I'm working on a module that will change the color of the guage depending upon where it is,

Ex: from blue to red when the tach hits redline, when temp guage is out of norm, and when low on fuel (also when speedo is over x mph).

Now I have the Schematic drawn up on paper, the only problem with that is I was wondering if the input to the tach and speed is a voltage or a square wave. I know the output of the speed sensor is a square wave, and I know the output from the ignition is a square wave, but is it a square wave all the way to the guages? or does it get converted to a voltage?

I'd take apart my dash and try and find out, but right now it's the only car I can drive and I don't have too much time to play.

Thanks!