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 or other machine.
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 measuring.
The slotted switch can be replaced with any sensor or device that produces pulses up to 1MHz.
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.