// multitasking 
#include <TimerOne.h>

int led_1 = 12;
int led_2 = 13;

void setup() {
  pinMode(12, OUTPUT);
  pinMode(13, OUTPUT);
  Timer1.initialize(500000); //microseconds,1/2 sec
  Timer1.attachInterrupt(blueBlink);
}

void blueBlink () {
  digitalWrite(12, !digitalRead(12));// viceversa of 1 if 0 ,0 if 1 state
}

void loop() {
  digitalWrite(13, 0);//LOW
  delay (200);
  digitalWrite(13, 1);//HIGH
  delay(200);
}