Hey so I am going to do a project where I am going to take the car badge on my trunk and put some diffuser material around it and then I am going to run sum rgb LED's to light it up. I was planning on using the pro mini board to do this. In the specs it says it can take 12V and through some digging people brought up a good point of the temperature increase but no one every says if they had problems with it or not. Since i would be using only LED's I was going to get the 3.3V version. But that's a lot of volts turned into heat. So my question is has anyone used the 3.3V pro mini with a 12V source, and if so did you have any temperature problems? ALso if any one has a way that would be better than this i am open to ideas.
Thanks
i don't have any specific information to your question. I do want to throw in that when your car is running the voltage is likely ~14.5 VDC. Just wanted to make sure you were compensating for that in your design.
I was not. I was going to tap into the wire that goes to the light for my license plate and the bulbs in those are rated at 12V. So is that true for all those lines too.
Your car battery outputs between 11 and 12.5 volts when the car is not running.
In order for the alternator to charge the battery and run the electronics in the car it outputs a slightly higher voltage. So any time your engine is running, any lines that have 12V from the battery will likely read about 14 volts. Quick way to tell is with a voltmeter. Red lead to one of the bulb wires (when the bulb is on) and black lead to somewhere that has continuity to the frame of the vehicle.
Most automotive devices have a range from ~10 volts to ~16 volts for this reason.
I have not had any experience with the pro mini but i ran a Mega1280 to control some led's in my truck for a long while. I ended up using an old car power adapter that output 9Vdc and just used it as an input supply for the Mega to make sure i didn't have a heat issue on the board. Worked great until i scavenged the board for other project 
I will be damned, you weren't wrong. That light was putting out about 13.6V. So i guess that means i need to step it down before the arduino.
and that's at idle. Step on the pedal a bit and it will ramp slightly. Might still be doable.... but just didn't want it to catch you off guard.
Not only do you have to deal with slightly higher voltages, but you'll also have to deal with fluctuations in voltages, as well as potential spikes; granted, these will likely be smaller at the rear of the auto vs the front (well, assuming your engine is at the front of the vehicle). You might want to read the following Atmel app notes for such applications:
http://www.atmel.com/Images/doc9108.pdf (*)
The one app note marked (*) is done so because Atmel noted it as "mature" and "not recommended for new designs"; it is probably still worth a look - I don't know about the other two, but you might want to review Atmel's documentation section - they have a lot of good stuff in there that may help you on your project.