| ///M3 smg style shift light 1/2 way done!
I've been working on this for hte past week and finally finished the schematic for it. I have this posted on NAM, thought I'll share with you guys here and see if I can get any inputs from you guys and maybe someone to check my work and tidy up the design a bit.
I'll basically post what I posted there to save me some time...
Ok finally finished w/ the schematic. It is a basic design as any other LED tachs out there with just some adjustments to the resistance and capacitance values. I don't understand the freq to voltage chip fully, so I won't fully know what will happen until I build a prototype and test it out.
To explain the schematic...*deep breath*
The chip on the left (LM2917) does the frequency to voltage conversion. Main things you have to worry about on the chip are R1, and C1. If you change them, the output voltages will change linearly. For example, if you increase R1 and C1, the output voltage will increase in small increments. You don't want to make these values very big because you'll get inaccuracies in the conversion (I'm still not too sure what the threshold is). R1 is a variable resistor to fine tune the output voltage to make sure the LEDs light up when they should.
I have set C1 and R1 to match the output voltage. The frequency for the lower end of the threshold is 213Hz @6.4kRPM (when the LEDs will start lighting), and for the upper end its 230Hz @ 6.9kRPM (when ALL LEDs should be lit). Using Vout=Vcc x C1 x R1 x Fin. Vcc is 12.4V(batt voltage) and Vout I set at 12.4V for 6.9kRPM and 11.49V for 6.4kRPM. The reason why I made the output voltages so high is because I needed the biggest voltage difference between the two freqencies as possible. 17Hz difference isn't alot to work with so I needed to expand the voltage as big as I can for such a narrow frequency range. I don't know how else to increase the difference so I just increased the Vout at 6.9krpm. This gives me 0.90V to work with which HOPEFULLY will be enough to fully distinguish each 10 LEDs. The problem that might arise from such a low voltage difference between the upper and lower RPMs is that the chip loses accuracy and might start "bleeding" current into other LEDs. For example, when LED2 should be lit, LED3 might glow faintly. Hopefully it'll work though :\
The output voltage comes from pin10, which ends up being the input to the chip on the right (LM3914). This side is fairly simple, as the only important part is resistor 7 which controls the current to the LEDs therefore controls their brightness. Again I put a variable resistor for easy adjustment. Pin 9 is an another adjustment someone might want to fiddle with. This controls whether the LEDs light up in bar mode (what I have it set to) or dot mode. I'm not exactly sure what the dot mode is, but it's very easy to just put in a single throw switch to have the pin be open for dot mode. For bar mode just put it directly to Vcc.
So there ya go. I'm going to fiddle w/ the capacitors and resistors a bit to get them to match the values that are available, as right now I have some weird #s. Or just make them all variable resistors and just fiddle w/ it to get what I need. I'm going to try the circuit on a breadboard by next week and hopefully it'll work. Any inputs and anyone who can clean up the circuit would be appreciated.
Sorry the schematic is such poor quality. I tried to upload my original but it was too big (exceeded 800x800), PM me if you want the original, and there is a small error in it also. Pin 11 and pin 9 on LM2917 shouldn't be connected. That dot on pin 9 by the 470 Ohm resistor is for the resistor and not the line going from pin 11 to ground.
OK to add some details that was not mentioned. The signal I'm using is from the rpm signal that goes to the OBD II plug. It is a digital signal at about 13.5V and frequency is 33.333Hz per 1000RPM. This is same frequency as i'd get if I use the coil, but digitized which is why I don't have any filters on the signal input side.
Car is only good as the driver. | 
well what you said then with ehh??? 
