Hello,

On Thu, 14 Jan 1999, Brad Stockdale wrote:

> At 10:41 AM 1/14/99 -0800, you wrote:
>
> >Tricky part seems like it would be handling the vast change
> >in ambient light level.  Say a basement computer room going
> >from bright flourescent lighting (pulsing at 120 Hz in US)
> >to nearly pitch blackness.  Maybe a grain of wheat bulb on
> >the PIC sensor would be a good idea...
>
> Well, an idea that comes to my mind is to modulate the photodiode (say at
> 40 Hz or something common like that -- that's how the IR remotes for TV's
> work). Then, you use a discriminator type circuit to look for the 40 Hz
> 'signature' and go from there.

In fact, nearly all electronically commutated fan motors can be tapped for
rotation signal off any phase, using an AC coupling and a single
transistor to shape the signal. This requires 3 * R, 1 * C, 1 * Trz. They
are usually biphase so the signal should be within the Intel spec. AC
driven fans are a different matter, but they can be sensed using a SMT
Hall sensor IC and an inexpensive SMT op-amp near the rotating permanent
magnet edge. Many muffin fans have a special hole in the outer rim for a
reflective photocoupler. This is for photocoupler sensing as you are
trying to use.

hope this helps,

        Peter