Sonic Eye is a very simple yet useful troubleshooting gadget that will find many uses in both studios and live sound setups. Essentially it is a battery-powered, buffered level meter built onto the end of a jack plug with three LEDs denoting -16dBu, -22dBu and -32dBu. Power comes from a small internal battery, so, to conserve power, the unit is only active as long as the black button on its surface is held down.
Plug it into any line-level or instrument-level output and immediately you can tell if there's signal present. It even works with electric guitars and basses, and of course it's ideal for plugging into a patchbay (when the expected signal doesn't materialise) or into a console effects send to check for signal. Other uses include checking assignable outputs on synths and soundcards, effects pedals, rack gear — in fact just about anything except speaker-level signals and microphones (and even some high-output, high-impedance dynamic mics might trigger it).
Basic though it is, I think the Sonic Eye could save the typical studio owner an enormous amount of head-scratching! This is a wonderfully simple piece of test gear to help find where that missing signal went.