//  timer grundlage für 7,8 hz   'schumann frequenz'
//  OCR1A = ((CPU frequ/Prescaler * ISR frequ)) -1      isr frqu  =  gewünschte frequenz

void setup()
{
 pinMode(11,OUTPUT);            // led pin 50 hz    ausgang

// start timer setup
// timer setup for highly preceised timed measurements

  cli();      // stop all interrupts

  // turn on CTC mode

  TCCR1A = 0;         // set entire TCCR1A register to 0
  TCCR1B = 0;         // same for TCCR1B
  TCCR1B |= (1 << WGM12);

  // set CS11 bit for prescaler 8

  TCCR1B |= (1 << CS11);

  // initialize counter value to 0

  TCNT1 = 0;

  // set timer count for 50 hz increments

  OCR1A = 39999;      // = (16*10^6) / (50*8) -1

  // enable timer compare interrupt

  TIMSK1 |= (1 << OCIE1A);


  sei();      // allow interrupts

  // end timer setup
  
}

ISR(TIMER1_COMPA_vect)
{
  digitalWrite(11,HIGH);
  delay(15);
  digitalWrite(11,LOW); 
}

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

}