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photoresistors

I

Isaac

Jan 1, 1970
0
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
 
L

Lord Garth

Jan 1, 1970
0
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


Better re-read what you wrote Isaac!

To me, reduce mean the same as decrease.

Tell us what your application is and perhaps we can suggest alternatives.
 
J

John Popelish

Jan 1, 1970
0
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...

All LDRs decrease resistance as illumination rises.
 
E

Eeyore

Jan 1, 1970
0
Isaac said:
a silly question that google can't seem to answer... are any LDRs
available that will _reduce_ resistance as light _increases_?...

You mean dynamic impedance of course ?

Graham
 
P

Phil Allison

Jan 1, 1970
0
"Eeysore the Pommy Fucking Tenth Wit"
You mean dynamic impedance of course ?


Graham Stevenson : ASD fucked egomaniac & public menace.



** Isaac has it quite right.



....... Phil
 
J

Jamie

Jan 1, 1970
0
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
what is that an oximoron?
 
B

Bob Pownall

Jan 1, 1970
0
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.)
 
I

Isaac

Jan 1, 1970
0
i guess i shouldn't drink and post!... i was interested in a photo
resistor that functions the opposite of those that i purchased from
radio shack... but, i've discovered that a common, and elementary
circuit, has answered my question - the night light, such as here:
http://www.oconee.k12.sc.us/walmid/teachers/Crawford/labs/night_light.htm

however, i'm assuming that because R1 in that circuit is a static
value, then the light level will remain the same as long as the
circuit has switched to its path?..

thanks, all..
 
I

ian field

Jan 1, 1970
0
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

Its quite common to see the LDR in series with a resistor across the supply
to form a voltage divider, and usually feeding one or other input of an
op-amp. To reverse the effect of changing light simply turn the voltage
divider upside down - or use the other input on the op-amp.
 
N

Nobody

Jan 1, 1970
0
i guess i shouldn't drink and post!... i was interested in a photo
resistor that functions the opposite of those that i purchased from
radio shack... but, i've discovered that a common, and elementary
circuit, has answered my question - the night light, such as here:
http://www.oconee.k12.sc.us/walmid/teachers/Crawford/labs/night_light.htm

however, i'm assuming that because R1 in that circuit is a static
value, then the light level will remain the same as long as the
circuit has switched to its path?..

The LED will typically be either fully on (with its current determined by
the battery voltage divided by R) or fully off. The region where it
is only partially illuminated will be quite small.
 
J

John Larkin

Jan 1, 1970
0
(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.)

Actually, it was the "photoelectric effect", which is the emission of
electrons from illuminated metallic surfaces into vacuum.

John
 
P

Phil Allison

Jan 1, 1970
0
"Daniel Mandic"



** Stop posting FALLACIOUS quotations !!!


- you vile, autistic pile of shit.




......... Phil
 
B

Bob Pownall

Jan 1, 1970
0
John Larkin wrote:
Actually, it was the "photoelectric effect", which is the emission of
electrons from illuminated metallic surfaces into vacuum.

John
Yeah, I remembered that shortly after I posted. Should have checked my
memory against a more reliable source, obviously.

Thanks for the correction.

Bob Pownall
 
I

ian field

Jan 1, 1970
0
Daniel Mandic said:
thanks for the compliment!

No, not really ;)


?




Best Regards,

Daniel Mandic

Just ignore Phil - he was raised by dingoes!
 
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