Isaac said:
a silly question that google can't seem to answer... are any LDRs
available that will _reduce_ resistance as light _increases_?... or is
this a circuit type that i should learn to build?... i bought a few
radio shack and have tested them, but they all decrease resistance as
light increases...
thanks,
isaac
Your question, as written, doesn't make much sense to me.
I'm assuming that what you meant to ask is if there are any materials
where the resistance increases as light increases. (As someone else
pointed out, "reduce resistance" and "decrease resistance" mean the same
thing.)
Most materials, if their resistivity changes at all under illumination,
experience a decrease in resistivity. This is commonly known as
photoconductivity. (Conductivity = 1/resistivity, so conductivity goes
up as light goes up.) (FWIW, to exhibit photoconductive behavior, a
material has to be a semiconductor, not a metal or an insulator. And
there are some pretty odd semiconductors...)
Google on "negative photoconductivity" for information on materials
where the conductivity goes down (resistance goes up) as the incident
light power goes up. (If you find anything by a Richard Bube or an
Albert Rose, that would be worth paying attention to. I don't know if
they've done any work in this area, but they're two of the leading names
in photoconductivity.)
I doubt you'll find anything of use to you, because (AFAIK) the
materials which exhibit negative photoconductivity generally require
special preparation or special conditions.
Never-the-less, it's an interesting enough phenomenon that there's a
moderate amount of research going on to better understand it.
Bob Pownall
(Random trivia - If I'm not mistaken, Einstein's Nobel Prize came not
for his work on special or general relativity, but for his work in
explaining photoconductivity in terms of quantum mechanics.)