Okay, on my Chrysler, a couple of the inside lights have burned out, along with the lights on top of the fenders that let you know your blinker’s on. I figured it’d be kind of cool to replace them with LEDs, since the lights are a bit of a PITA to get to. Now, no one makes (AFAIK) LED replacements for these, so I’m wondering if I can just go down to Rat Shack, by some LEDs of the right color (too bad they don’t make “black light” LEDs, it’d be cool to paint the numerals on the speedometer with Day-Glo[sup]TM[/sup] paint so they’d really glow at night) and just wire them in, or am I going to need to hook up a resistor because the average LED won’t be able to handle the juice running through it and the car will burst into flames when I turn the lights on?
At a minimum, you are going to need a current limiting resistor. LEDs are also polarity sensitive.
Link just an example that they exist. I think the prices there are pretty excessive, I paid about $2 CDN for my little UV LED, meant for detecting counterfeit cash.
I figured that they were polarity sensitive. How about hooking up a couple of them in series? Since I’m not sure if one’s going to be bright enough (especially with the dash lights, since they can be dimmed) would hooking up a couple in series work? Or would the first one blow, then the second one blow, then the third, etc., etc.,?
Say “Hello” to my new dash from Funkytown! 
You’ll definitely need a current limiting resistor. If, for example, the LED specs say 2V 10mA and if you want to run it off a 12V source, you need a resistor that creates a 10V voltage drop at 10mA current. That means 1 kohm.
There’s nothing wrong with hooking up LEDs in series, but I’d still put a resistor in series. LEDs do not behave gracefully when the voltage is even slightly higher than the rated value (i.e. current rises very fast). A resistor in series will make it behave a bit better.
From my experience, LED’s subjected to way too much voltage tend to explode with a flash and a puff of smoke, so take my advice as you will…
I suggest that if you want more than one LED, you should wire them in parallel and the current limiting resistor in series with the LED array. This way, the voltage across all of the LED’s is the same so they are more likely to be equally bright.
Say you have N LED’s and they are rated at 15 ma and 2V. Then the total current running through the current limiting resistor should be 0.015N amps. Assuming a 12 volt car battery is powering this rig, and that we require 2 volts across the LED’s, then the resistor R should drop 10 volts.
By Ohm’s well known Law: R = V / I = 10 / ( 0.015N) = 666.66 / N (ohhh the resistance of the beast!)
Thus if you want 4 LED’s you need about a 160 ohm resistor
Something we need to remember is that unlike a bench power supply regulated at 12 VDC ± 0.25, the voltage swings of an automotive system are greater. 13.6 across battery terminals with a vehicle running isn’t unusual.
They are getting pretty cheap now; I bought a pack of two hundred (just the LEDs as plain components, not packaged into a flashlight or anything) on eBay for twenty quid or so; the seller was in Hong Kong.
There are a variety of methods for powering an LED array:
1. Stick all the LEDs in parallel, and use one current-limiting (series) resistor for all the LEDs.
Advantages: Voltage fluctuations in the power source will not have a tremendous impact on the overall current through the LED array. Also, an open-circuited LED will not cut power to the other LEDs. (But the others will get brighter, which may decrease the reliability.) This design is also the simplest to implement and has the lowest parts count.
Disadvantages: It’s an inefficient design, in that the current-limiting resistor must dissipate quite a bit of power. And because of this, the resistor must also be physically large. And unless all the LEDs have the same I-V characteristics (i.e. unless they’re matched), some LEDs will be brighter than others. Finally, a short-circuited LED will cut-out all the others.
Overall, this is not a very good design.
2. Stick all the LEDs in parallel, but use one current-limiting (series) resistor for each LED.
Advantages: Voltage fluctuations in the power source will not have a tremendous impact on the current through any of the LEDs. An open-circuited or short-circuited LED will not cut power to the other LEDs, nor will it change the brightness of any other LED (assuming a voltage source is powering the array).
Disadvantages: It’s an inefficient design, in that the current-limiting resistors (taken together) will be dissipating quite a bit of power. And all the resistors take up room.
This is an extremely robust design, but the part count is kind of high due to all the resistors. And it’s not all that great from an electrical efficiency standpoint.
3. Stick all the LEDs in series, and use one current-limiting (series) resistor for all the LEDs.
Advantages: It’s an efficient design, in that the current-limiting resistor will have a relatively small voltage drop across it (the resistor will not be dissipating much power). Furthermore, all the LEDs will have the same current. This means the intensity from each LED will be better matched vs. having a constant voltage across each LED (as in design #1).
Disadvantages: Voltage fluctuations in the power source may have a significant impact on the current through the LEDs. Also, an open-circuited LED will cut power to the other LEDs.
Overall, this is a pretty good design as long as the voltage source is relatively constant or regulated.
