Home > Circuit Blox > Tachometer

UPDATED:.00:41 03 January 2019

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

-Spike Generator:
Set negative polarity, VR1 to minimum

+Spike Generator:
Set positive polarity, VR1 to minimum.



The heart of the system is the Astable which acts like a clock. It provides the Timing Frame for measurement.

The output from the Astable connects to the Enable of the Counter, so it only counts when the Astable is low. This is the sample time.

The ouput from the astable also provides logic timing. It is next inverted using a transistor inverter.

This inverted signal then triggers a spike generator. It is configured to produce a low pulse on a falling input - in this case when the astable goes high (and is inverted going low). The resulting low pulse generated latches the display with the counter value.




The output from this spike generator also triggers another one configured to produce a high pulse from a rising input. This pulse resets the counter ready for the next sample count. The whole process repeats on the next cycle from the astable.

The clock pulses for the counter are produced by the slotteed opto-switch.

The display can be expanded as many times as required - 2 stages are shown above

The unit can be calibrated by adjusting the Timing Frame duration.


Timing and measurement through sampling and gating.

How some things are measured relative to time

MPH = miles per hour
RPM = revolutions per minute
Hz = cycles per second

This can be used with many inputs:

  • slotted wheels,
  • hall effect sensor
  • square wave inputs

A variety of projects could be developed with little or no modification:

  • Wind speed meter
  • Frequency meter
  • Flow rate meter
  • RPM meter
  • Speedometer