ORP12 LDR stability

[Andrew Gabriel]

I have a couple of ORP12's which are used for detecting when it's
dark outside, connected up to homebrew circuits. One thing I've found
is they are not very stable long term. One I have which faces the
midday sun has increased it's light and dark resistance by about a
factor of 10 over 4 years verses a second bought at the same time
which has been kept indoors used. I had another one stop working
(could have been the same symptom -- I didn't check at the time)
after about 6 years in a different location, but possibly more
shielded from the direct sun.

Is this a known issue with these devices? I could probably relocate
the sensors so they are not in direct sunlight if that's the issue,
or maybe install them under a UV filter if that would help.

Another thing is replacement ORP12's seem to be much harder to find
than they were a few years ago -- none of the electronic component
suppliers I normally use stock them anymore. So if anyone can point
me to someone who sells in very small quantities in the UK I'd be
interested.

--
Andrew Gabriel

 

[Ross Herbert]

On 4 Aug 2004 10:52:46 GMT, andrew@cucumber.demon.co.uk (Andrew
Gabriel) wrote:

|I have a couple of ORP12's which are used for detecting when it's
|dark outside, connected up to homebrew circuits. One thing I've found
|is they are not very stable long term. One I have which faces the
|midday sun has increased it's light and dark resistance by about a
|factor of 10 over 4 years verses a second bought at the same time
|which has been kept indoors used. I had another one stop working
|(could have been the same symptom -- I didn't check at the time)
|after about 6 years in a different location, but possibly more
|shielded from the direct sun.
|
|Is this a known issue with these devices? I could probably relocate
|the sensors so they are not in direct sunlight if that's the issue,
|or maybe install them under a UV filter if that would help.
|
|Another thing is replacement ORP12's seem to be much harder to find
|than they were a few years ago -- none of the electronic component
|suppliers I normally use stock them anymore. So if anyone can point
|me to someone who sells in very small quantities in the UK I'd be
|interested.


LDR's such as ORP12 (I'll bet that is not the original one from
Philips/Mullard circa 1950's) are usually made of Cadmium Sulphide. If
you look at the characteristics of light sensor devices shown here
http://www.engr.udayton.edu/faculty/jloomis/ece445/topics/egginc/tp4.html
you will note that these types are inherently unstable, so i think
this answers your question.

 

[Watson A.Name - \"Watt Su]

"Andrew Gabriel" <andrew@cucumber.demon.co.uk> wrote in message
news:ceqf5u$7kn$1@new-usenet.uk.sun.com...

I have a couple of ORP12's which are used for detecting when it's
dark outside, connected up to homebrew circuits. One thing I've found
is they are not very stable long term. One I have which faces the
midday sun has increased it's light and dark resistance by about a
factor of 10 over 4 years verses a second bought at the same time
which has been kept indoors used. I had another one stop working
(could have been the same symptom -- I didn't check at the time)
after about 6 years in a different location, but possibly more
shielded from the direct sun.

Is this a known issue with these devices? I could probably relocate
the sensors so they are not in direct sunlight if that's the issue,
or maybe install them under a UV filter if that would help.

The ORP12 is a CdS photocell, as far as I can determine. I've often
asked others about these, because they are often specified in projects
and in the U.S. they're not readily available. But Radio Snack in the
U.S. carries (or did carry) a package of four CdS photocells for a few
dollars, which seem to be a reasonable substitute.

One issue with all CdS photocells is they don't handle high temps very
well. I've seen a number of the outdoor daylight sensors fail over the
years. These use a large CdS photocell which during daylight lets
current thru a resistance wire wrapped around a bimetallic strip, which
heats up and opens the contacts to keep the lights turned off. At
night, there is almost no current, the strip cools off and closes,
turning on the light. When they fail, usually the lights come on all
day long, indicating the photocell is open.

Another thing is replacement ORP12's seem to be much harder to find
than they were a few years ago -- none of the electronic component
suppliers I normally use stock them anymore. So if anyone can point
me to someone who sells in very small quantities in the UK I'd be
interested.

I would try to find a replacement that doesn't have the reliability
problems of a CdS photocell. One possible replacement is a Photovoltaic
cell. Another might be the IR sensor out of an old mouse. But I don't
know how these would work with the IR from the sun. In any case, put a
big capacitor across the input to prevent a bird's shadow or similar
from triggering the sensor during the daytime.

