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The Electronic Goldmine  has a rare to find component.
"Tiny audio optocoupler is about the size of a pencil eraser and has 4 leads. These are used extensively in audio compressors, audio level controls, audio limiters, expanders/noise gates, guitar tremolo effects, guitar amplifiers and music effect boxes. Inside the black epoxy case is a 2 lead photocell (not a photodiode or photo-transistor) and a 2 lead LED. This is why the device finds extensive use in audio applications (photocell instead of photodiode). The LED requires about 10-16mA current and the resistance of the photocell varies from 150K up to megohms when the LED is off to under 400ohms when the LED is on."




4 years ago

So most will think this is an add for the Goldmine and your probably correct !

BUT I'm not related to this electronic store and this component is the

Answer to a recent Question and as an EE this is a good deal even if

you need to buy 50 for $20 that is 40 cents each for your parts bin.

This post needs an At-A-Boy or Cool or I wont bother next time.

I guess if I can buy a UV laser for under $3 this isn't so exciting and I'm just an old EE who keeps a good spare parts bin



Not exciting, per se, but useful. Thanks for the link.

I've got a handful of LDRs I ordered a few years ago. They were once in vogue (as noted) for use in amplifier channel switching and for tremolo circuits. Some are difficult to replace--the old ones used tiny filament light bulbs--and LEDs work fine, but don't replicate the rise/fall of the LDR exactly.

Used to make my own with CdS photoresistors and a light bulb or an LED. Stick 'em in a piece of heatshrink tubing. Great for those old Craig Anderton compressor circuits...

Thanks, the true audio compressors did use the rise to glow and fall of an incandescent integrating filament to work the gain structure of an amplifier.

I know I'm always threatening to design and build one for our main TV because of those annoying higher volume over-modulated commercials that blare at me.

Now that I'm thinking ! ? ! ... I could try to add pi RC or even an LC front end to the LED to simulate the beneficial integral effect of a filament using that LED=>LDR device.

Feel free to disagree if you don't agree :-)

If a starting point would help, here's a schematic for the old Anderton compressor. It's an LED type LDR. It's from Guitar Player mag, looks like 1970's vintage.

The "original" schematic I used "way back" was in his book "Electronic Projects for Musicians" (1975) and uses two opamps, not three (the first schemo) but still has the LED LDR (CLM6000).

I think CdS photoresistors might be so slow that there aren't too many negatives using LEDs vs filament bulbs, beyond a certain frequency. Maybe at average audio frequencies it's not bad.

I have an Acoustic G60T amplifier (60 W tube, early 1980's) that uses filament LDRs for channel switching--there isn't any delay circuity for the switching, but it's very soft. If I had to guess, I'd say the channels switch in 200 to 400 milliseconds. From clean->gain it creates a short "swell." LED LDRs would need additional components to replicate that "organic" delay.

I'm sure with an opamp and a well-placed cap or two you can make a little "sample and hold" circuit that keeps the LEDs on for a few milliseconds...