LED Christmas lights

One of the bummers of Christmas is trying to get the strings of lights to work. The biggest culprit are the strings that are connected in series. These are usually the strings of so-called miniature Christmas lights with the push-in bulbs. You've probably read on the box that when one light goes out the rest stay lit. True, the light bulbs have an internal "shunting device" so that if one burns out, it doesn't break the circuit and the rest of the lights do stay lit. But the problem is that if the circuit should become broken at any other point, the whole string goes out. The usual place this happens is between the lamp and the socket, especially if the string is used outdoors. This is because water in the socket causes the thin copper lamp wires to oxidize, which prevents them from conducting electricity.  Sometimes the wires on the lamp can become completely eaten away. At any rate, it breaks the circuit and the string doesn't work.

Finding the broken connection is no easy matter, especially when you consider that most strings have at least 35 lamps. I've seen every trick in the book used to get a bad string to work, using an ohmmeter to find the bad connection, even running a high voltage through the string which causes an arch to form at the point of the broken connection. To make matters worse, there's usually more than one broken connection. Whatever method is used can be time consuming. You may be wondering, if series strings are such a hassle, why don't they run the lamps in parallel, like they do with the strings that use 120 volt light bulbs.

Many of you are probably not old enough to remember the so-called transformer light strings of the 50's. They were usually strings of low voltage blinking lights that had to be run in parallel, because when each light blinked, it would break the circuit. The transformer  was about the size of the type that ran model electric trains (probably made by the same transformer company), except it was a fixed voltage (I think 6 volts) and was probably about 10 amps. When lamps are in parallel, the current adds up, so it took a pretty hefty transformer to run the lamps, which added to the cost of the string. Because of the high current, the wire size of the string also had to be a heaver gauge. This is probably the main reason they don't make these strings anymore. The miniature strings of lights could be made with 24AWG wire and no transformer.

Enter the light emitting diode. Although they've been around since the late sixties, it has only been in recent years that they have become suitable for Christmas light strings. The first problem was the lack of colors, red was all you could get. By the mid-70s, you could get three colors, red, yellow and green, but the color shades weren't that great and lack of briteness was another problem. 

By the 1990s, all these problems had been eliminated, and LEDs looked as good as miniature Christmas lights, and also had a lot of advantages. They use about a tenth of the current of normal miniature Christmas light bulbs, they have a life of 100,000 hours, they have much thicker wires than miniature Christmas light bulbs and they now come in all the colors of the rainbow and are much brighter than LEDs were only a few years ago, and you can even buy LEDs that blink.

Here's the two lamps in a side by side comparison, the lamp on the left is a conventional miniature Christmas light. Between the two wire supports is the filament that gives off the light. The lamp on the right is the light emitting diode. It is made out of plastic instead of glass and it is not hollow. That makes them virtually unbreakable. The metal substrate holds the semiconductor material that gives off the light. They generate very little heat. Both lamps are mounted in the plugs that fit the miniature Christmas lamp sockets. Unlike regular incandescent lamps, LEDs don't have to be colored to produce colored light. It's the semiconductor material that determines the color of the lamp. Since making this material produce more than a narrow wavelength of light is difficult, white LEDs are produced to generate ultraviolet light, which causes a phosphor coating to glow, much like a fluorescent lamp, only without any poisonous mercury.

Converting an old miniature light string to work with LEDs is fairly easy for anyone who knows how to soldier. It is also a good project to learn how to soldier for those who don't, but you should practice on scrap wire first, before trying to build the string. It involves cutting a series string apart and rewiring it in parallel, with a series resistor for each of the LEDs, which limits the current of each LED to 20ma. This usually works out to be 470 ohms for a 12 volt power supply.   Below are the power supply requirements according to the numbers of LEDs in the string for a LED current of 20 mA. 

I begin by cutting the light sockets off the string about three inches from the socket and I store them in a box. There is usually a wire that goes from one end of the string to the other (two such wires if you're lucky to have a string that has an electrical socket at the end of the string), which I wind onto a spool. Sometimes you can find wire the right shade of green wire at electronics stores. I find that 22AWG size stranded wire (make sure it is not solid) works the best. Also get some 1/8th inch heat shrink tubing to insulate your connections. It usually comes in four foot lengths.  It may take two of these lengths to do an entire string. You will also need a heat source to shrink the tubing. If you don't have a heat gun, a lighter can be used if you are careful.

After you get the string cut apart, you should strip about a half-inch of insulation off the socket wires and soldier a 470 ohm resistor to one of the two wires. It should look like this. The color code for a 470 ohm resistor is Yellow, Violet, Brown.

To build the string, I cut the green wire into 7 inch lengths. When soldiered, that gives about a 6 inch spacing between sockets, close to that of the original string. When the 7 inch wires are attached, it looks like this:

Two of the four wires go to the previous lamp in the string (or to the power supply if it's the first lamp in the string) and the other two wires go to the next lamp in the string. Since LEDs are polarized, (electrons will only flow through them in one direction because they're diodes) you should wire the string so that all the resistors are in the same line (I choose +). That makes things easier when plugging the LEDs into the string. Although LEDs can operate on AC, I have misgivings in doing so and prefer to operate them on filtered DC. For one thing, it protects the LEDs from power surges, because the filter capacitors in the power supply tend to filter out any spikes on the AC line.

The next step is to cover the connections with heat shrink tubing and to shrink it with your heat source. Before you do this, you should smooth out any sharp burrs (such as can be seen on the top wire above) in the connections with needle nose pliers. Otherwise they could poke through the heat shrink tubing. This wiring process is repeated for each lamp socket in the string. Now for a look at the finished connection:

Notice how much nicer the connections are than if you were to cover them with electrical tape. You don't have to worry about the tape peeling off. The heat shrink tubing is there to stay. After shrinking the tubing over the connection, the two wires between the sockets is twisted together just like is done with a normal string of Christmas lights.

Now it's starting to look like a light string. The two wires heading down go to the light socket (not shown).

You can also construct line voltage strings that will work with neon indicator lamps, often known as NE-2. Like the miniature Christmas lights, these lamps have wire leads and fit the same sockets. The only change you have to make in the string construction is to use 100k series resistors rather than the 470 ohm ones required for LEDs. Also, you can get green "neon" lamps the same size of the orange ones. These are actually argon glow lamps with a fluorescent coating that glows green from the ultraviolet light that the argon discharge produces. You used to be able to get the argon lamps without the fluorescent coating, but I don't think they make them anymore. These argon lamps may need a different resistor value than the neons.

Instead of having two wires, these strings have five, one for each circuit and a common return wire. I refer to this common wire as the backbone of the string, since it is the only wire that connects to every socket. One wire from each socket connects to the return wire through the resistor, and the other socket wire connects to one of the four circuit wires, the first socket in the string to circuit 1, the next to circuit 2, and so on. I use 5-pin DIN connectors to connect the string to the chase controller.

Copyright  © 2002, Colin Pringle  (colin@wild-bohemian.com)
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