--
Andrew Gabriel

 

[Andrew Gabriel]

In article <imk1h0d45doibid44mn59v3qb4b7fpmnj0@4ax.com>,
Ross Herbert <rherber1SPAMEX@bigpond.net.au> writes:

LDR's such as ORP12 (I'll bet that is not the original one from
Philips/Mullard circa 1950's) are usually made of Cadmium Sulphide. If

It's marked NSL (nothing else). That would seem to imply it's a
Silonex part, http://www.silonex.com/optoelectronics/optophotoc.html
and it's one of the TO-8 Hermetic Package types, although the
electrical characteristics are not a good match with any listed on
that page. ORP12 had a very wide dynamic range, with resistance in
the Mohm's when dark, and a couple of hundred ohms when bright, and
the one unused NSL LDR I have seems to match that (and I'm pretty
sure it was sold as an ORP12 or equivalent at the time).

you look at the characteristics of light sensor devices shown here
http://www.engr.udayton.edu/faculty/jloomis/ece445/topics/egginc/tp4.html
you will note that these types are inherently unstable, so i think
this answers your question.

Interesting web page, thanks.

--
Andrew Gabriel

 

[Andrew Gabriel]

In article <10h1lr2cfltmsa4@corp.supernews.com>,
"Watson A.Name - \"Watt Sun, the Dark Remover\"" <NOSPAM@dslextreme.com> writes:

The ORP12 is a CdS photocell, as far as I can determine. I've often
asked others about these, because they are often specified in projects
and in the U.S. they're not readily available. But Radio Snack in the

They were readily available all through my youth (some ~30 years ago;-)
in the UK, and up until perhaps 5 years ago when they started getting
harder to find. There are plenty of other LDR's available, generally
physically much smaller, but without anything like the dynamic range
of resistance values the ORP12 offered.

I would try to find a replacement that doesn't have the reliability
problems of a CdS photocell. One possible replacement is a Photovoltaic
cell. Another might be the IR sensor out of an old mouse. But I don't

I have a few streetlamp photocells of various types (including the
thermal type you mention) but they all use what look like CdS photocells.
One set of instructions for them did say the cell should point to the
Northern horizon, so I might try that, such that it doesn't get direct
sunlight.

know how these would work with the IR from the sun. In any case, put a
big capacitor across the input to prevent a bird's shadow or similar
from triggering the sensor during the daytime.

The associated circuit has a time constant of a couple of minutes
built in to avoid such issues.

--
Andrew Gabriel

 

[Graham W]

"Andrew Gabriel" <andrew@cucumber.demon.co.uk> wrote in message
news:cet5dg$g33$1@new-usenet.uk.sun.com...

In article <10h1lr2cfltmsa4@corp.supernews.com>,
"Watson A.Name - \"Watt Sun, the Dark Remover\"" <NOSPAM@dslextreme.com
writes:

The ORP12 is a CdS photocell, as far as I can determine. I've often
asked others about these, because they are often specified in projects
and in the U.S. they're not readily available. But Radio Snack in the

They were readily available all through my youth (some ~30 years ago;-)
in the UK, and up until perhaps 5 years ago when they started getting
harder to find. There are plenty of other LDR's available, generally
physically much smaller, but without anything like the dynamic range
of resistance values the ORP12 offered.

I would try to find a replacement that doesn't have the reliability
problems of a CdS photocell. One possible replacement is a
Photovoltaic
cell. Another might be the IR sensor out of an old mouse. But I
don't

I have a few streetlamp photocells of various types (including the
thermal type you mention) but they all use what look like CdS
photocells.
One set of instructions for them did say the cell should point to the
Northern horizon, so I might try that, such that it doesn't get direct
sunlight.

Unless it's used in Oz!

But to be serious for a moment, the north pointing bit helps to ensure
that both the evening and morning illumination are evenly balanced.

[In your other post you said]

ORP12 had a very wide dynamic range, with resistance in
the Mohm's when dark, and a couple of hundred ohms when bright

I think some designs didn't account for the very low on resistance of
the ORP12 and finished up pumping tens of milli-amps into the base
of the first transistor which didn't like it!


--
Graham W http://www.gcw.org.uk/ PGM-FI page updated, Graphics Tutorial
WIMBORNE http://www.wessex-astro-society.freeserve.co.uk/ Wessex
Dorset UK Astro Society's Web pages, Info, Meeting Dates, Sites & Maps
Change 'news' to 'sewn' in my Reply address to avoid my spam filter.

 

[Watson A.Name - \"Watt Su]

"Andrew Gabriel" <andrew@cucumber.demon.co.uk> wrote in message
news:cet2qb$f27$1@new-usenet.uk.sun.com...

