Introduction: Red LEDs for Car (for Star Parties, Etc.)

Picture of Red LEDs for Car (for Star Parties, Etc.)
I've been embarrassed at star parties by the bright white light trunk and dome lights in our Taurus damaging people's night vision.

I did something about it, in my usual cheap way. I made red LED lights to replace the central dome bulb (the one that turns on whenever the doors open) and the trunk light.  The instructions here should work for any car with festoon-style bulbs.

Safety:  Car batteries and the electricity stored in them can be dangerous.  Safety is your own responsibility.  

  • LEDs
  • resistor (correct resistance and wattage needs to be calculated based on the LEDs forward voltage, forward current and your car's voltage--remember that it's higher when the car is running, probably around 14-14.5 volts)
  • 3/8" dowel (exact size may differ depending on vehicle)
  • Electrical tape
  • Needlenose pliers to twist and cut wires
  • Multimeter to check polarity in car light sockets
  • Drill with small drill bit
  • Optional (but useful): Punch to start the holes
Note on LED polarity: LEDs have two ways of marking their plus and minus sides.  Most LEDs have a flat side on the minus side and a longer lead on the plus side.  Some LEDs have no flat side, but they will still have a longer lead on the plus side.

I will assume that you're using three LEDs.  It's easy to vary this for fewer or more LEDs.

1. Use the multimeter to identify which side of your car's bulb socket is plus and which side is minus.  Be careful--don't short out the socket (i.e., don't connect both ends of the socket together with a wire or probe) and don't give yourself a shock.
2. Pull out the fuse (yes, cars have a fusebox--I didn't know this before this project) that governs the light you are working on.  Verify with multimeter that no electricity is flowing.
3. Cut dowel to a length that matches the car's festoon bulb that you're replacing (40mm in my case).  
4. Mark one end minus and the other plus.
5. For three LEDs, drill three holes through the dowel, all in a line, spaced at at about 1.5x the LED lead spacing.  Start about 1/4" to 3/8" from the minus end.  Then drill one more hole, nearer the plus side, leaving about 1/4" or 3/8" to the edge.  (This hole is not shown in the picture--it's from a newer design that I don't have a photo of.)
6. Identify the plus and minus sides of the LEDs.  (See note above.)
7. Insert the LEDs into the holes, with the minus sides facing the minus side of the dowel, and neighboring LEDs sharing holes for the leads that are side-by-side.  So, the hole on the minus side of the dowel will have the minus lead of one LED, the next hole will have that LED's minus lead and the plus lead of the second LED, the next hole will have the the minus lead of the second LED and the plus lead of the third LED, and the third hole will have the plus lead of the third LED.  (In the photo, this goes right to left.)
8. Insert one end of the resistor in the hole closest to the plus end.  Insert the other lead where the plus lead for the LED closest to the plus end of the dowel goes.
9. Twist together leads that share a hole.  Cut them to about 3/8" length below the bottom of the dowel.
10. Curve the twisted together leads around one side of the dowel.  Make sure there is no contact between pairs of leads that are not in the same hole.  Cover the side of the dowel on which these leads sit with electrical tape.  Make sure that the leads can't move in such a way as to make any contacts except for those leads that are in the same socket.
11. Put the remaining lead of the resistor through the remaining unoccupied hole near the plus side of the resistor.  (This hole isn't in the photo, but I marked where it should be.)  Wrap the part that comes out of the hole around the end in such a way that when you insert the dowel in the socket, it will make contact with the socket's electrical part.  You will want to cut it down and maybe drill a retaining hole for its end.
12. The LED closest to the minus side of the dowel has a lead that isn't connected to anything.  Wrap it around the end in such a way that it will make electrical contact with a socket, and cut it down as needed.  Again, you might want to make a retaining hole for its end.
13. Carefully insert in socket, with fuse still out, and make sure the wires are contacting the socket..  
14. With multimeter, check the resistance between the plus and minus ends of the socket.  The resistance should be at least as great as the value of your resistor.  If not, take it out and diagnose.  
15. Carefully insert fuse and see if it works.  If not, immediately disconnect fuse and diagnose.

Works just fine for me. My first version is too dim, so I later bought some brighter LEDs on ebay (alas, these spill over too much into the orange part of the spectrum, but so do my usual red keylights that I use for astronomy and nobody has complained about those).

Moreover, I think the LED system probably draws less than the 0.7A the incandescent bulb drew. So less danger of running out the car battery if I leave it on overnight.

My reading of Texas law (though I am not a lawyer) suggests that alas I am not allowed to do this for the puddle lights ("running board courtesy lights" must be amber or white--maybe this only applies if they're on while the car is going, but why take risks?), so when I go observing, I'll just put some tape over the puddle lights.

There was one unexpected cost. I blew a fuse when removing the original bulb. That's why I say to keep the fuse disconnected when doing this, except for steps 1 and 15.


dsandds2003 (author)2011-08-14

I think this is a neat idea. This idea could also be used in older cars and trucks to replace the glass dome bulbs.

arpruss (author)dsandds20032011-08-15

For the record, you can also buy LED festoon lights that replace those, and they're not expensive.

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