120VAC when the motor is at the limit? Maybe tap that 120VAC, plug it into a small wall-wart like power supply and send that output to the controller?
Past that, I would think a series current limiting resistor along with a voltage divider (and maybe a ZENER for protection) would do the trick...or a voltage divider that divides it down to +12v (or whatever) and feed that into a 7805 regulator.
I do like your light bulb idea though...simple, effective, practically zero thought/effort required
Did something like that awhile back. Needed to switch 120VAC with a PIC. Didn't want to mess with the AC, so I took a PIC, a servo, a chunk of threaded rod and a standard wall light switch...programmed the PIC to move the servo to move the light switch handle to turn power on/off. Almost perfect isolation...
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