In article <imk1h0d45doibid44mn59v3qb4b7fpmnj0@4ax.com>,
Ross Herbert <rherber1SPAMEX@bigpond.net.au> writes:
LDR's such as ORP12 (I'll bet that is not the original one from
Philips/Mullard circa 1950's) are usually made of Cadmium Sulphide.
If

It's marked NSL (nothing else). That would seem to imply it's a
Silonex part, http://www.silonex.com/optoelectronics/optophotoc.html
and it's one of the TO-8 Hermetic Package types, although the
electrical characteristics are not a good match with any listed on
that page. ORP12 had a very wide dynamic range, with resistance in
the Mohm's when dark, and a couple of hundred ohms when bright, and
the one unused NSL LDR I have seems to match that (and I'm pretty
sure it was sold as an ORP12 or equivalent at the time).

For some reason I get the sense that the Brits are fixated on the ORP12,
Always call it a LDR, and never specify a CdS photocell. Clairex made
many CdS photocells and it seems that the trend has been to go to a
cheap ceramic substrate with clear epoxy over the sensitive layer. This
cheap encapsulation is one reason for their instability, IMHO. Early
CdS photocells were in a hermetically sealed TO-5 size metal transistor
can, with a glass window. A few of those might still be around, but at
a probably much higher price.

you look at the characteristics of light sensor devices shown here

http://www.engr.udayton.edu/faculty/jloomis/ece445/topics/egginc/tp4.htm

l

you will note that these types are inherently unstable, so i think
this answers your question.

Interesting web page, thanks.

Reminds me of Win Hill's "Ghastly" electrolytic tolerance assessment in
AoE.
;-)

--
Andrew Gabriel

 

[Watson A.Name - \"Watt Su]

"Graham W" <graham@his.com.puter.INVALID> wrote in message
news:4112738d$0$59491$ed2619ec@ptn-nntp-reader03.plus.net...

"Andrew Gabriel" <andrew@cucumber.demon.co.uk> wrote in message
news:cet5dg$g33$1@new-usenet.uk.sun.com...
In article <10h1lr2cfltmsa4@corp.supernews.com>,
"Watson A.Name - \"Watt Sun, the Dark Remover\""
NOSPAM@dslextreme.com
writes:

The ORP12 is a CdS photocell, as far as I can determine. I've
often
asked others about these, because they are often specified in
projects
and in the U.S. they're not readily available. But Radio Snack in
the

They were readily available all through my youth (some ~30 years
ago;-)
in the UK, and up until perhaps 5 years ago when they started
getting
harder to find. There are plenty of other LDR's available, generally
physically much smaller, but without anything like the dynamic range
of resistance values the ORP12 offered.

I would try to find a replacement that doesn't have the
reliability
problems of a CdS photocell. One possible replacement is a
Photovoltaic
cell. Another might be the IR sensor out of an old mouse. But I
don't

I have a few streetlamp photocells of various types (including the
thermal type you mention) but they all use what look like CdS
photocells.
One set of instructions for them did say the cell should point to
the
Northern horizon, so I might try that, such that it doesn't get
direct
sunlight.

Unless it's used in Oz!

But to be serious for a moment, the north pointing bit helps to ensure
that both the evening and morning illumination are evenly balanced.

[In your other post you said]

ORP12 had a very wide dynamic range, with resistance in
the Mohm's when dark, and a couple of hundred ohms when bright

I think some designs didn't account for the very low on resistance of
the ORP12 and finished up pumping tens of milli-amps into the base
of the first transistor which didn't like it!

Well, you might need tens of mA into the base of the transistor if it's
driving a relay that needs a hundred mA from the transistor. The
transistor having a gain of 10 when it's saturated.

--
Graham W http://www.gcw.org.uk/ PGM-FI page updated, Graphics
Tutorial
WIMBORNE http://www.wessex-astro-society.freeserve.co.uk/ Wessex
Dorset UK Astro Society's Web pages, Info, Meeting Dates, Sites &
Maps
Change 'news' to 'sewn' in my Reply address to avoid my spam filter.

 

[Graham W]

Watson A.Name - "Watt Sun, the Dark Remover" wrote:

"Graham W" <graham@his.com.puter.INVALID> wrote in message
news:4112738d$0$59491$ed2619ec@ptn-nntp-reader03.plus.net...

"Andrew Gabriel" <andrew@cucumber.demon.co.uk> wrote in message
news:cet5dg$g33$1@new-usenet.uk.sun.com...
In article <10h1lr2cfltmsa4@corp.supernews.com>,
"Watson A.Name - \"Watt Sun, the Dark Remover\""
NOSPAM@dslextreme.com
writes:

The ORP12 is a CdS photocell, as far as I can determine. I've
often asked others about these, because they are often specified
in projects and in the U.S. they're not readily available. But
Radio Snack in the

They were readily available all through my youth (some ~30 years
ago;-) in the UK, and up until perhaps 5 years ago when they
started getting harder to find. There are plenty of other LDR's
available, generally physically much smaller, but without anything
like the dynamic range of resistance values the ORP12 offered.

