GlitchBoy:
If you haven't yet, you should read up on the LM1815 chip as it is designed specifically for what you're doing.From the LM1815 datasheet:
The LM1815 is an adaptive sense amplifier and default gating
circuit for motor control applications. The sense amplifier
provides a one-shot pulse output whose leading edge
coincides with the negative-going zero crossing of a ground
referenced input signal such as from a variable reluctance
magnetic pick-up coil.
When i messure signal in a car i use 74HC14 HEX SCHMITT-TRIGGER inverters chip to get a square great signal.
But from the PC blower i have a nice 100% squre signal not missing a beep at looking at the scoope ?
But suddently the Interupt pin fires i between signals giving me a readout of way to high RPM