void setup()
{
  pinMode(3, OUTPUT);                                       //D3
  TCCR2A = _BV(COM2B1) | _BV(COM2B0) | _BV(WGM20);            //PWM
  TCCR2B = _BV(WGM22) | _BV(CS22) | _BV(CS21) | _BV(CS20);    //div 1024
  OCR2A = 156;                                                //20mS period
  OCR2B = 148;
}

void loop()
{
 OCR2B = 140;
 delay(1000);
 OCR2B = 148;
 delay(1000);
}