So I decided to make a replacement bulb for my dome light. I know, it has been covered here before, but I figured that I'd document as I went along and share my experiences.
36 3mm Red LEDs
6 100 Ohm Resistors
1 small piece of acrylic
Some scrap solid-core copper wire
1 Appropriately-size wood dowel
1/8" drill bit
3/32" drill bit
Pliers (needle-nose or regular or both)
Step 1: Get the Overhead Light Outta There!
Once you've got the cover off, you should be able to see the bulb. Get it lit, and use a multimeter to check polarity and voltage. On mine, ground was nearer the switch, so I used a sharp pointed object to scribe a small "+" on the positive side.
NOTE: Be sure to check the voltage with the engine running. Most car's are 12 volt systems that are actually somewhat higher. Mine is 11.7V when the engine is off, and 13.8V with the engine at idle (and probably a bit higher at wide-open throttle).
Now you can pull out the entire dome light assembly. Again, look online for instructions specific to your vehicle. For mine, I had to pry a few clips to remove the trim, then pull the whole light assembly down and remove a plug.
Step 2: Make a Template
Begin by cutting out the basic shape by eye. Remember, you can always trim a little more off later, but it's harder to add back to it, so leave it a little big. Then take the rough shape and press it into the opening, creasing around the edges. Take out the paper and cut out the creased areas. Repeat until you get a good fit.
On my light, there are a few areas where the backing is raised up. I marked and cut these in order to transfer them to my cardboard template. I used them to keep the acrylic raised up from the backing, to leave space for the wiring on the back of the acrylic.
Once your paper template is complete, transfer it to a piece of cardboard and cut it out. I used cardboard that was slightly thicker than the acrylic I planned to use. This accounted for the acrylic and the slight bit of LED that would stick out the front.
Test fit the cardboard in the light assembly, and take the time to put the cover on. If it doesn't fit, modify your design now, before it gets more challenging (this is the entire purpose of the template).
Step 3: Cut Out the Plastic
For the more intricate cuts, I used a high-speed rotary tool with a cutting head (because I ran out of cut-off discs). I began by trying a jigsaw, but it just mangled my acrylic. I cut slightly outside the lines.
Once the rough shape has been cut, I smoothed out the edges with a sanding drum. Again, test-fit the piece now (and often). Mine required slight modification to fit.
Step 4: Make the Bulb Stand-in
I needed to find a bulb-sized, non-conductive replacement. It just so happens that a leftover dowel from a kitchen remodel several years ago fit the bill (yeah, I'm one of those people who throw bits into a coffee can instead of getting rid of them). Other options I considered included a pencil, the center of one of those rolls of bags for picking up dog-doo, and the body of a pen. My first choice just happened to work. You can use what works in your particular case.
I needed to get power from the contacts to the LEDs - that's where the solid core copper wire comes in. I stripped some out of old coaxial cable (once again, never throw anything out!) and used that. I marked the dowel where the contacts are, and then chucked the dowel into my drill. Then I spun the dowel against a hacksaw blade to create a small groove. Wrap the wire around the grooves, grab 'em in a pair of pliers, twist 'em together, and you're ready to move on to the next stage!
Step 5: Decide on Your Array
I used the handy-dandy LED array calculator at http://led.linear1.org/led.wiz
I simply plugged in my values to find an appropriate sized resistor. At 13.8V supply voltage, and a forward voltage of 2V on my LEDs, the calculator suggested 6 LEDs in series, with a 100 ohm current-limiting resistor. I liked this value, because my LEDs are rated at 1.9-2.2 fV at 20 mA forward current. That means that this array will work through the whole range of values, from 11.7V up to more than 14V.
I had originally thought of 4 clusters of 9 LEDs each, but layout proved to be more difficult. Instead, I decided on a nice 6 by 6 array. Really, this was just for convenience's sake - I could've done any number that I wanted (and you can, too!).
Step 6: Layout Time!
I have grid paper (graph paper, square paper, whatever you want to call it). The intersection of lines are spaced nicely to allow two LEDs to sit next to one another. I decided to do 6 rows with each row two squares away from its neighbour. This allowed even spacing, with space in the center for my power supply lines coming off the dowel.
To find the right size drill bit, I took a LED and used the holes in my drill bit kit to determine the size where the LED would fit, but not go all the way through. For a 3mm LED, 1/8" seemed to work fine.
I took a small piece of graph paper, taped it to my piece of acrylic, and marked each hole location with a small x in pencil. I then drilled each x slightly to begin the hole. I found it easier to start each hole, then remove the paper, and completely drill the holes without the paper in the way.
Be sure to hold the acrylic firmly when drilling. It tends to flex a lot as the drill pierces the far side, and with all those holes so close by, it would probably snap quite easily. I didn't have that problem.
Step 7: Wire It Up!
For me, the easiest way to make the connections was to bend the negative lead down at 90 degrees, and clip the positive lead short. Then solder the negative lead from one LED to the positive on its neighbour. Continue in this manner until you've got your array all wired up.
If you don't know how to solder, check out the great tutorials here or anywhere on the web. Just be sure to keep the iron clean, move quickly, and try not to fry the LEDs.
Your technique may vary. Whatever you do, don't forget the resistors!
I periodically checked each LED using the diode function on my multimeter. I only had one defective LED, but it was enough to put an entire row out of commission. That's what happens when you pay about $3 for 100 LEDs on ebay (still a heck of a lot cheaper than my local electronics stores).
Now insert the dowel supply wires. I put the wires through, bent them around, and sent them back through the acrylic a second time. It's important that the connection between dowel and acrylic is solid, because it's what's going to hold the whole thing in place.
Once all the connections are made, test it as a stand-alone bulb (by powering up the positive and negative supply lines) and test it inserted into the holder (by powering up the holders in the dome light assembly).
Step 8: Glue It!
I originally considered hot glue, but the inside of a car gets mighty hot in the summer, and may be enough to liquefy the glue again.
I opted for 5 minute epoxy. I mix it in a small zip-top bag, then cut a tiny corner off and use it as a dispenser. A dab on each LED around the edges, and enough under the dowel to secure it to the acrylic.
Step 9: Put It in the Car, and Let 'er Rip!
Again, I know it's been done before, but I thought it was neat to have a DIY LED replacement bulb for the overhead light. In total, this project cost me $3, and I have 54 LEDs left over. Not bad for a significant lighting improvement.
Plus, I really prefer red lights at night.
Good luck with yours! Next on the list is the trunk light, and some foot-well lighting with a daylight sensor and/or door sensor activation.