This was just a quick fun project that I used to test whether a UV nail varnish hardener could be used to expose UV sensitive PCB laminate. (It worked fine.) It's basically a small PCB designed to push into a standard USB socket with two pads where the +5V and 0V connections are. It uses the 5V supply to power three parallel LEDs via a 100 ohm resistor. (Just 20mA shared between them.) The only issue I had with the design was that the standard 1.6mm thick laminate is just a bit too thin and needed some layers of tape applied and then carefully trimmed to make it a snug fit against the electrical connections.