int toggle = 0;

void setup() {
  // put your setup code here, to run once:
    pinMode(2, OUTPUT);


cli();//stop interrupts


//set timer1 interrupt at 1Hz
  TCCR1A = 0;// set entire TCCR1A register to 0
  TCCR1B = 0;// same for TCCR1B
  TCNT1  = 0;//initialize counter value to 0
  // set compare match register for 1hz increments
  OCR1A = 15624;// = (16*10^6) / (1*1024) - 1 (must be <65536)
  // turn on CTC mode
  TCCR1B |= (1 << WGM12);
  // Set CS12 and CS10 bits for 1024 prescaler
  TCCR1B |= (1 << CS12) | (1 << CS10);  
  // enable timer compare interrupt
  TIMSK1 |= (1 << OCIE1A);

  sei();//allow interrupts

}

ISR(TIMER1_COMPA_vect){//timer1 interrupt 1Hz toggles pin 2 (LED)
//generates pulse wave of frequency 1Hz/2 = 0.5Hz (takes two cycles for full wave- toggle high then toggle low)

  if (toggle == 4){
    digitalWrite(2,HIGH);
    toggle = 0;
  }
  else{
    digitalWrite(2,LOW);
  }
  toggle++;//toggle increments untill it reaches 4 and resets the process
}

void loop() {
  // put your main code here, to run repeatedly:

}