tdp124

Overview:

I wanted to add some blue lighting in my foot wells but didn’t just want to slap on a cold cathode tube. Instead, I decided to go the LED route, properly powered via a PWM signal with a duty cycle controlled by a pot… in layman’s terms, LEDs that are dimmable in a linear fashion.

This wasn’t originally intended to be a DIY tutorial, but it should give you an idea of how to do something like this yourself should you desire to do so. All images are thumbnails and can be clicked to view their full, larger resolution versions.



LEDs:

To start, I purchased a bunch of LEDs at 17 cents each from *gasp* eBay (though apparently the seller has a website too where you can get them for even cheaper). The LEDs I bought are 5mm in size, can sustain anywhere from 3.0-3.4V @ 24mA max, and are rated at 6000mcd equally spread over 180° (important). This means there shouldn’t be any “hot spots” or rings where the light is more intense/focused than other places. I think you guys who spend dollars per each LED are crazy, but that’s just me. I decided to use 4 equally spaced LEDs in each foot well to get the most even light spread (yeah--only 68 cents per side!) These were soldered together as a chain and the exposed metal parts were covered with heat shrink tubing. A dab of hot glue was added to the back of each led just to cover up any more exposed metal leads.





Switch/Dimmer:

Next, I decided to work on the switch. I would be using one of the 4 empty switch holders to the left of the steering wheel and planned on putting an illuminated push button (to turn them on and off) and a potentiometer (“pot”) for a dimmer ****. To power the LED in the button I used two 10k Ohm resistors in series to make a 20k Ohm resistor. When powered, this gave me the perfect amount of illumination I was looking for: not blindingly bright, but just enough to easily see where the button is in the dark to push it. I came across a big problem--I couldn’t fit the pot up close enough to the switch panel to mount it because of the way those slots in the car are designed. The pot would need to be mounted about an inch behind the front of switch, but how? As you can see in these two pics:



…I cut a piece of a pipe, whipped out the tapping kit, and made an extension. Inside, I tapped about a 1/4 of an inch in with a threading SIMILAR TO but DIFFERENT than the threading on the pot’s mounting cylinder. I then re-tapped the pot half way down. This allowed for me to screw on my pipe attachment enough to make it secure, and then enough to tighten it really well (it tightened well because it became cross-threaded due to the different threading spacing). I made a couple of nuts that would be used on either side of the plastic to secure it in place, and painted the very thin one black (to be used on the visible side).



The PWM Circuit & Wiring:

Now that the switch and LEDs were made, I designed a simple PWM circuit to power the LEDs. For those of you that don’t know, PWM stands for Pulse Width Modulation, and it’s essentially turning the power on and off very fast (much faster than our eyes can detect) while maintaining a constant voltage--none of that voltage divider crap! This is the proper way to power an LED. Also, since LEDs don’t have an intensity linearly corresponding to voltage, this is the best way to make LEDs dim smoothly and linearly.

I won’t go into depth about designing or building the circuit (you can find schematics online or buy them prebuilt I bet), but after it was finished and tested on a breadboard, I soldered it together on a mini proto-board from Radioshack (they come in 2 packs for a couple of bucks). The 555’s output (“555” = the integrated circuit responsible for generating the PWM signal in this circuit) was fed into a generic NPN transistor to switch it on and off very rapidly to power the LEDs. Basically, the purpose of this transistor is to not pull current through the 555 timer chip to power the lights, but instead pull the current directly from the power source.



I used a single small resistor (I forget--but I think about an ohm or so) for each LED strip because it made the voltage drop per LED just about perfect—roughly 3.3V. Each strip of 4 LEDs (in series) pulled ~23mA and was being powered by the car’s power source (when the engine is running this is the alternator), which on my car is between 14.00 to 14.10 volts. The 555 circuit pulls 12mA to generate the PWM signal and flip-flop the transistor, and the little push button LED in the switch draws only ~3mA. So in total, that’s 23mA per side + 12mA + 3mA = 61mA. At 14V, the entire circuit consumes only 854mW of power (P=IV). That’s less than a watt… in comparison, the little bulb in the middle-console cubby hole is a 5W bulb if I recall correctly! So essentially, this entire mod uses practically no power!



Grabbing Power from the Car:

On to the next step: I wanted to power everything from the combination switch that controls the headlights/dashboard lights/etc. After reading over the wiring diagrams for an hour, I decided to just go check it out. I saw that the cubby hole light in the center console only turned on when the dashboard lights were on, which was the first switch position when turning the headlight ****. This was perfect for my purpose.

I pushed the light in with my thumb so it fell into the back of the console, easily folded back the carpet on the passenger side toward the center console (down in the foot well) after removing the one plug holding it in place, and found the wires going to the light I pushed through. I peeled back the electrical tape on the wires to expose two wires: a green one and a white one with a black stripe. According to the wiring manuals, I remembered that white-with-black-stripe meant ground, so I spliced a wire into the green wire and ran it over to the other side. I mounted my circuit to the left wall of the fuses next to the steering wheel/under the auxiliary switch spots because there was plenty of room:




I ran the power cable I spliced into over to my main switch and to both sets of LEDs. The other end of each LED strip was grounded through the transistor connected to the output of the 555 circuit. Here’s a picture of each side all wired up before mounting the LEDs:





Results:

So all this work for nothing because I bought cheapo 17 cent 5mm LEDs on eBay, right? Well here’s the final result, with dimmer set to 95%+ duty cycle (max brightness), I’m quite pleased!



The only thing left to do is find a nice dial to cover the ugly pot shaft (suggestions welcome).

night87

iTrader: (5)

good write up and thank you for the diy
more pics next time if possible~

januslym

Looks like a type of project I want to take on this summer. Feels like the documentation I've been reading on other sites like oznium say that a fuse is required... yet you didn't seem to use one. I'm pretty much a noob with this stuff, could you or anybody possibly explain when a fuse would be required and why yours didn't require one?

SquallLHeart

very impressive work. i approve.

he tapped into a power source that was fused.

you ALWAYS need a fuse, otherwise melting... sparking.. fires.. yea.
BAD.

tdp124

yup if you dont use a fuse, your thinnest gauge wire will act as the fuse. That's bad... That means the wire would turn red hot before breaking/melting apart, and possibly cause a fire. I thought about using my own fuse, but since I was drawing less than 1/10th of a Watt I just used one of the existing interior lighting fuses (like SquallLHeart said) because this amount of power surely wouldn't overload the existing circuit.
The benefit of having an additional fuse here for my circuit (I would place it between where I spliced into the car for power and my main switch) would be if something of mine shorted out, the rest of the interior lighting (or dashboard lights or whatever was on that fuse) wouldn't go out--so long as the fuse I picked was less than the car's.