bool led_State= true;

void setup() {
  pinMode(5, OUTPUT);
  noInterrupts();   //set interrupts off
  TCCR1A=0;         //set control registers to 0;
  TCCR1B=0;

  TCCR1B |=  1<<CS12;  //set cs12 to 1 for prescallar 256
  TIMSK1 |=  1<<OCIE1A;//set OCIE1A to 1 to compare match A

  OCR1A = 45000;
  interrupts();   //turn interrupts on
}

void loop() {
}
ISR(TIMER1_COMPA_vect){
  TCNT1=0;    //set timer back to 0
  led_State =!led_State;
  digitalWrite(5,led_State);
}