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

long led_red_previous_millis = 0;
char led_red_status = LOW;
int led_red_interval = 1000;

long led_yellow_previous_millis = 0;
char led_yellow_status = LOW;
int led_yellow_interval = 2000;

long led_green_previous_millis = 0;
char led_green_status = LOW;
int led_green_interval = 500;

void setup()
{
  pinMode(led_red, OUTPUT);
  pinMode(led_yellow, OUTPUT);
  pinMode(led_green, OUTPUT);
}

void loop()
{
  long current_millis; // referee to keep track of time

  // current time
  current_millis= millis();

  //check if it is red led's turn
  if (current_millis-led_red_previous_millis >= led_red_interval)
   {
    // update led red's time
    led_red_previous_millis = current_millis;

    //LOW --> HIGH or
    //High --> LOW
   if (led_red_status == LOW)
    led_red_status= HIGH;
   else
    led_red_status= LOW;
    // control the red led
    digitalWrite(led_red, led_red_status);
   }

  //check if it is yellow led's turn
  if (current_millis-led_yellow_previous_millis >= led_yellow_interval)
   {
    // update led yellow's time
    led_yellow_previous_millis = current_millis;

    //LOW --> HIGH or
    //High --> LOW
    if (led_yellow_status == LOW)
    led_yellow_status= HIGH;
    else
      led_yellow_status= LOW;
      // control the yellow led
     digitalWrite(led_yellow, led_yellow_status);
   }

  //check if it is green led's turn
  if (current_millis-led_green_previous_millis >= led_green_interval)
   {
    // update green red's time
    led_green_previous_millis = current_millis;

    //LOW --> HIGH or
    //High --> LOW
    if(led_green_status == LOW)
    led_green_status= HIGH;
    else
      led_green_status= LOW;
      // control the green led
     digitalWrite(led_green, led_green_status);
   }


}