Using an optocoupler to drive a pulse counter
I suspect that the signal is very weak and can't drive much of a load. If it can drive 100k, something like this should work. I plain NPN optocoupler should work, I don't see an advantage here to a MOSFET Opto.
If the 50 V threshold is important, this won't be accurate. If you are only building a few, you could hand-tune it. If a 50 V threshold is important, you should consider a pulse transformer like Tony recommended.



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