All it takes is one bad bulb to keep all of the other Christmas lights from lighting up. The trouble is, how do you find that one bad bulb? This gadget is supposed to help you find it.

A bad bulb breaks the circuit, which is why a string of lights might not all light up.

You may have seen the LED Keeper and LightKeeper Pro gadgets in hardware stores by the Christmas trees. LED Keeper is for strings of LED lights, Lightkeeper Pro is for incandescent light strings.

Here’s how they work. They both check the circuit to help find the bad bulb. With the Lightkeeper Pro, you remove one of the bulbs from the strand that isn’t working and plug the empty socket into the Lightkeeper Pro. When you pull the trigger, it sends a pulse of electricity through the wires to have them light up. Then, you can spot the bad bulb, the one that doesn’t light up. Replace that with a new bulb, and the strand is repaired.

For LED lights, the LED Keeper requires you to insert the wire into the gadget and pull the trigger. When one doesn’t light up… “This is the bad bulb.”

There’s a bulb tester built in. Toss the old bulb, replace it with a new bulb, and the strand is repaired.

It takes patience and you may need to watch the YouTube instructional videos a few times, but they do work.