LDR reads low (10K) in darkness?

[Bill Bowden]

I repaired an outdoor dusk/dawn LED lamp that had a defective LDR used to switch the LED on/off. Problem was the LDR resistance read low (10K)in darkness, so I replaced it with one I had laying around and got it working. Since then (don't know how long) the same problem has returned with the LDR resistance again reading low in darkness. I checked a few LDRs in my spare parts and found one that also read low in darkness, which had never been used. Do these things go bad over time, and do they deteriorate sitting in the sun all day?

-Bill

 

[defaullt]

On Mon, 18 Nov 2013 17:19:19 -0800 (PST), Bill Bowden
<bperryb@bowdenshobbycircuits.info> wrote:

I repaired an outdoor dusk/dawn LED lamp that had a defective LDR used to switch the LED on/off. Problem was the LDR resistance read low (10K)in darkness, so I replaced it with one I had laying around and got it working. Since then (don't know how long) the same problem has returned with the LDR resistance again reading low in darkness. I checked a few LDRs in my spare parts and found one that also read low in darkness, which had never been used. Do these things go bad over time, and do they deteriorate sitting in the sun all day?

-Bill

They absolutely most certainly do go bad just sitting around. They
are incredibly sensitive to moisture. Potting in polyester resin,
epoxy etc. or using hermetically sealed photocells is the only way to
get around the problem. Humidity will kill them while they sit in a
parts bin

The cells with just a thin clear coating of epoxy don't deal with
water very well at all.

Photo transistors make lots more sense - one photo transistor, one
resister, and one mosfet and you have a dawn to dusk light control
(for DC lights). I've found that when the lights come on that the
"linear region" amounts to less than 30 seconds (as the light grows
dim - a change I can't even distinguish with my eyes) And the circuits
(three of them) have worked for years now.

 

[Frnak McKenney]

On Tue, 19 Nov 2013 09:01:01 -0500, defaullt <defaulter_@defaulter.net> wrote:

On Mon, 18 Nov 2013 17:19:19 -0800 (PST), Bill Bowden
bperryb@bowdenshobbycircuits.info> wrote:

I repaired an outdoor dusk/dawn LED lamp that had a defective LDR
used to switch the LED on/off. Problem was the LDR resistance read
low (10K)in darkness, so I replaced it with one I had laying around
and got it working. Since then (don't know how long) the same
problem has returned with the LDR resistance again reading low in
darkness. I checked a few LDRs in my spare parts and found one
that also read low in darkness, which had never been used. Do
these things go bad over time, and do they deteriorate sitting in
the sun all day?

-Bill

They absolutely most certainly do go bad just sitting around. They
are incredibly sensitive to moisture. Potting in polyester resin,
epoxy etc. or using hermetically sealed photocells is the only way
to get around the problem. Humidity will kill them while they sit
in a parts bin

The cells with just a thin clear coating of epoxy don't deal with
water very well at all.

While not ignoring the idea of a photodiode/phototransistor, I'm
curious as to whether anyone could comment on what kind of protective
coating could be applied to an existing (e.g.) CdSe LDR to protect it
and lengthen its useful lifespan.

Would something as simple as Krylon spray acrylic or polyurethane do
the job without reacting to the CdSe or creating an excessively low
leakage path in parallel with the sensing "ribbon"?

Jes' curious.


Frank McKenney
--
Any man with a vital knowledge of the human psychology ought to have
the most profound suspicion of anybody who claims to be an artist
and talks a great deal about art. Art is a right and human thing,
like walking or saying one's prayers; but the moment it begins to be
talked about very solemnly a man may be fairly certain that the
thing has come into a congestion and a kind of difficulty.

-- G.K. Chesterton: On the Wit of Whistler (1905)
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney aatt mindspring ddoott com

 

[defaullt]

On Tue, 19 Nov 2013 09:17:24 -0600, Frnak McKenney
<frnak@far.from.the.madding.crowd.com> wrote:

On Tue, 19 Nov 2013 09:01:01 -0500, defaullt <defaulter_@defaulter.net> wrote:
On Mon, 18 Nov 2013 17:19:19 -0800 (PST), Bill Bowden
bperryb@bowdenshobbycircuits.info> wrote:

I repaired an outdoor dusk/dawn LED lamp that had a defective LDR
used to switch the LED on/off. Problem was the LDR resistance read
low (10K)in darkness, so I replaced it with one I had laying around
and got it working. Since then (don't know how long) the same
problem has returned with the LDR resistance again reading low in
darkness. I checked a few LDRs in my spare parts and found one
that also read low in darkness, which had never been used. Do
these things go bad over time, and do they deteriorate sitting in
the sun all day?

-Bill

They absolutely most certainly do go bad just sitting around. They
are incredibly sensitive to moisture. Potting in polyester resin,
epoxy etc. or using hermetically sealed photocells is the only way
to get around the problem. Humidity will kill them while they sit
in a parts bin

The cells with just a thin clear coating of epoxy don't deal with
water very well at all.

