The tach signal (IGN from the diagnostic box under the bonnet, or on the back of the instrument panel) is a square wave, and the period of the high level of the pulse is inversely proportional to RPM. I took my PCB, and added a few extra components to clean up the signal. I added a large smoothing cap to the power supply (too large, but it does the job for testing purposes), an optoisolator to safely consume the signal, and a schmitt trigger to clean it up nicely.
I then added 50RPM hysteresis to the code, so that it doesn't flicker the LEDs, and it works very well!
Of course, all the added hardware means it doesn't fit on the PCB any more, so there are a couple of extra breakout PCBs added. I've almost completed the design of a new PCB which will have all of the extra components. I'll then get a housing 3d printed up and it should be sweet
Here, you can see the shape of the original PCB, it matches the size and mounting holes of the sunvisor mounting. I'll run the cables up inside the A pillar trim;
Shiftlight by CaffeineAU, on Flickr
This is the layout of the LEDs, for testing I have them come on at the following RPM;
1000,1100,1200,1400,1600,1800,2000,2200
Shiftlight by CaffeineAU, on Flickr
