Are these optos useful? Datasheet.

[blackieNYC]

http://www.cel.com/pdf/datasheets/ps2502.pdf
How do these compare to a vactrol?  The output is a darlington. I don't get from the datasheet whether the output is 2 state logic or variable output. Found a couple.  Is the LED functionally just a base for the darlington?
I suppose I'm thinking about some LFO purpose, but are they of any use in our little world?
Thanks

[blackieNYC]

Moved and bumped

[PRR]

Vactrol output is a resistor.

This is, as you say, a light-input transistor, essentially a controllable Current Source, not a resistor. Same current no matter what C-E voltage.

It sure is not pop-in replacement in 99% of the places we use Vactrols.

> whether the output is ...variable output.

"CTR", current transfer ratio. As an over-simplification, output current is proportional to input current. For the single transistor optos, a number like 10% or 50%. If you put 10mA into the LED, you could be sure of 1mA out of the transistor, which is enough to switch the worst logic-chip.

These are Darlingtons so the CTR is about an Hfe higher. 2,000% is the stated "typical". But look at top-right page 7. The true CTR varies a LOT from device to device, and with operating current. Like any device, gain is small at small current, should get higher, but ultimately chokes-off. With a Darlington this trend is doubled. CTR rises swiftly to a peak and then goes away. And we can't really predict that peak.

Aside from switching, these are used in line-operated regulated power supplies. You need to know the voltage on the user side, without a copper connection to the line side. Use a Zener about 1.2V lower than your desired output. This drives the LED. Pure light couples that light-dark to the Darlington, which outputs a more/less signal to the regulator. But it is "linear" only for VERY small changes. That's OK in a regulator where the idea is to stay very close to a specific voltage. For audio it probably sucks; and sucks in ways we can do easier many other ways.

They are also typically hissy.

And being a weak Darlington, it is barely fast enough for audio even at quite high current.