photoresistors

On Thu, 08 Nov 2007 08:27:25 -0800, Isaac wrote:

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?..
Click to expand...
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.
 
"Isaac" <isaacdover@gmail.com> wrote in message
news:1194488814.704097.32020@z9g2000hsf.googlegroups.com...
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
Click to expand...
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.
 
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..
 
Isaac wrote:
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.
Click to expand...

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.)
 
Isaac wrote:
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?
Click to expand...


--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
 
"Eeysore the Pommy Fucking Tenth Wit"
Isaac wrote:

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 Stevenson : ASD fucked egomaniac & public menace.
Click to expand...


** Isaac has it quite right.



....... Phil
 
Isaac wrote:

a silly question that google can't seem to answer... are any LDRs
available that will _reduce_ resistance as light _increases_?...
Click to expand...
You mean dynamic impedance of course ?

Graham
 
"Isaac" <isaacdover@gmail.com> wrote in message
news:1194488814.704097.32020@z9g2000hsf.googlegroups.com...
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
Click to expand...

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.
 
Isaac wrote:
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...
Click to expand...
All LDRs decrease resistance as illumination rises.
 
I

Isaac

Guest
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
 
On Wed, 07 Nov 2007 22:36:04 -0700, Bob Pownall
<repownall@netscape.net> wrote:


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

John
 
"Daniel Mandic"



** Stop posting FALLACIOUS quotations !!!


- you vile, autistic pile of shit.




......... Phil
 
John Larkin wrote:
<snip>
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
Click to expand...
memory against a more reliable source, obviously.

Thanks for the correction.

Bob Pownall
 
"Daniel Mandic" <daniel_mandic@aon.at> wrote in message
news:4733e2db$0$15037$91cee783@newsreader02.highway.telekom.at...
Phil Allison wrote:

- you vile, autistic pile of shit.

thanks for the compliment!

No, not really ;)

........ Phil

?




Best Regards,

Daniel Mandic
Click to expand...
Just ignore Phil - he was raised by dingoes!
 
"ian field" <dai.ode@ntlworld.com> wrote in message
news:g82Zi.1830$1x.1111@newsfe4-win.ntli.net...
"Daniel Mandic" <daniel_mandic@aon.at> wrote in message
news:4733e2db$0$15037$91cee783@newsreader02.highway.telekom.at...
Phil Allison wrote:

- you vile, autistic pile of shit.

thanks for the compliment!

No, not really ;)

........ Phil

?




Best Regards,

Daniel Mandic

Just ignore Phil - he was raised by dingoes!
In my best Johnny Carson impersonation,
Click to expand...
"I did not know that"
Mike
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top