T&L Publications

Can someone tell me what a Light-dependent resistor is? It was mentioned in a book I was reading, but the book doesn't say what it is. It also mentions its light coefficient. What's that?<p>Thanks for any help,<p>Brad

Brad:
How about the most common unit , known as… the cadmium sulfide “photocell”. Run a resistance check across it in the dark and get a certain resistance or put it under a bright light and get a MUCH lower resistance. They are manufactured to different tolerances and will exhibit diffferent ratios on the light/vs/dark resistances …thus the coefficient. This is basically established in manufacture by variance to the width/length of the serpentine path the element is usually laid out in, as well as the doping of the mix.
Check: This snippet of info as well as prime manuf sites…esp Clairex …..at one time.
http://www.wikipedia.org/wiki/Light-dependent_resistor <p>73's de Edd
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Are photoconductive cells the same thing?<p>Brad

More light = less resistance. <p>Photo conductive is just "another word" for the same thing. There are many different ways to make cells, that light makes changes, but general terms like photo conductive/resistive cover the expression well.

Hi, Brad. Can you tell us what kind of book you got that term from? If it's an intro-type electronics book, they might just be referring to the ratio between R(on) and R(off). A higher ratio would mean the photocell would be easier to use as an on/off indicator.<p>By the way, found out that Clairex is getting out of photocells. Good deals direct from the manufacturer on inventory -- might be worth looking into.

Clairex and Vactec were both heavy duty into LDRs and sold off their assets a few years ago. Perkin-Elmer picked up a bunch of stuff from Clairex, I believe.<p>Clairex was the king, in my opinion, because they also made optoisolators that used LDRs as the receiving element, putting them in a special spot where you could interface easily to a voltage divider.<p>"LDR" (vs photocell, etc.) is the term that Heathkit popularized if no one else did, as they used them a lot in various courses, especially their old "Optoelectronics" course. An LDR can have a "dark" resistance that exceeds 100M ohms and "light" resistance that is less than 100 ohms, giving them a dynamic range that is huge. They can be used to detect the tiniest of light leaks in a light-tight box.<p>Dean

The book I was reading is Applied Electricity and Electronics lab manual by Bayne.<p>Brad

There is a term called the absorption coefficient which could refer to photoresistors, but unless your book was a college-level physics textbook, it's not what you're talking about.

I found thisweb pagethat mentions light coefficient as related to a LDR. I would think that it would be highly nonlinear and have units something like ohms/lumen if specified. I suspect that the term is usually used only in the general sense, as in "positive light coefficient" or "negative light coefficient".