Over what voltage range will the voltage inputs on the DAM and DVM trigger?

The circuit is set up for 24 Volts, although the optos (with the 1K series resister installed) will operate down to 11.5 volts. This results in about 10mA, which is at the bottom of the opto's spec. so any noise could cause false triggering. I'd say keep it to a minimum of 12 volts in a clean and noise-free environment, meaning that no motors or contactors are around, and the wires are kept shorter than 3 feet or so...