X10 device to detect open circuit or voltage drop

It might help if you tell us the specifics of what you want to do. No X10 module is designed to do exactly what you want but we may be able to come up with suggestions on how to use them or other devices to accomplish your goals.

For example, many of the Smarthome modules will send an x10 code when the load they control is turned off.

I have snap style temperature sensors that are normally closed. I want to detect when the circuit opens, and be able to send an x10 signal to a red light. The problem is that the powerflash module only sends a lights ON signal when a circuit closes. I do not have a computer available in this location to interpret an OFF signal to mean ON. What are my options ?

What country? What kind of loads?

If you are in N. America and your loads are 120v and not too high wattage, you could plug the sensors into Smarthome modules - they will send X10 codes with all load changes.

Would you be able to elaborate further, since I am only familiar with very basic electronics and am using 3-snap sensors (normally closed) and is curently in series. How does this get wired ?