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How To Make LED In-Car Christmas Lights

What This Is

For some years now, I have had a homemade string of Christmas lights in my car which was adapted to run from a cigarette lighter adapter. I updated them recently to use LEDs; here's how to do it.

The lights are alternating red and green LEDs with a ping pong ball around each LED. They are bright enough to see well at night, but not so bright that they interfere with your vision or attention while driving. They can be turned off simply by unplugging the adapter from the cigarette lighter socket.

The string of lights is Y-shaped. The "tail" of the Y has the power plug which goes into the cigarette lighter socket. This powers the lights when the car is on. A bit of tape ensures that the wire stays out of the way and helps ensure that the results are sufficiently ugly and home-made looking.

The "arms" of the Y go from behind the rear-view mirror, across the base of the sunshades, and then back across the tops of the doors. I tuck the wires under plastic borders to the upholstery and use twist ties wherever necessary to hook or otherwise attach the string of lights in place. In total, there are 12 lights (6 on each "arm") which reaches just far enough around the car's interior to reach the rear windshield.

How They Are Made

The following items are needed:

  1. 12 LEDs (I used 6 red and 6 green). The normal diffused ones are better than the clear high-brightness ones for this application.

  2. A length of 2-conductor cable long enough to go around your car's interior. I made the one shown out of green and red (Christmas colors).

  3. A 47 Ohm resistor - It should be a beefy, high-wattage resistor since it will be getting quite warm in this configuration. I had a 10W one handy which I used.

  4. 12 Ping Pong balls.

  5. A car accessory plug.





Power comes from the accessory plug (about 12V or a little more), will go through the 47 Ohm resistor, and the LEDs will be wired up in parallel.

If you wish to play around with the number or type of LEDs, or the wiring, you should make sure your resistor is appropriate and your wiring layout is good. An LED calculator (there are many on the web - here is one of them - can help you here.)

Assembly is simple, but requires attention to detail since a mistake can - at best - result in a Christmas light string that partially or wholly doesn't work - or at worst, a short circuit being plugged into your car's electrical system.

The Schematic

Click for a larger version.
This design wires the LEDs in parallel which is easier to construct but requires a (physically) big resistor since the resistor needs to dissapate a lot of heat from the current it will be handling.

Building the LED String

First, it may be useful to measure out the cable inside your vehicle and mark off where you'd like the bulbs to be and how long you'd like the cable to reach. If you don't particularly care, you can just space them more or less evenly.

First, select a spot for your first LED, separate the wires if required, and strip a piece of the two wires so that the conductors are exposed. You don't need to actually remove the insulation from the wires entirely - you should be able to just push them aside enough with a wire stripper so that you can solder the LED to the conductors.

Next, bend the leads of your selected LED as shown. The longer lead goes to what will be the "-" wire. It is critical that you are consistent! All short leads of all LEDs should go to the same wire (and conversely, all longer leads should go to the other wire).

Then, solder the LED onto the wire. Make sure it's securely soldered.

Now, cut a small hole in a table tennis ball - big enough for the LED to poke through.

Finally, secure the ball and the LED together (and cover any exposed wire and LED leads) with hot glue.

Done one light! Repeat until you have all 12 LEDs installed and secured. Then you can test by hooking the string up to a +12V source through the resistor. If all of them light up, you're set. (Otherwise, inspect the connections of any that don't light up.)

Building the Accessory Plug

There's not much to say here - just put the resistor in series with the "+" terminal wire (putting it onto the "-" wire will also work, but I like to put it on the "+" personally).

Then the + and - should connect to the appropriate wires in the middle of the LED string we made. This results in a "Y" shaped string of lights, with the accessory plug hanging from the middle of the string. Remember that the resistor is expected to get warm (perhaps quite warm) so don't use anything that will melt as part of insulating or anchoring it - hot glue would be a no-no.

Hook it Up, Cruise Around!

Light it up by feeding +12V into the adapter plug, and after one last check that all the LEDs light up properly, Install it into your car!

(From past experience these get attention when going through a drive-through.)

And speaking of LED Xmas lights, you may also wish to check out my miniature USB-powered LED Xmas Tree with blinking lights!

Happy Holidays!

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