I would try to find a replacement that doesn't have the reliability
problems of a CdS photocell. One possible replacement is a
Photovoltaic cell. Another might be the IR sensor out of an old
mouse. But I don't

I have a few streetlamp photocells of various types (including the
thermal type you mention) but they all use what look like CdS
photocells. One set of instructions for them did say the cell
should point to the Northern horizon, so I might try that, such
that it doesn't get direct sunlight.

Unless it's used in Oz!

But to be serious for a moment, the north pointing bit helps to
ensure that both the evening and morning illumination are evenly
balanced.

[In your other post you said]

ORP12 had a very wide dynamic range, with resistance in
the Mohm's when dark, and a couple of hundred ohms when bright

I think some designs didn't account for the very low on resistance of
the ORP12 and finished up pumping tens of milli-amps into the base
of the first transistor which didn't like it!

Well, you might need tens of mA into the base of the transistor if
it's driving a relay that needs a hundred mA from the transistor. The
transistor having a gain of 10 when it's saturated.

Yes, but the ORP12 didn't survive the high current!


--
Graham W http://www.gcw.org.uk/ PGM-FI page updated, Graphics Tutorial
WIMBORNE http://www.wessex-astro-society.freeserve.co.uk/ Wessex
Dorset UK Astro Society's Web pages, Info, Meeting Dates, Sites & Maps
Change 'news' to 'sewn' in my Reply address to avoid my spam filter.

 

[Watson A.Name - \"Watt Su]

"Graham W" <graham@his.com.puter.INVALID> wrote in message
news:41155952$0$531$ed2619ec@ptn-nntp-reader02.plus.net...

Watson A.Name - "Watt Sun, the Dark Remover" wrote:
"Graham W" <graham@his.com.puter.INVALID> wrote in message
news:4112738d$0$59491$ed2619ec@ptn-nntp-reader03.plus.net...

"Andrew Gabriel" <andrew@cucumber.demon.co.uk> wrote in message
news:cet5dg$g33$1@new-usenet.uk.sun.com...
In article <10h1lr2cfltmsa4@corp.supernews.com>,
"Watson A.Name - \"Watt Sun, the Dark Remover\""
NOSPAM@dslextreme.com
writes:

The ORP12 is a CdS photocell, as far as I can determine. I've
often asked others about these, because they are often specified
in projects and in the U.S. they're not readily available. But
Radio Snack in the

They were readily available all through my youth (some ~30 years
ago;-) in the UK, and up until perhaps 5 years ago when they
started getting harder to find. There are plenty of other LDR's
available, generally physically much smaller, but without anything
like the dynamic range of resistance values the ORP12 offered.

I would try to find a replacement that doesn't have the
reliability
problems of a CdS photocell. One possible replacement is a
Photovoltaic cell. Another might be the IR sensor out of an old
mouse. But I don't

I have a few streetlamp photocells of various types (including the
thermal type you mention) but they all use what look like CdS
photocells. One set of instructions for them did say the cell
should point to the Northern horizon, so I might try that, such
that it doesn't get direct sunlight.

Unless it's used in Oz!

But to be serious for a moment, the north pointing bit helps to
ensure that both the evening and morning illumination are evenly
balanced.

[In your other post you said]

ORP12 had a very wide dynamic range, with resistance in
the Mohm's when dark, and a couple of hundred ohms when bright

I think some designs didn't account for the very low on resistance
of
the ORP12 and finished up pumping tens of milli-amps into the base
of the first transistor which didn't like it!

Well, you might need tens of mA into the base of the transistor if
it's driving a relay that needs a hundred mA from the transistor.
The
transistor having a gain of 10 when it's saturated.

Yes, but the ORP12 didn't survive the high current!

I think the Clairex photocells that are about 6 to 9 mm diam. are rated
for a hundred or so milliwatts max. Keep the voltage low across the
photocell, and it might not dissipate too much.

Better yet, use a second transistor to drive the relay transistor. This
should reduce the needed current to a few hundred microamps.

For a phtocell that turns off the light in the daytime, it might be best
to use it to turn off the driver transistor, which allows the relay
transistor to go on. This is so that if the photocell fails, it would
allow the light to be on all the time, sort of a failsafe mode.

--
Graham W http://www.gcw.org.uk/ PGM-FI page updated, Graphics
Tutorial
WIMBORNE http://www.wessex-astro-society.freeserve.co.uk/ Wessex
Dorset UK Astro Society's Web pages, Info, Meeting Dates, Sites &
Maps
Change 'news' to 'sewn' in my Reply address to avoid my spam filter.