4. A parallel-series combination of LEDs.
By designing an LED array using series-parallel arrangements of LEDs, you can create a design that may be acceptable in terms of robustness and efficiency (but not optimized for either).
5. Stick all the LEDs in series, and use a current source to power the LEDs.
Advantages: Because LEDs like to have a constant current through them, it makes sense to power them with a true current source vs. the (much more common) voltage source w/ current-limiting resistor (which can be thought of as a “poor-man’s current source”).
Disadvantages: You have to design and build a current source. And it’s more expensive to implement verses designs #1 through #4.
6. Use a chip that’s specifically designed to drive LED arrays.
This is the Cadillac approach. These chips provide all the bells & whistles to safely and efficiently power an LED array (charge pumps, current drivers, closed-loop power controllers, over-voltage / over-current protection, etc.). It’s also the most expensive way to power LEDs. Sipex and Maxim make some neat ones.
Yeah, but will they allow me to dim the dash lights or will it override the voltage input from the dimmer?
Okay, that’s not really clear. I can’t tell if I can use the existing dimmer control, or if I’ll have to wire up a second one.
With that particular driver, you’d need an analog-to-digital converter to use the dimmer. But, that was just an example I found in a quick search. I’m pretty sure I’ve seen them with analog inputs for brightness. I’ll keep looking.
Incidentally, I just won an eBay auction for 50 UV LEDs (~380 nm)–more than I’ll ever need. If you want a couple, email me with an address, and I’ll shoot 'em off to you when I get them. Specs are here.
To add to this great post what I can: this option seems like the best to me. Whipping up a current source to drive LEDs isn’t that big of a deal, we don’t need it to be that precise. Bias a transistor with a zener and a resistor, say a 5.1V zener in parallel from base to ground, slap a 220 ohm resistor in the emitter, 1/4W should be fine, and you can get two LEDs in series in the collector without worrying about saturation. Put more [matched] LEDs in parallel by lowering the resistor but increasing its power rating. For a weaker design you could use a rheostat with a series resistor instead of a zener across the base, it is probably easier to control the brightness this way than by tweaking the emitter resistance. The nice thing about this is that it scales pretty well. Only one component really needs to be manipulated to add more and more LEDs, given a decent transistor. For more scaling, additional transistors can be paralleled to handle the extra current more parallel banks would add.
If you don’t want the LED brightness to change when the voltage changes, you pretty much have to go this route with the zener. That leaves you with controlling the current through the emitter resistance, which is still doable.
Depending on resistor tolerances and how many you have in series, normal resistor tolerances make 2 kind of a poor option in terms of trying to achieve the same brightness. But offhand I don’t know how well LEDs match brightness in the first place. Don’t do much with them where I care about anything other than “are they on?” 
For multiple LEDs, I would use a voltage regulator. For the OP’s automotive application, there’s two benefits…No need to account for the voltage swings and no need to worry about resistor power dissipation. The power dissipated for this project is not trivial either. Say you need to drop 10 volts and are running the LED at 10mA (that’s sort of low for high performance LEDs, too). Thats 100mW per LED. If you run a few of these, you might be beyond what is easily available at your local Radio Shack.
I’d recommend a voltage regulator circuit. You’ll need an adjustable one to get the most output form your LEDs. I’ve had nothing but success using the LM317 adjustable regulator. Here’s what I use. Since the LEDs are pretty forgiving of quick little current spikes, you can probably safely leave the capacitors out (I always do). The parts (regulator, fixed resistor, potentiometer, wire, perf board, heatsink) for this entire project are available at any Radio Shack. The dimming with the dashboard lights might be a little tricky though.
Its worth pointing out, too, that many modern LED chemistries have a nominal forward voltage of 3.5-4.0 volts and run at 25-30mA. This is especially true of blue, violet/UV, white, cyan and many green types.
I’d also use recommend the LM317, but I’d wire it as a current source.
like this:
http://linear1.org/gm/archives/00000147.php
Just put your LEDs in series, add up the forward voltages, and feed the LM317 a couple volts more. Increasing the resistance will reduce the current and dim the LEDs.
The Luxeons the above article talks about are worth looking into also. IMHO, the ideal replacement for auto interior bulbs (kinda spendy though).
Finally, more than you ever wanted to know about LEDs:http://www.candlepowerforums.com/ubbthreads/ubbthreads.php
Be careful if you’re thinking about using Luxeons - they run hot by design - the place you’d be putting them may not be adequately ventilated etc and they could overheat and fail or set fire to your car.
Never heard of Luxeons until now, so no plans to use 'em (though I wonder if they’d work in the house, gets cold in this place in the winter). No idea of what the dimmer in the car is, if it’s a simple rheostat or something else. Have to check my service manual and see what it says.
The dash dimmer is a rheostat, one component of the parking/headlight switch assembly.