While not ignoring the idea of a photodiode/phototransistor, I'm
curious as to whether anyone could comment on what kind of protective
coating could be applied to an existing (e.g.) CdSe LDR to protect it
and lengthen its useful lifespan.

I can't comment on spray coatings. I made a "solid state relay" with
an LED/CDS cell and potted it in a cube of epoxy and it never failed
in 10+ years. I potted a pair of them in small block of polyester
resin then painted the outside, masking windows for the light, and
they also lasted.

One other technique (sounds like cheating) is to just keep them out of
the rain and run enough current through them to keep them dry (or dry
them out once in awhile). Those street lamp thingy's used relays with
several milliamps of current or a heater/bimetallic switch in series
with the cell.

Would something as simple as Krylon spray acrylic or polyurethane do
the job without reacting to the CdSe or creating an excessively low
leakage path in parallel with the sensing "ribbon"?

Jes' curious.


Frank McKenney
--
Any man with a vital knowledge of the human psychology ought to have
the most profound suspicion of anybody who claims to be an artist
and talks a great deal about art. Art is a right and human thing,
like walking or saying one's prayers; but the moment it begins to be
talked about very solemnly a man may be fairly certain that the
thing has come into a congestion and a kind of difficulty.

-- G.K. Chesterton: On the Wit of Whistler (1905)

 

[Bill Bowden]

On Tuesday, November 19, 2013 6:01:01 AM UTC-8, defaullt wrote:

On Mon, 18 Nov 2013 17:19:19 -0800 (PST), Bill Bowden wrote:

I repaired an outdoor dusk/dawn LED lamp that had a defective LDR used to switch the LED on/off. Problem was the LDR resistance read low (10K)in darkness, so I replaced it with one I had laying around and got it working. Since then (don't know how long) the same problem has returned with the LDR resistance again reading low in darkness. I checked a few LDRs in my spare parts and found one that also read low in darkness, which had never been used. Do these things go bad over time, and do they deteriorate sitting in the sun all day?



-Bill



They absolutely most certainly do go bad just sitting around. They
are incredibly sensitive to moisture. Potting in polyester resin,
epoxy etc. or using hermetically sealed photocells is the only way to
get around the problem. Humidity will kill them while they sit in a
parts bin

The cells with just a thin clear coating of epoxy don't deal with
water very well at all.

Thanks for that. The cell I replaced was encased in silicon rubber or something like that. I have one cell that appears to be encased in epoxy, so I might use that. I have some clear roofing sealer that is somewhat elastic when dry. Might try encasing a bare cell in some of that. The circuit switches at a resistance of about 27K ohms and the dusk reading of the cell is about 77K ohms. So, I was thinking of adding a 15K resistor in series with the cell so if it goes bad and only reaches 15K at dusk, the thing should still work?

-Bill

Photo transistors make lots more sense - one photo transistor, one
resister, and one mosfet and you have a dawn to dusk light control
(for DC lights). I've found that when the lights come on that the
"linear region" amounts to less than 30 seconds (as the light grows
dim - a change I can't even distinguish with my eyes) And the circuits
(three of them) have worked for years now.

 

[defaullt]

On Tue, 19 Nov 2013 17:54:37 -0800 (PST), Bill Bowden
<bperryb@bowdenshobbycircuits.info> wrote:

On Tuesday, November 19, 2013 6:01:01 AM UTC-8, defaullt wrote:

On Mon, 18 Nov 2013 17:19:19 -0800 (PST), Bill Bowden wrote:

I repaired an outdoor dusk/dawn LED lamp that had a defective LDR used to switch the LED on/off. Problem was the LDR resistance read low (10K)in darkness, so I replaced it with one I had laying around and got it working. Since then (don't know how long) the same problem has returned with the LDR resistance again reading low in darkness. I checked a few LDRs in my spare parts and found one that also read low in darkness, which had never been used. Do these things go bad over time, and do they deteriorate sitting in the sun all day?



-Bill



They absolutely most certainly do go bad just sitting around. They
are incredibly sensitive to moisture. Potting in polyester resin,
epoxy etc. or using hermetically sealed photocells is the only way to
get around the problem. Humidity will kill them while they sit in a
parts bin

The cells with just a thin clear coating of epoxy don't deal with
water very well at all.


Thanks for that. The cell I replaced was encased in silicon rubber or something like that. I have one cell that appears to be encased in epoxy, so I might use that. I have some clear roofing sealer that is somewhat elastic when dry. Might try encasing a bare cell in some of that. The circuit switches at a resistance of about 27K ohms and the dusk reading of the cell is about 77K ohms. So, I was thinking of adding a 15K resistor in series with the cell so if it goes bad and only reaches 15K at dusk, the thing should still work?

I think you'd just be delaying the inevitable by padding the
resistance.

