Hi,
I am attempting to design a continuity tester that can test continuity on a single wire without having return wire, Eg plug one box into one connector on a control unit in a factory say and plug another box into maybe another control box across the other of a factory to check continuity of a wire between the 2 if there is continuity maybe a led light comes on or a simple buzzer.
I have very little electronics knowledge so any help would be greatfull. I was thinking of trying to transmit rf down the wire to a reciever but I don't really no..

 

It's called a "signal tracer". https://www.google.com/search?q=sig...la:en-US:official&client=firefox-a&channel=sb

 

This design by Charles Wenzel might also be helpful:
http://www.techlib.com/electronics/c_beeper.html

Alan Yates has some demo videos of this device on his relevant web page:
http://vk2zay.net/article/266