So now that I have the LED portion figured out I need to figure out how to interpret the speed signal from my car. It's a sine wave that grows in frequency and amplitude with speed. I want to take this and depending on the vehicle's speed have the lights do certain things. How would I go about interfacing with this signal? :-/
DH,
There may be a few ways to do this.
one you could ignore the voltage and just read the frequency, or the other ignore the frequence and read the voltage.
Reading the Frequency: You could pass the the signal through a diode which will then give you half of that signal.
Pretty much giving you a clock signal that the arduino can read. From there, I would create a look up table for the speeds. This of course will take some experimentation and calibration.
Reading the voltage. It has been a while since I fussed with electronics (until recently again) But here I think you would use a bridge rectifier to convert the AC signal to a DC one. Though I am not sure how you would clip the signal. And here too you would probably need to create a look up table.
Not sure if this helps, but may give you a direction to look in... or maybe a direction to look away from.http://www.arduino.cc/yabbfiles/Templates/Forum/default/wink.gif
Cheers
ñ
if the amplitude is changing, and the speed is an AC signal, a variant of this circuit should give you a varying DC voltage that can be read by an Arduino analog input. Wire it up as shown, put a voltmeter across the capacitor, and then fiddle with the values to get the response you need. You might have to reduce R2 if the capacitor doesn't discharge fast enough. Drive safely.
Google "zero crossing detector". Here's one example: Zero Crossing Detector Packet Modem
feed the output of the op-amp into a digital pin, and time it, e.g. with the pulseIn() function.
This project http://www.knology.net/~gdion/whereavr.html uses a zero crossing detector, too. It's on an ATmega8 (although not arduino) and uses fewer components but more features of the AVR. The bottom left corner http://www.knology.net/~gdion/images/ax25sch.gif labeled "SPK IN" is the AC (audio, in this case) input.
-j
So what happened to your vehicle speed sensor project? I'm quite interested to it, I really hope you have a DIY for it. Thanks again!. 
Anyways I have already found some DIY in some auto books for my vehicle speed sensor project. I will scan it and hopefully send or post it here.
Did you make any progress on this so far. I like the idea of reading voltage since it seems as if Arduino would better interpret this. Then I would create a calibration using the analog to digital to find the digital voltage coming out of the thing and calibrate it. Kinda hard to explain but message me and I will be gladly to help since I have some code up! 