Check point

control voltage indicator

Check point is a passive voltage indicator. Touch the check point with the tip of your cable, the LEDs will give you instant visual indication of the incoming voltage’s amplitude (brightness) and polarity (+ or -). The module is connected to ground through the mounting screws and the rail in your eurorack case. In some instances this connection may not be reliable and the module won’t work properly. Rails with threaded strips proved to perform the best.

http://doboz.audio/check-point/