We've all seen the problem: One bulb in a string of Christmas lights goes out, and half the string goes dark with it. And Murphy's Law says that it's probably going to happen after you've strung up a few hundred lights.

But Murphy's Law can't hold a candle (so to speak) to a Roanoke Valley inventor who built a device that fixes those broken Christmas lights. Pull out any dark bulb on the string, plug in the $19.95 LightKeeper, pull the trigger, and the lights come on. (Except the burned-out bulb, of course, which is now easy to identify and replace.)

It's one of those wonderfully simple tools that solves an annoying problem in an elegant way.

Sure, you can get a string of Christmas lights for a buck or two on December 26. But it's not about the money; it's about the hassle.

"If it's in a tree and you've got all your decorations and tinsel up, you don't care that you got your lights for a dollar the day after Christmas," said Rich Frederick, the LightKeeper's inventor. "You want your lights back."

Plug, pull, presto - That's what the LightKeeper does.

Frederick invented the device out of sheer necessity. Living in Chicago in 1999, he found himself "55, broke and an unemployed inventor and engineer" after his business partner proved not to be the most business-savvy of partners.

His way out, he figured, was some Christmas-related devices he had built but never sold. In a scene out of a 1950s movie, he "went cold to the National Hardware Show in Chicago" to show his inventions to the vendors there.

One was interested in some of his ideas: The Ulta-Lit Tree Co., which makes prelit Christmas trees. The company was discussing some marketing possibilities when, according to Frederick, "One of the guys said, 'It would be really nice if someone figured out a way to fix Christmas lights.'" (Even prelit trees occasionally have a bad bulb.)

So Frederick did.

Four patents later, he demonstrated a crude, but working, prototype. The Ulta-Lit people were happy, and they signed an agreement: Frederick would handle the product design and development, Ulta-Lit would handle sales and marketing.

Frederick worked with industrial designers and came up with the look for the LightKeeper that's available in stores today. "It looks like a phaser," he said. "But it also looks like something that goes 'boom' and fixes your lights."

The LightKeeper debuted in 2001, when it sold about 3,000 units in the Chicago area. Since then, it's been picked up by retailers such as Lowe's, Sears and Target, and sales have been solid. (Frederick will only say that he sells "thousands," but his share of those sales make up most of his income these days.)

In February, "tired of two-and-a-half-hour traffic jams," Frederick and his wife scoured the eastern half of the country looking for a place to settle. They ended up in the Roanoke Valley, in a ridge house in Windy Gap with a full electronics lab - a place he spends 10 or 12 hours a day being what he calls "semiretired."

Besides updating and improving the LightKeeper - this year's model, the LightKeeper Pro, sports an easier-to-use plug, for example - Frederick is working on other Christmas-related inventions. Coming soon, in fact, is the SnowDrift: an outdoor, lighted tree skirt that looks like a pile of snow.

As nice as that's likely to be, for sheer usefulness it's hard to beat the LightKeeper, especially if you're the one on the ladder in the cold.

Frederick isn't getting rich on the LightKeeper, but he said that wasn't his goal in the first place. "I like that I made Christmas a little better."