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DPRG: biasing & reading a phototransistor

Subject: DPRG: biasing & reading a phototransistor
From: Dennis Draheim dld at cyberramp.net
Date: Wed Aug 18 19:19:47 CDT 1999

Mike,

Thanks for the incredibly detailed explanation on comparators.  I've
also downloaded an application note from National, and it pretty much
agrees with what you said.  I'll try out the values you suggested and
let you know how it works.

MIKE MCCARTY -- 93789 wrote:

8< snip >8

> Paper is notoriously transparent to IR.
> 
> )With my disk (printed on an ink jet printer, backed with white card
> )stock) in front of sensor, the best I get is something like 25 microA.
> )
> )You're right, I guess my discs are pretty bad.  I'll try a more
> )reflective paper, & printing on a laser printer instead of an ink jet
> )for a better black.
> 
> Maybe better, maybe not. Germanium (yes, the semiconductor) is
> *transparent* to IR. Wouldn't guess it from looking at it. It looks
> just like lead. So is iodine, and it's so black it's hard to tell it's
> actually purple :)

So, has anyone else made reflective encoder discs, and what did you
make them out of?

> I hope I included enough example math for you to figure out any further
> experiments you might want to try.

You certainly did!  I'm actually starting to remember things from my 
analog electronics class 17 or 18 years ago.  [Unfortunately, I had the 
world's worst teacher for it.  He was a genius, but he taught way over 
the heads of everyone in the class, even the Ph. D. students.  I think 
I got a 60% on the final, and that was one of the higher scores...at the 
end of the class, he announced, "Well, I can't flunk all of you, so I 
guess I'll give you all B's."]

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