Like the cool underglow kits on riced out cars but don't want to break the bank? This will show you how to make your own using LED strings.
-This instructable requires some knowledge of electronics and auto body assembly.
-This might involve taking apart large sections of your car's console, if this could void a warranty, do so at your own risk.
-In some states and municipalities, it is illegal to have non-signal lights on while your vehicle in motion. Check with your local authorities before using on the street.
Time: 3-6 Hours
Cost: $20-50 (depending mainly on how many lights you get)
Difficulty: Decently easy
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Step 1: Gather Materials
-One or several waterproof, 12V LED strings - $10 ea.
-Lots of wire. Two-strand polarized wire is great (red-black or white-black)
-SPST switch - Free-$3 (I got a lit-when-on one)
-555 timer - $0.10-$1.50 if you went to Shady-o Rack
-10000-15000 uF worth of 10v capacitors (you can string them in parallel to add the capacitance)
-15kOhm resistor - the bigger the resistor the slower the pulse - alternatively you can use a potentiometer
-Project box of some sort
-Scrap thin medium-density fiberboard (MDF) or sheet metal
-Soldering iron and solder
-Hot glue gun
-Various size drill bits
And of course a car. It's a good idea to know your way around the inside of your car a bit before beginning.
Step 2: Take Apart Your Console
Easier said than done. You have to basically remove enough panels to get at the wiring of the car's cigarette lighter or auxiliary power. Next, find a suitable spot for putting in the switch. I like using one of the blank buttons near the headlight controls. It's a good idea to drill the hole for the switch while everything is all apart.
Then, since you're probably already under the steering column, look for an unused plug in the firewall, right by the fuse box on most models. This is for running extra wiring (like we're doing) and, once you pop out the plug, should lead to somewhere near the left front wheel well. Save the plug, you'll need that later.
Step 3: Add an Auxiliary Extension
Find your 12V power line. This is usually the red or white wire that goes into the housing for the cigarette lighter socket. Then, find the ground, a black wire most likely attached to the outside metal of the socket. Cut or expose the insulation from both and splice in the red (to 12V) and black (to ground) wires from the double strand. Cut the wire so that it's long enough to reach the switch and then some just in case. It's easier to cut wire than to extend metal and plastic using telekinetic ability :-).
Step 4: Install the Switch
Take either your positive wire or your ground, and wire it to one of the switch terminals. I used the positive just out of force of habit. Then, add about 6" of double-stranded wire to this (red to the switch and black to the other black wire). This will later connect to the controller circuit.
Step 5: Affix LED Strands to the Bottom of Your Car
The layout is pretty much up to you, you can have a strand for front, back, left, or right, sides only, front and back only, all sides, whatever.
Make yourself some 2" squares of MDF, two per strand, and epoxy one to each end of each strand. Let that cure.
In the meantime, you can pick out suitable spots to place the strings under your car. Anything that is directly part of the chassis is great. Be careful not to put it too close to the engine or exhaust line! If you want a more temporary attachment, you can bolt the MDF to it or hot glue it for semi-permanent. A liberal amount of hot glue seems to bond the MDF to the metal really well; I've driven 100 miles with my setup and it hasn't fallen off yet. I wouldn't recommend using epoxy or J-B weld in case you have to take it off for whatever reason.
Step 6: Wire the LED Strands
Remember that hole in the firewall from step 2? We're going to use that now. If your car doesn't have a vacant one already, either piggyback one that already has wires going through it or drill your own hole.
Since every car is different, you're going to have to come up with your own way to snake some double-strand wire from the cabin to the bottom of your car. Generally though, there should be an opening straight to the bottom near the front left wheel well (for countries that drive on the right, it'd be the right wheel if you're across the pond :-P). Tie a loop of string around the end of the wire, then feed about a foot of the wire through the hole. You should be able to see it when you look under the hood.
Take a coat hanger, or perhaps using your hand, grab the end of the wire from underneath your car and pull it through. Duct tape it to the bottom of your car to keep it from going anywhere (you can take off the loop now).
Run wire from each of the LED strands and wire them all in parallel (red to red, black to black) and solder the whole mess together. I'd recommend using liberal amounts of electrical tape as well. Once again, stay away from hot components.
Make sure to leave 1-2 feet of wire inside the cabin.
Step 7: Circuit Time!
If you're at all savvy with circuit boards this should be cake. This circuit creates the pulse effect, if you want just a continuous light, you can skip this.
The idea is the 555 timer generates a square wave which charges up the capacitors in the on phase. In the off phase, the capacitors light the LEDs, depleting over time and creating the fade out effect. Theoretically, during the on phase, the current used to charge the capacitors should detract from the LEDs, creating a fade in effect, but often the current is so large that this is nearly instantaneous. Alternatively, you can put in a switch that bypasses the pulse circuit (so that the ground from the LEDs goes straight to the mains ground) so you can go from pulse to steady on.
I more or less used capacitors and resistors that were lying around. You can customize the frequency using the 555 timer calculator. Alternatively, you could sub in a potentiometer for RB to get a variable rate.
The 12000 uF capacitor is just a bunch of large 12V capacitors wired together. You may need more or less depending on the number and rating of the LED strings to get the right effect.
Test out the circuit on a breadboard connected to the car's electronics before putting it onto a PCB. And then test it when you've soldered it before boxing it up.
Step 8: Installing the Project Box
I didn't feel like buying a project box from Shady-o Rack, and since this thing is gonna be tucked up under my dashboard, I frankly don't care. So I made one out of MDF and hot glue :-P.
I put two screws on either ends for the power, ground, power for lights, and ground for lights, so I could just screw on the wires. Make sure to leave enough on the inside to account for the twisting (and start with the screws nearly fully tightened when you solder).
Find a nice comfy spot somewhere up under the console to put it. Screw down each of the wires to their respective contact, and tape the box securely so it won't rattle around.
Step 9: End.
Wait till night falls and enjoy your handiwork.