The Light Keeper Pro is a Christmas “mini lights” maintenance tool. It facilitates finding bad bulbs in series strings. It’s multiple tools in one, but the main feature is a piezoelectric zapper that sends a high voltage pulse down an energized string that activates dead shunts in burned out bulbs, thereby enabling all the good bulbs to light up and allow one to easily find and replace the dead bulb(s).
It also contains a battery operated individual bulb tester, and a built in special purpose contactless voltage sensor like the previously reviewed Fluke Voltalert.
I have a pre-lit tree and it lit up a dead string on the first try. A word of caution, if you have too many dead shunted bulbs on a string, the line voltage is applied across the remaining good bulbs and can cause a cascade failure, burning out all the remaining bulbs on the string.