On Sun, May 03, 2009 at 09:05:54PM -0400, Gene Heskett wrote:
> I used 2 pieces of brass tubing from the model hobby store that slip inside
> the next larger size. 1/4" & the next size smaller. I cut a pair of windows
> in the outside 1/4" tube such that a one piece opto interrupter (ir led on
> one
> side, ir photo-transistor looking at the led across the air gap) fit by
> friction across the slots. Then in the smaller tube I cut a longer pair of
> matching windows in, using diamond wheels in a dremel cuz that stuff bends
> easily, too easily to saw, filled the top with solder so a ball point pen
> spring could rest on it pushing it down, made a steel pointed plugin for the
> bottom of it, and slid a piece of a black cable tie inside it to act as the
> light valve. The outside tube is swaged in at the top just enough to trap
> the
> ball point pen spring.
Gene, that's a super clear word picture. Good enough to build from. I
hadn't imagined a simple shutter (the piece of black cable tie),
transitting the light beam from one side (below), cutting off the light
beam in anything but a gradual manner.
The little bagful of Sharp opto interrupters I bought last week have a
0.5 mm slit on the detector side of the slot. Ahh ... with a schmitt
trigger (or just a comparator with minimal hysteresis) on the detector
output, we can trigger on a (reasonably) fixed light level, and so a
fixed point in the 0.5 mm beam transit.
Many thanks for the description. (I'll still be in the audience when the
pictures are published. :-)
Cheers,
Erik
--
Anyone who has had a bull by the tail knows five or six more things
than someone who hasn't.
-- Mark Twain
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