LDR reads low (10K) in darkness?

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.

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

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.

o switch the LED on/off. Problem was the LDR resistance read low (10K)in da rkness, so I replaced it with one I had laying around and got it working. S ince 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 us ed. Do these things go bad over time, and do they deteriorate sitting in th e sun all day?

Thanks for that. The cell I replaced was encased in silicon rubber or somet hing like that. I have one cell that appears to be encased in epoxy, so I m ight use that. I have some clear roofing sealer that is somewhat elastic wh en dry. Might try encasing a bare cell in some of that. The circuit switche s at a resistance of about 27K ohms and the dusk reading of the cell is abo ut 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 stil l work?

-Bill

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?

mething 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 swit ches 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 s till work?

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 s o. Came out almost a perfect sphere like a little marble. Now, all I have t o do is mount it through the top surface so water doesn't get into the main assembly.

-Bill

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.