Other DIY Testers

Simple circuit continuity tester

You can build a simple circuit continuity tester with just a few parts. This would allow testing a cut or broken extension cord to discover which wire connects to the wide terminal at the plug end. (Of course, disconnect the extension cord from the wall socket first!). You could also use it to test the ends of a communication cable to discover which wire connects to which pin. If there's "continuity" (i.e. "a connection") then the LED would light. If not, the LED would remain dark.