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From: Tim Williams on 22 Mar 2010 23:33
"Phil Hobbs" <pcdhSpamMeSenseless(a)electrooptical.net> wrote in message
news:4BA80B04.2000500(a)electrooptical.net...
> I don't know the origin of the photoresponse hysteresis of CdS and CdSSe,
> but a SWAG would be that there are long-lived trap states that get
> saturated in very bright light, and that this effect makes the carrier
> lifetime longer.

Oh, ya know, for a physics lab we exposed CdS photocells to a strobe light
and watched the "exponential" decay. Now, theory claims recombination is
proportional to concentration, so the exponential feeds back into itself
effectively as a varying tau. We were supposed to measure this 1/R
correspondence (where R is inversely proportional to carrier concentration,
hence the measurement), but when I ran through all the plots and such, it
looked stupid. On a whim, I took sqrt(tau) instead and that was linear. I
pretty well stumped the professor with that one. ;-)

Weird? Ya...

Oh, and don't forget you can use CdS photocells as MOSFETs.
http://sparkbangbuzz.com/cds-fet/cds-fet.htm

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


From: Jasen Betts on 23 Mar 2010 03:53
On 2010-03-22, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote:
> On Mon, 22 Mar 2010 10:07:56 -0500, "Tim Williams"
><tmoranwms(a)charter.net> wrote:
>
>>"Jim Thompson" <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote in
>>message news:hoveq5paaq3k2b28unqe67u0ojd7cdom8n(a)4ax.com...
>>> My task is to produce a PWM output proportional to a photodiode
>>> current.
>>>
>>> I guess I need to find a typical one and measure it myself.
>>
>>Well, you didn't specify what kind of proportionality, so I guess I win
>>firsties.
>>http://webpages.charter.net/dawill/Images/Photodiode_PWM.gif
>>
>>Tim
>
> Except that's current-to-frequency :-)

It doesn't have to be, feed a square wave clock through a small
capacitor to the gate if there's a particular frequency you want.



--- news://freenews.netfront.net/ - complaints: news(a)netfront.net ---
From: Wimpie on 23 Mar 2010 07:57
On 22 mar, 23:52, hal-use...(a)ip-64-139-1-69.sjc.megapath.net (Hal
Murray) wrote:
> In article <4BA7797C.7020...(a)electrooptical.net>,
>  Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> writes:
>
> >  Night lights use CdS or CdSSe photoconductors, but they don't have to
> >have any kind of stability or repeatability, and they don't.  (CdSSe's
> >resistance can vary 5X due entirely to previous illumination history.)
>
> What is the mechanism for that history quirk?
>
> What is the time scale?  How long does it remember the history?
>
> --
> These are my opinions, not necessarily my employer's.  I hate spam.

Hello,

Go to www.perkinelmer.com and search for "photocell", or follow the
links. When at the photocell page, on the right side you have an
application note link. They have a very nice document on the LDRs that
includes the memory effect also.

Best regards,


Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me
From: Wimpie on 23 Mar 2010 08:32
On 23 mar, 12:57, Wimpie <wimabc...(a)tetech.nl> wrote:
> On 22 mar, 23:52, hal-use...(a)ip-64-139-1-69.sjc.megapath.net (Hal
>
> Murray) wrote:
> > In article <4BA7797C.7020...(a)electrooptical.net>,
> >  Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> writes:
>
> > >  Night lights use CdS or CdSSe photoconductors, but they don't have to
> > >have any kind of stability or repeatability, and they don't.  (CdSSe's
> > >resistance can vary 5X due entirely to previous illumination history.)
>
> > What is the mechanism for that history quirk?
>
> > What is the time scale?  How long does it remember the history?
>
> > --
> > These are my opinions, not necessarily my employer's.  I hate spam.
>
> Hello,
>
> Go towww.perkinelmer.comand search for "photocell", or follow the
> links. When at the photocell page, on the right side you have an
> application note link. They have a very nice document on the LDRs that
> includes the memory effect also.
>
> Best regards,
>
> Wim
> PA3DJSwww.tetech.nl
> without abc, PM will reach me

The document that I have is no longer present on the Perkinelmer
website. Here:
www.tetech.nl/divers/BRO_PhotoconductiveCellsAndAnalogOptoiso.pdf
you can find the document about LDR. I will leave it there for about
10 days.

Best regards,

Wim
PA3DJS
www.tetech.nl
From: George Herold on 23 Mar 2010 09:59
On Mar 23, 8:32 am, Wimpie <wimabc...(a)tetech.nl> wrote:
> On 23 mar, 12:57, Wimpie <wimabc...(a)tetech.nl> wrote:
>
>
>
>
>
> > On 22 mar, 23:52, hal-use...(a)ip-64-139-1-69.sjc.megapath.net (Hal
>
> > Murray) wrote:
> > > In article <4BA7797C.7020...(a)electrooptical.net>,
> > >  Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> writes:
>
> > > >  Night lights use CdS or CdSSe photoconductors, but they don't have to
> > > >have any kind of stability or repeatability, and they don't.  (CdSSe's
> > > >resistance can vary 5X due entirely to previous illumination history..)
>
> > > What is the mechanism for that history quirk?
>
> > > What is the time scale?  How long does it remember the history?
>
> > > --
> > > These are my opinions, not necessarily my employer's.  I hate spam.
>
> > Hello,
>
> > Go towww.perkinelmer.comandsearch for "photocell", or follow the
> > links. When at the photocell page, on the right side you have an
> > application note link. They have a very nice document on the LDRs that
> > includes the memory effect also.
>
> > Best regards,
>
> > Wim
> > PA3DJSwww.tetech.nl
> > without abc, PM will reach me
>
> The document that I have is no longer present on the Perkinelmer
> website. Here:www.tetech.nl/divers/BRO_PhotoconductiveCellsAndAnalogOptoiso.pdf
> you can find the document about LDR. I will leave it there for about
> 10 days.
>
> Best regards,
>
> Wim
> PA3DJSwww.tetech.nl- Hide quoted text -
>
> - Show quoted text -

Thanks for the document, page 35 talks about shot noise in LDR's. Are
they just throwing equations around or is there shot noise in LDR's?

George H.
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