int ledPin = 5; 
volatile bool LED_STATE = 0; // setting initial led state to low
volatile int count = 0;

void setup(){
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, LED_STATE);
  cli(); // Stops interuppts until all settings are made
  TCCR1A = 0; //Timer/Counter 1 control register A (PWM) - SET TO 0
  TCCR1B = 0; //Timer/Counter 1 control register B (PRESCALER) - SET TO O
  TCCR1B |= B00000101; //set prescaler to 1024
  TCNT1 = 0; // reset timer to 0
  TIMSK1 |= B00000010; //enables OCIE1A to enable compare match A
  OCR1A = 39063; //Sets compare register A to this value - (2.5 sec)
  sei(); // Enable back the interuppts

}
void loop (){

}

ISR(TIMER1_COMPA_vect){
  TCNT1 = 0; // resets timer to 0
  count += 1; // after every 2.5 sec the count is inreased
  Serial.print(count);

  if (count == 2){ // when the count is 2 that is after 5sec the LED is toggled
   LED_STATE =! LED_STATE;
   digitalWrite(ledPin,LED_STATE); //Toggles the pin 
   count = 0;
  }

}