char led_red = 23;
char led_yellow = 19;
char led_green = 18;



char led_red_state = LOW;
char led_yellow_state = LOW;
char led_green_state = LOW;



long led_red_previous_millis = 0;
long led_yellow_previous_millis = 0;
long led_green_previous_millis = 0;



int led_red_interval = 1000;
int led_yellow_interval = 2000;
int led_green_interval = 500;



void setup() {
  // put your setup code here, to run once:
  pinMode(led_red, OUTPUT);
  pinMode(led_yellow, OUTPUT);
  pinMode(led_green, OUTPUT);
}

void loop() {
  long current_millis = millis();

  if (current_millis - led_red_previous_millis >= led_red_interval)
  {
    led_red_previous_millis = current_millis;

    // now reverse the led's state
    if (led_red_state == LOW)
      led_red_state = HIGH;
    else 
      led_red_state = LOW;

    // control the led accordingly
    digitalWrite(led_red, led_red_state);
  }


  if (current_millis - led_yellow_previous_millis >= led_yellow_interval)
  {
    led_yellow_previous_millis = current_millis;

    // now reverse the led's state
    if (led_yellow_state == LOW)
      led_yellow_state = HIGH;
    else 
      led_yellow_state = LOW;

    // control the led accordingly
    digitalWrite(led_yellow, led_yellow_state);
  }



  if (current_millis - led_green_previous_millis >= led_green_interval)
  {
    led_green_previous_millis = current_millis;

    // now reverse the led's state
    if (led_green_state == LOW)
      led_green_state = HIGH;
    else 
    {  
      led_green_state = LOW;
    }
    // control the led accordingly
    digitalWrite(led_green, led_green_state);
  }

}