Not sure you get it, Charlie. You don't have to
resolve the pulse width, only the timing of the
relationship between the reference pulse and the
detector pulse. And as long as you dgtect at the
same point for each cell most of the time, you
should be OK. It doesn't matter (really) how sharply
defined the pulse is, even, as long as it is
detected consistently. And even if not so consistent,
the sample rate of 60 samples per minute should
avaerage out the noise and allow precise timing.

------Original Message------
From: "Charles R. Patton" 
To: psn-l@..............
Sent: May 30, 2001 4:53:02 PM GMT
Subject: Re: Not so simple photoelectrics, or are they?


Thomas W Leiper  wrote:
"You could put a cheap linear filament bulb (like fish tank bulbs)
inside a rotating can with a vertical slit (aligned with the bulb
filament). Put one photocell close to the can for a reference signal. As
the can rotates you get a timing pulse."

My thoughts:
On the surplus market, in the motors section of the various catalogs
there are often motors with the multi-faceted mirror attached.  These
units have come out of laser scanner units.  If you wanted to try Tom's
method, I think it would be much simpler to start with one of these as
they're balanced, and have good optical first surface mirrors. 
Personally I think it would be a difficult challenge to obtain
satisfactory results for the following reason.  Start with a reasonable
rotation rate of 3600 rpm on the scanner motor.  Assume that your
optical path length is 10 ft long and that your detectors are 0.1 inch
in dia.  Your first number is the pulse width on the detector.  0.1 in /
{10 ft * 12 in/ft * 2 * pi * (3600rpm/60 sec/min)} = pulse width in
seconds = 2.210485320721E-6   Or about 2 us.  You now need to resolve
this to parts in 2^16.  It's not going to happen.  The fastest optical
diodes available today are in the 10 GHz range which would only get you
to 2^7 and those diodes and circuitry are not cheap -- they are state of
the art.  In fact, it would take a bit of doing to even see, much less
decimate, the 2 us pulse with standard silicon solar cells.  Some
photo-diodes have responses beyond this region but you have left the
easy designs and are starting to step into interesting territory.

Regards,
Charles R. Patton
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