Cadmium sulphide photosensitive resistors have poor sensitivity to light wavelengths outside their design range. This means a typical cell designed to have a peak response to yellow/green light will have very poor response to red and blue.
ldr - What Colors or wavelengths are Photoresistors sensitive to? - Electrical Engineering Stack Exchange
ldr - What Colors or wavelengths are Photoresistors sensitive to? - Electrical Engineering Stack Exchange
Yes; but an LED nailed to the face of an LDR is "BRIGHT" enough for many purposes. I did a lot of LDRs when LEDs only came in red. Further the historical trend of brighter-every-year works in our favor.
OTOH CdS is falling out of fashion, toxic material. Also became a commodity. Production has shifted from big companies to small shops. Reports are that performance is variable.
Try-and-see becomes the best rule.
In the old days I also fell for the match-the-spectral-peak trap and used green Leds (CDS peak is yellow-green to the human eye) .. got meh! results.
Surprisingly, most optocouplers (at least all I ever dissected) used RED Leds ... wtf?
Talking about that with a friend, an Engineer specialized in fiber optics and such, told me this:
"human eye has reduced sensitivity to Red wavelengths, so Led makers dope the hell out of them, to radically increase photon-per-mA output so we can actually see them well. ... end result is that photon Tsunami drives CDS cells very well, simply because of sheer abbundance".
So I retorted to high efficiency Red Leds ... instant bliss
But test them yourself: build 2 optocouplers with same batch LDR, one with a high efficiency Red Led , the other with the best "any other colour" you can get.
Pass, say, 10mA through each and measure DCR with a standard multimeter at the CDS end ... lower is better
Please post results.
Surprisingly, most optocouplers (at least all I ever dissected) used RED Leds ... wtf?
Talking about that with a friend, an Engineer specialized in fiber optics and such, told me this:
"human eye has reduced sensitivity to Red wavelengths, so Led makers dope the hell out of them, to radically increase photon-per-mA output so we can actually see them well. ... end result is that photon Tsunami drives CDS cells very well, simply because of sheer abbundance".
So I retorted to high efficiency Red Leds ... instant bliss
But test them yourself: build 2 optocouplers with same batch LDR, one with a high efficiency Red Led , the other with the best "any other colour" you can get.
Pass, say, 10mA through each and measure DCR with a standard multimeter at the CDS end ... lower is better
Please post results.
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