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  • Digital Tachometer Using Arduino

  • Created: Oct 01, 2014

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Summary

Tachometer is a dedicated device that measures the number of revolutions of an object in a given interval of time such as the engine shaft in a car. The device reveals the revolutions per minute (RPM) performed by the object with the support of Arduino and a 2x16 LCD screen. The Arduino Uno is a microcontroller board based on the ATmega328. It has 14 digital input/output pins, 6 analog inputs, and a 16MHz ceramic resonator used for counting the RPM. This project can be practically used in various types of equipment for counting, speed monitoring and in process control applications.


An IR photo transistor and IR LED formed the sensor. The use of IR phototransistor avoids other light interferences from the environment. Resistor R2 limits the current through the IR diode and a reflective strip is glued on the rotating object (shaft, disc or fan) in line with the sensor. When the reflective strip passes in front of the sensor, IR waves are reflected back to the photo transistor. RPM can be determined by counting the number of upward shoots in a given interval of time. In addition, the Arduino is used for counting the RPM where the emitter of the photo transistor is connected to the Interrupt 0 (digital pin 2). The arduino interrupt is configured to be rising edge triggered and the result will be an interrupt for every upward shoot in the emitter waveform. The number of interrupts occurred in a given time is counted by incrementing a variable using the interrupt service routine. The time elapsed during the counting cycle is determined using the millis() function which returns the number of milli seconds passed since the arduino board is switched ON. Calling the millis() function before and after the counting cycle and the taking their difference gives the times passed during the counting cycle. The (number of interrupts/time in milliseconds)*60000 will give the revolutions per minute (RPM). Moreover, a provision for controlling the motor speed using a potentiometer is also included in the circuit. Transistor Q1 is used for driving the motor. Its base is connected to pwm pin 9 of the arduino through the current limiting resistor R1 while the wiper of the speed control POT R4 is connected to analog pin A0 of the arduino. The voltage at this pin is converted into a value between 0 and 1023 using the analog Read function. The result will be a square wave at pin 9 whose duty cycle is proportional to the value written using the analogWrite function. The rpm and duty cycle are displayed on the LCD screen using the standard LiquidCrystal library.


Generally, the tachometer has a simple interface that can be integrated with an existing industrial motor or system. This can be used for various types of equipment in a number of settings, such as conveyors, windmills, rotary feeders, grinders, dryers, cooling equipment, augers and elevators. Some common industries that use these devices include power plants, recycling facilities, chemical plants, automotive plants, material handling, food/beverage facilities, paper mills and many others. The device can also help prevent damage to equipment as well as reduce loss of material.

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