Frequency response of LDRs

[Joel Kolstad]

I'm thinking of using some of the combined LEDs/light dependent resistors as
'voltage controlled resistors' in an op-amp circuit. Would anyone care to
take a stab at what sort of frequency response I can get with such an
approach? I.e., does something like a CdS cell LDR have significant
parasitics that will seriously degrade the performance of an op-amp filter?

I'm looking at items such as Silonex (e.g., NSL32). I couldn't find the
Vactec web site (didn't look much, but it's not Vactec.com) and Clairex
doesn't seem to make these any more.

Thanks,
---Joel Kolstad

 

[Keith Wootten]

In message <KsKdnRbcBaRblZbfRVn-sA@comcast.com>, Joel Kolstad
<JKolstad71HatesSpam@Yahoo.Com> writes

I'm thinking of using some of the combined LEDs/light dependent resistors as
'voltage controlled resistors' in an op-amp circuit. Would anyone care to
take a stab at what sort of frequency response I can get with such an
approach? I.e., does something like a CdS cell LDR have significant
parasitics that will seriously degrade the performance of an op-amp filter?

I don't know the specifics, but they're certainly fine in the tens of
kilohertz region. The problems are more to do with the 'Light History
Effect' where their resistance depends not only on their current
illumination, but also on their past illumination, so making predictable
settings difficult.

Http://www.aicl.com.tw/cds/p6-12.htm is a good guide. Look at the
'Light History Effect' section.

Cheers
--
Keith Wootten

 

[Tony Williams]

In article <KsKdnRbcBaRblZbfRVn-sA@comcast.com>,
Joel Kolstad <JKolstad71HatesSpam@Yahoo.Com> wrote:

I'm thinking of using some of the combined LEDs/light dependent
resistors as 'voltage controlled resistors' in an op-amp
circuit. Would anyone care to take a stab at what sort of
frequency response I can get with such an approach? I.e., does
something like a CdS cell LDR have significant parasitics that
will seriously degrade the performance of an op-amp filter?

I used them many years ago, in an analogue multiplier.
Clairex? did a pair of cells in one can. I used one
light source and servo'd the control voltage on one
cell. The other cell then controlled the signal channel.

ISTR that they were fairly slow at low light levels,
presumably due to the high resistance and stray capacitance.

--
Tony Williams.

 

Have a look at the Fairchild H11F1 and H11F3 parts.

http://www.fairchildsemi.com/ds/H1/H11F1.pdf

Wost case turn-on and turn-off times are listed as 25usec, but the
resistive element is a FET rather than some horrible lump of CdS.

-----------------
Bill Sloman, Nijmegen

 

[Joel Kolstad]

Hi Jim,

"Jim Thompson" <thegreatone@example.com> wrote in message
news:75vm01pvja84fhjh3ikmdld661pbuuva6v@4ax.com...

If it's uP-controlled, why not use a ladder of R's switched with
analog switches?

Since I'm trying to minimize parasitics. This is part of the 'can you build
an active 30MHz filter with a GHz op-amp?' idea.

---Joel