blinkInterrupts.ino

int ledpin = PORTB5;

void setup() {
  // put your setup code here, to run once:
  DDRB = _BV(ledpin);

  TCCR1A = B00000000; //Normal Mode, no automatic pin toggling
  TCCR1B = _BV(CS12) | _BV(CS10); //Set up x1024 prescaler
  TIMSK1 = _BV(TOIE1); //Enable overflow interrupt

  interrupts(); //Enable interrupts globally
}

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

}

ISR(TIMER1_OVF_vect){   //Timer1 Overflow ISR
  PORTB ^= _BV(PORTB5);
}