A tachometer (revolution-counter, tach, rev-counter, RPM gauge) is an instrument measuring the rotation speed of a shaft or disk, as in a motor.
The circuit below senses the slots in a slotted disc rotating on a shaft breaking the light beam in the Opto Sensor. This can be anything that rotates and needs the speed measuring.
The slotted switch can be replaced with any sensor or device that produces fast clean pulses.
Set Low Range, (0.7-18Hz). Use an oscilloscope to set the "space" time to 0.6 second
Set negative polarity, VR1 to minimum
Set positive polarity, VR1 to minimum.