/elec/propeller-clock

void setup()

{

  // set up interupt 0 (which is pin 2) and turn on the pull-up resistor as we

  // would if the pin had been set to be an input. This is so that when the

  // switch on the pin is open, it is not a floatin signal and has 5V across it

  // (albeit through a high-resitance resistor to heavily constricy the

  // current).

  attachInterrupt( 0, rpmCounter, RISING );

  digitalWrite( 2, HIGH );

  // init serial comms

  Serial.begin( 9600 );

}

static unsigned long fan_rotations = 0;

void rpmCounter()

{

  fan_rotations++;

}

void loop()

{

  // get milliseconds

  unsigned long start_millis = millis();

  // display fan speed. We divide by 2 because the fan's "sense" pin actually

  // pulses twice per revolution. We multiply by 60 because fan_rotations will

  // have counted the number of rotations in a second and we want RPM.

//  Serial.println( fan_rotations );

  Serial.print( "RPM: " );

  Serial.println( fan_rotations / 2 * 60 );

  fan_rotations = 0;

  // wait until the remainer of a second has elapsed

  delay( start_millis + 1000 - millis() );

}