It's from the notch filter in my hm-250 distortion meter. There are a few of them.
Sorry to say that i haven't measured it yet. That have to wait another week.
I have got a few suggestions - optocoupler and reed relay.
Resistance value between the output side of the CdS cell current flows to the input side of the LED changes. Where the output is resistance rather than a constant current is miso (relation between voltage and current is linear).
Some discontinued direction because photo coupler of CdS output is using harmful Cd (cadmium). There is for now no element to the same function without using harmful substances. Can be re-design the circuit from the root to the circuit without a photo coupler of CdS output but circuit becomes complicated.
There seem to be very little information on the mcd5221. I found a datasheet regarding other moririca optocouples in the mcd range. Well the ones in my DM is still working so for now I'll leave them as is.