sdturner:
I question the tachometer circuit. It's output will be a signal between 0 and 15V, either a pulse width modulated signal or an analog voltage (depending on filtering). I think the VR signal conditioner on the front end needs some protection and it may not work at slow speeds. I don't understand what the block labeled "charge pump" does unless it is a switched cap amplifier to isolate the output from the input.
That "Charge pump" is internal of LM2917
Ok Here my try
I'm adopting http://www.ti.com/lit/ds/symlink/lm2907-n.pdf Page 10.
It say 66 Hz / Volt ... thats how my max Volt will be very High at first.
Then I try to put Voltage devider to the output ... and feed the devided voltage to arduino
OOpppsss ... My apologize ... I don't know the code to shrink down the image for display
R3 and R4 are the voltage devider.
MPU is : Magnetic PickUp
TACH Pin is the one that connected to Arduino Analog Pin.
Whith 132 tooth wheel the freq/RPM is : 2.2 Hz/RPM
And with that Volt devider, Hopefully the Volt/RPM is arround : 0.00105922002889 Volt
Sincerely
-bino-