Lightkeeper Pro is a device that is used to fix miniature Christmas light sets that no longer light up due to a failed light bulb. It works by sending a pulse through the set, finding the malfunctioning bulb, and fixing its shunt.
A problem with miniature light sets is that if one bulb malfunctions, the entire set stops working. To counteract this problem, bulbs are equipped with a "shunt." This is a mechanism that shorts out a malfunctioning bulb to enable the rest of the set to light up. Shunts work most of the time; however, sometimes they fail to work, resulting in a dead light set.