If you put the tachogenerator signal through a series resistor through a pair of back-to-back diodes you'llget a nice 1.2V peak to peak signal whose voltage is relatively insensitive to the speed. Try 10k or so.
This solution will give a ramp from zero volts up to the clip level. How will an Arduino react to this on a digital pin?
1. What type of diode for the clipping? Do diodes come in different sizes as do resistors and caps? (sorry...I am very new...)
2. To make a ST do I just buy a ST chip and add the resistors to make my own complete unit? There has been mention of starting with an op-amp but I have only found bare bones chips not nice complete boards. Any links to parts gratefully accepted lol