Simple Tachometer Circuit

Last Updated on March 16, 2024

To identify the Rotation speed or Revolutions per second (RPS) of rotating shaft or wheel we need tachometer, here simple tachometer circuit designed by using easily available elements slotted opto Isolator module MOC7811 and two seven segment display unit, by using this circuit we can measure spinning shaft or disc speed in RPS. This circuit is able to count RPS from 00 to 99 if you want more value then add another  decade counter stage.

About MOC7811

The MOC7811 is an opto Slotted Coupler / Interrupter Module device widely used in electronics and communication applications. This component consists of an infrared emitter and phototransistor, arranged in a configuration that enables efficient detection of objects or interruptions in the emitted infrared light. It is commonly employed in position sensing, object detection, and rotational encoding applications.

Circuit Diagram

Construction and Working 

This circuit contains IC555, MOC 7811, IC CD4081, IC CD4069 and IC 4033 and finally seven segment display unit LTS 543. At first timer IC 555 configured as mono stable multivibrator and it produce timing pulse when switch S2 pressed, the green LED1 indicates the sensing duration.

The MOC 7811 IC2 is contains IR transmitter and photodiode to produce changing logic levels depends on the blocking or interrupting IR light beam. AND logic gate N1 enables Johnson decade counter (CD 4033) and this counter drives LTS 543 seven segment display.

There are two decade counter and two seven segment display to show RPS from 00 to 99.

This illustration shows how to make a rotating interrupter, measure the slot width and depth of MOC 7811 and make the spinning shaft nose,One interruption to the IR beam by the nose will taken as one count and total count of rotation is RPS, by the way we can multiply 60 to RPS to know Revolution Per Minute (RPM).

Seven segment LED


This circuit is for hobby purpose and not suitable for industrial level measurement.

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