURN ALL RELAYS OFF
// ==========================================

void allRelaysOff()
{
  digitalWrite(RELAY1, LOW);
  digitalWrite(RELAY2, LOW);
  digitalWrite(RELAY3, LOW);
  digitalWrite(RELAY4, LOW);
}