PSN-L Email List Message

Subject: Re: Not so simple photoelectrics, or are they?
From: "David A. Latsch" blottobear@..........
Date: Wed, 30 May 2001 12:05:36 -0600


"Charles R. Patton" wrote:

> 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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