I've been trying to get my hands on some vactrols (opto-variable resistor) for a long time, unsuccessful. Not even Digi-Key has them.
Some examples: VTL5C2, VTL5C3, NSL32
I can't see them as "too expensive to manufacture" considering you can improvise one using an LED+LDR+tape, but it's in no way as "linear" or robust as the actual components.
Does anyone know the story behind it?
Edit: So there are places where you can get them, but at least where I live it's something almost unheard of. They look useful in many ways (at least to me,) yet I've never seen them anyhere except some old audio equipment.
Edit2: What could be an alternative to a vactrol if I require a significant resistance change controlled by voltage? A MOSFET as a resistor is not an option for me because of the very low resistance range. Is there such a device/equivalent circuit that allows all this with the simplicity of the vactrol? This is a hypothetical question, based on many compressor units I have seen. Itseems hard to achieve such functionality in such a simple way. The closest I've seen are OTA based for closed loop circuit, but they aren't nearly as simple as vactrols.