I've been doing some math, trying to figure out how I'm going to get this optocoupler set up.

The resistor that Stan uses... 100 Ohm Clarostat VP25K [$8.33]

http://www.newark.com/jsp/search/productdetail.jsp?SKU=10M5288&CMP=AFC-TL10000001appears to be too low a value and too low a wattage rating.

Dynodon demonstrated the setup running up around 80 volts, which will put 800 mA through the H11D1 optocoupler, which is rated for 100 mA, so say 50% duty cycle, it's still getting 400 mA. Also that resistor, as large as it is at 25 watts, would be getting 64 watts.

Re-doing the math for what I would expect it to be, this should be a 1000 Ohm resistor, possibly missing a zero on the posted value?

This resistor,

http://www.newark.com/ohmite/l25j1k0/power-resistor/dp/02F2291?matchedProduct=VP25K+1000&whydiditmatch=rel_defaultlisted as "Alternative for VP25K 1000" (compared to VP25K 100), is probably what is required.

This will give 100 mA to the opto at 100 volts, with 10 watts (now within the 25 watt rating), and 10 mA to the opto at 10 volts, the collector needs about 4 mA to work, so this would probably work fine from 10 volts to 100 volts on the variac.

Or... considering a 50% duty cycle all the time, we could drop the resistor value by half...

a 500 ohm resistor:

http://www.newark.com/ohmite/l25j500/power-resistor/dp/02F2288