A Tachometer is a device which measures the speed of a rotating object like an electric motor or a crank shaft of a vehicle engine. Speed of an electric motor is determined by the number of revolutions made by the motor in one minute. In other words, speed is measured in RPM (Revolutions per Minute). Here, in this project, we designed a simple Non – Contact or Contactless Digital Tachometer using 8051 Microcontroller, which can measure speed with an accuracy of 1 rev/sec.
Principle Behind the Circuit
The basic principle behind the Contactless Digital Tachometer involves a simple embedded system with a sensor, a controller and an actuator. The sensor used here is Infrared (IR) transmitter – receiver pair, the controller used is the 8051 Microcontroller loaded with a compiled code and the actuator is a display device, for displaying the speed of the motor.
The sensor senses the speed of the motor without actually being in contact with it by the principle of light transmission and reflection and generates a signal. This signal is converted into an electric signal and fed to the microcontroller, which is programmed to calculate the speed in terms of number of motor revolutions in one minute. This speed is displayed on the 7-segment display.
Construction and Output Video
Contactless Digital Tachometer Circuit
A Tachometer is basically used to measure angular speed of a motor. It can be mechanical device with a warm gear and spindle arrangement or an electrical device which converts the angular speed into electrical signal. The electrical tachometer in turn can be an AC tachometer or a DC tachometer.
While a conventional tachometer is a contact tachometer, which can produce erroneous results due to change in contact parameters, a Contactless digital tachometer is preferred which doesn’t requires any contact with the device whose speed is to be measured.
It basically works on the principle of retro reflective scanning, wherein a light source device like LED transmits light signal to the retro reflective target device which reflects the light, which is in turn received by the light detector.
Circuit Diagram of Contactless Digital Tachometer
- 8051 Microcontroller
- 8051 Development Board
- 8051 Microcontroller Programmer
- IR Sensor Module (Reflective Type)
- 4 – Digit 7 – Segment Display
- 4 x 2N2222 NPN Transistors
- 4 x 470Ω Resistors (1/4 Watt)
- 8 x 100Ω Resistors (1/4 Watt)
- If Development Board is not used, then you need
- 11.0592 MHz Quartz Crystal
- 2 x 33pF Ceramic Capacitors
- 2 x 10 KΩ Resistor (1/4 Watt)
- 10 µF Capacitor (Polarized)
- Push Button
- 1 KΩ x 8 Resistor Pack
How to Design Contactless Digital Tachometer?
Sensor Circuit Design
The sensor circuit consists of an IR transmitter and an IR receiver. An IR LED is used as the transmitter and a photo diode is used as the receiver. A reflective type of IR sensor is used in this project. In this type, the IR transmitter and receiver are placed side -by – side.
The IR transmitter circuit is very simple. The anode of the IR transmitter is connected to 5V supply and the cathode is connected to ground through a current limiting resistor of 150Ω. Thus, the IR transmitter starts emitting infrared rays.
IR receiver used in the project is a photo diode and it must be connected in reverse bias. The negative terminal or cathode is connected to 5V supply and the positive terminal or anode is connected to ground through a current limiting resistor of 10KΩ.
And finally, the output of the IR receiver is given to the comparator. The comparator compares the input from the IR receiver with a reference value (which is given through a 10KΩ Potentiometer). If the input from the IR receiver is greater than the reference value, the output of the comparator will be HIGH or else, the output will be LOW.
Controller Circuit Design
The controller circuit consists of the microcontroller and its interfaces. Here, we chose AT89C52 microcontroller. Microcontroller reset circuit consists of a push button, a resistor and a capacitor with values such that the voltage at the reset pin doesn’t falls below 1.2V and the timing constant is never less than 100ms. Here, we selected a 10 KΩ resistor and 10µF capacitor.
Since the oscillating frequency of the microcontroller is 11.0592MHz, we selected the values of ceramic capacitors to be 33pF. The EA’ pin must be pulled high via a 10KΩ resistor as we are not using any external memory.
Read more: contactless tachometer circuit with code