I had a suspected short circuit in a cable I made (I think I used too much heat on the heat shrink and caused insulation to melt) and what I needed was a continuity tester. My multimeter is bollocks for continuity so I knocked together my own:
I used a green 5mm LED, a 68 ohm resistor, two AA batteries, some banana sockets, and a bit of wire. I used my rotary tool with drill bit, burr, and countersink to score the holes in the case.
p.s. the cable under test was shorted! Turns out its a bad idea to put heat shrink tubing over electrical tape! And now we know.