Digital Tachometer Circuit Operation using Microcontroller and their Types

A digital tachometer is a digital device that measures and indicates the speed of a rotating object. A rotating object may be a bike tyre, a car tyre or a ceiling fan, or any other motor, and so on. A digital tachometer circuit comprises LCD or LED read out and a memory for storage. Digital tachometers are more common these days and they provide numerical readings instead of dials and needles.

Digital Tachometer
Digital Tachometer

Digital tachometer is an optical encoder that determines the angular velocity of a rotating shaft or motor. Digital tachometers are used in different applications such as automobiles, aeroplanes, and medical and instrumentation applications.

What is a Tachometer?

The word tachometer is derived from two Greek words: tachos means “speed” and metron means “to measure”. It works on the principle of a tachometer generator, which means when a motor is operated as a generator, it produces the voltage according to the velocity of the shaft. It is also known as revolution-counter, and its operating principle can be electromagnetic, electronic or optical-based. Power, accuracy, RPM range, measurements and display are the specifications of a tachometer. Tachometers can be analog or digital indicating meters; however, this article focuses only on the digital tachometers.

Digital Tachometer Types

Digital tachometers are classified into four types based on the data acquisition and measurement techniques.

  • Based on the data acquisition technique, the tachometers are of the following types:
  1. Contact type
  2. Non Contact type
  • Based on the measurement technique, the tachometers are of the following types:
  1. Time measurement
  2. Frequency measurement

1. Contact Type Digital Tachometer

Contact type digital tachometer
Contact type digital tachometer

A tachometer which is in contact with the rotating shaft is known as contacttype tachometer. This kind of tachometer is generally fixed to the machine or electric motor. An optical encoder or magnetic sensor can also be attached to this so that it measures its RPM.

Digital Tachometers are capable of measuring low-speeds at 0.5 rpm and high speed at 10,000 rpm and are equipped with a storage pocket for the circumferential measurement. The specifications of this tachometer are LCD 5 digit display, operational temperature range of 0 to + 40oC, temperature storage range of – 20 to + 55o C and rotating speed of about 0.5 to 10,000 rpm.

2. Non-Contact Type Digital Tachometer

Non Contact type Digital Tachometer
Non Contact type Digital Tachometer

A tachometer that does not need any physical contact with the rotating shaft is called as noncontact digital tachometer. In this type, a laser or an optical disk is attached to the rotating shaft, and it can be read by an IR beam or laser, which is directed by the tachometer.

This type of tachometer can measure from 1 to 99,999 rpm; the measurement angle is less than 120 degrees, and the tachometer has a five-digit LCD display. These types of tachometers are efficient, durable, accurate, and compact, and also visible from long distance.

3. Time Measurement Digital Tachometer

A tachometer that calculates the speed by measuring the time interval between incoming pulses is known as time-based digital tachometer. The resolution of this tachometer is independent of the speed of the measurement, and it is more accurate for measuring low speed.

4. Frequency Measurement Digital Tachometer

A tachometer that calculates the speed by measuring the frequency of the pulses is called as frequency-based digital tachometer. This type of tachometer is designed by using a red LED, and the revolution of this tachometer depends on the rotating shaft, and it is more accurate for measuring high speed. These tachometers are of low-cost and high-efficiency, which is in between 1Hz-12 KHz.

Internal operation of these tachometers can be with the use of tachometer generator or purely with the electronic components that are described below.

Tachometer Generator

A micro-electric machine that is used to convert, the rotating speed and the shaft values of a machine into an electric signal is known as tachometer generator. The operation of the tachometer generator is based on the principle that the angular velocity of rotor is proportional to the generated EMF if the excitation flux is constant.

Tachometer Generator
Tachometer Generator

These tachometers are specified with generated voltage, accuracy, maximum speed, ripples and operating temperature. This kind of tachometer generators are used as sensors in various automobile and electro-mechanical computer devices.
The generators can be AC or DC types.

Electronic Tachometer

A tachometer made purely from electronic components and is used to measure the speed of an engine or any other moving object in revolutions per minute is known as an electronic tachometer. Electronic tachometers are used in the dashboard of a car for measuring the driving speed. These tachometers are light weight, easy to view and accurate under all conditions.

Electronic Tachometer
Electronic Tachometer

Block Diagram of Digital Tachometer

The operational set up of a digital tachometer consists of various blocks such as an optical or magnetic sensor, a signal conditioning unit, a microcontroller, a memory, a display and an external port, as shown in the figure.

Block Diagram of Digital Tachometer
Block Diagram of Digital Tachometer

Optical sensing: An optical sensor consists of an optical disk placed near the motor which generate pulses proportional to the rotating shaft. A slotted disk and IR emitter are used to generate these pulses.

Magnetic sensing: In this type of sensing, there is a possibility to use either Hall Effect sensors or magnetic sensors. Hall Effect principle generates the pulses proportional to the speed of the shaft and magnetic sensors are used to generate pulses by making use of variable reluctance.

Signal Conditioning: The output signals from the sensors are noisy, and therefore, are filtered, amplified and digitized so that the microcontroller recognizes these signals for further action.

Microcontroller: A microcontroller is used to analyze and process the readings from the sensors. It sends that information to a display device, and when the speed is reduced or increased to a predefined level, it alerts the user by taking appropriate action.

Memory: The memory unit stores the data from the microcontroller.

Display Unit: The function of the display unit is to view the stored values transmitted from the microcontroller.

Contactless Digital Tachometer by Using 8051

This non-contact tachometer is implemented to design a three-digit contact-less digital tachometer by using 8051 microcontroller which can be used for measuring the revolutions of a wheel, disc, shaft etc.

Digital Tachometer Circuit using 8051
Digital Tachometer Circuit using 8051

This circuit uses various components, such as microcontroller, photo transistors, op-amps, seven segments LED display, and other miscellaneous components. In addition to this, a sensor is placed near the reflective strip – for instance, an aluminum foil that is fixed on to the rotating surface. The LED directed from this device gets reflected as the strip is detected by the photo transistor.

Op-amp LM 324, as a comparator, compares this transistor collector’s voltage with the fixed voltage. Therefore, it generates continuous pulses for the rotation of the shaft. These trains of pulses are applied to the microcontroller, which then counts them and converts them into RPM as programmed. Furthermore, they are displayed in a seven-segment display, which is connected in the transistor-driven common anode configuration.

This is all about the digital tachometer circuit and its types. Hope you have got wholesome information about digital tachometers on a single platform. Furthermore, any queries about this topic and about designing the circuit, you can write to us by commenting in the comment section.

Photo credits

2 Comments

  1. Alan Michael says:

    Hello there, I’m trying to make a tachometer on my own. I’m going to use the circuit diagram on your website, but before I start, I wanted to know about the range of this tachometer.What is the maximum range of this tachometer to give the correct speed measurement value?

Add Comment