Summary This is an simple device used to check if a cable is live or not. It is a quick way to find the right cable in a bundle when there is no connector to probe directly on. Just push the sharp metal probe through the isolation and watch if the LED lights up. It is mainly for car electronics so it's mainly for 12V. Some soldering required and a small resistor around 300 Ohm and an LED. The hand drawn sketches shows the basic how it works.