Here's essentially what you're looking at. The circuit requires a comparator and a few resistors. R6 will need to be a combination of a fixed resistor and a variable "trimmer" resistor to get the level set correctly. You'll need a steady source of power. You might use a wall plug in the 120V circuit, and with good bypass capacitor, around 10uF or so, it should work during switchover. To set the circuit up, you should use a variable voltage you can adjust to your desired trip point. To make one, just use a fixed DC voltage and a variable resistor connected across the voltage and connect the wiper to the circuit. Using a 10K variable resistor will ensure you won't have over current and blow it up. Oh BTW the actual comparator don't have to be the same. Use whatever will drive you actuator, I just grabbed one from the library. You'll probably need a transistor between the comparator and your relay, or whatever you're planning on using.