scottdrums89:
Hi Lefty,Opto-isolators were on my radar as a possibility, but I'm not sure of a good way to current-limit the input to protect the diode given the wide range of input voltages. I had thought about using a resistor in series with the input and a Zener diode in parallel with the isolator to regulate the voltage, but I believe that would limit me to DC only.
Thanks,
Scott
No, your on the right path. It's just that every circuit you wish to monitor has to be evaluated as to the best way to interface the diode input of the opto. Without seeing each equipment's circuitry detail I can only state "where there is a will there is a way". 
Lefty