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, the extension cord would not be plugged into a wall socket at the time!!!). 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.