Hi, I noticed this long drawn out discussion, here is my bit, find in the attached schematic that if you use a zener diode in series with the input to the opto, the input will not conduct until the input is above the zener voltage.
Using 10mA as the starting current is just a suggestion.
Just some quick calcs and you don't need high wattage components. It will need some more work to check current at 80V, but hey there's the challenge.
A lot of industrial CNC and other control equipment use this method to check if all the supply rails in its system are present before commencing and continuing any sequence.