Pay attention to sealing where the leads enter the cell in the back,
as well as the surface,

It's been awhile since I used a CDS cell, but I seem to remember there
was a visible change in the moisture damaged cells versus new ones.
Take a magnifying glass to it... you may be able to see where the
moisture entered - comparing it to a new one. The silvery stuff turns
darker.

Providing you don't need a lot of current, a photo transistor can
replace a CDS cell in a DC circuit. Not good for operating relays
directly - but if your switch point is 27K you probably aren't using a
relay. For the hell of it I put a photo transistor across my
ohmmeter. Total darkness is >2 meg ohms, the light from my computer
screen ~8K, holding it close to a desk lamp ~100 ohms. I wonder that
someone doesn't already make CDS replacements with a couple of back to
back parallel photo transistors.

Back in the germanium age, we would hack the metal covers off power
transistors, pour in epoxy and make photo transistors that would
operate (sensitive) relays directly.

BTW Bill Bowden were/are you the same guy on the TC mailing lists some
time in the 90's?

 

[Bill Bowden]

On Wednesday, November 20, 2013 5:31:21 AM UTC-8, defaullt wrote:

On Tue, 19 Nov 2013 17:54:37 -0800 (PST), Bill Bowden

Thanks for that. The cell I replaced was encased in silicon rubber or something like that. I have one cell that appears to be encased in epoxy, so I might use that. I have some clear roofing sealer that is somewhat elastic when dry. Might try encasing a bare cell in some of that. The circuit switches at a resistance of about 27K ohms and the dusk reading of the cell is about 77K ohms. So, I was thinking of adding a 15K resistor in series with the cell so if it goes bad and only reaches 15K at dusk, the thing should still work?



I think you'd just be delaying the inevitable by padding the
resistance.

Pay attention to sealing where the leads enter the cell in the back,
as well as the surface,

It's been awhile since I used a CDS cell, but I seem to remember there
was a visible change in the moisture damaged cells versus new ones.
Take a magnifying glass to it... you may be able to see where the
moisture entered - comparing it to a new one. The silvery stuff turns
darker.

Providing you don't need a lot of current, a photo transistor can
replace a CDS cell in a DC circuit. Not good for operating relays
directly - but if your switch point is 27K you probably aren't using a
relay. For the hell of it I put a photo transistor across my
ohmmeter. Total darkness is >2 meg ohms, the light from my computer
screen ~8K, holding it close to a desk lamp ~100 ohms. I wonder that
someone doesn't already make CDS replacements with a couple of back to
back parallel photo transistors.

Back in the germanium age, we would hack the metal covers off power
transistors, pour in epoxy and make photo transistors that would
operate (sensitive) relays directly.

BTW Bill Bowden were/are you the same guy on the TC mailing lists some
time in the 90's?

No, I've never been on any mailing lists.

I encased a small LDR in epoxy yesterday. I mixed a glob of glue and rolled the LDR in it and then dried it slowly by rotating it every half hour or so. Came out almost a perfect sphere like a little marble. Now, all I have to do is mount it through the top surface so water doesn't get into the main assembly.

-Bill

 

[Oppie]

"Frnak McKenney" <frnak@far.from.the.madding.crowd.com> wrote in message
news:59idnVWWib2ZHBbPnZ2dnUVZ_qudnZ2d@earthlink.com...

While not ignoring the idea of a photodiode/phototransistor, I'm
curious as to whether anyone could comment on what kind of protective
coating could be applied to an existing (e.g.) CdSe LDR to protect it
and lengthen its useful lifespan.

Am I right that CdSe cells don't like DC operation? I seem to recall
something from long ago about them working better long term with AC. Aside
from that, these things were supposed to go extinct with RoHS, weren't they?

 

[defaullt]

On Wed, 27 Nov 2013 17:00:06 -0500, "Oppie" <Oppie@127.0.0.1> wrote:

"Frnak McKenney" <frnak@far.from.the.madding.crowd.com> wrote in message
news:59idnVWWib2ZHBbPnZ2dnUVZ_qudnZ2d@earthlink.com...

While not ignoring the idea of a photodiode/phototransistor, I'm
curious as to whether anyone could comment on what kind of protective
coating could be applied to an existing (e.g.) CdSe LDR to protect it
and lengthen its useful lifespan.


Am I right that CdSe cells don't like DC operation? I seem to recall
something from long ago about them working better long term with AC. Aside
from that, these things were supposed to go extinct with RoHS, weren't they?

I don't think there's any restriction about the direction of current
flow, but do remember reading something about "moisture sensors"
requiring AC to insure that the electrodes didn't try to migrate. So
photocells might do better in the presence of AC versus DC.

But they also go bad just sitting around in the presence of moisture.

As for the RoHS, who knows? There's so many restrictions and
exclusions that a team of Philadelphia lawyers might be needed to sort
that out.

The amount of cadmium, life of the parts, and spectral response would
seem to make a good case for an exclusion, but in general terms
cadmium is one of the metals considered hazardous.