Arduino RPM Counter Using an Optical Sensor

Building an RPM (revolutions per minute) counter is a classic and useful Arduino project. This guide walks you through creating an optical tachometer using an IR LED, an IR phototransistor, and a 16×2 LCD display. The result is a simple, accurate RPM counter suitable for motors, fans, or propellers.

Project Overview

This Arduino RPM counter works by interrupting an infrared beam with a rotating object (such as a propeller). Each interruption is detected by the Arduino, counted, and converted into an RPM value that is displayed on an LCD screen.

Key features:

Real-time RPM measurement
Optical (non-contact) sensing
LCD output for easy reading
Simple and low-cost components

Parts List

You will need the following components:

1 × Arduino board
1 × 16×2 LCD display (HD44780 compatible)
1 × 10kΩ potentiometer (LCD contrast control)
1 × 10kΩ resistor
1 × IR LED
1 × IR phototransistor
Jumper wires

Wiring Instructions

Follow these steps carefully to assemble the circuit. Each subsection explains exactly where every wire should go to avoid confusion.

Power Distribution
- Connect the Arduino 5V pin to the breadboard positive rail.
- Connect the Arduino GND pin to the breadboard ground rail.
- Make sure all components (LCD, potentiometer, IR LED, and phototransistor) share this common ground.
LCD and Potentiometer Connections (16×2 Parallel LCD)
- LCD Pin 1 (VSS) → Ground
- LCD Pin 2 (VDD) → 5V
- LCD Pin 3 (VO) → Middle pin of the 10kΩ potentiometer
  - Potentiometer side pins → 5V and Ground (used to adjust LCD contrast)
- LCD Pin 4 (RS) → Arduino digital pin 7
- LCD Pin 5 (RW) → Ground (LCD set to write mode)
- LCD Pin 6 (E) → Arduino digital pin 8
- LCD Pin 11 (D4) → Arduino digital pin 9
- LCD Pin 12 (D5) → Arduino digital pin 10
- LCD Pin 13 (D6) → Arduino digital pin 11
- LCD Pin 14 (D7) → Arduino digital pin 12
- LCD Backlight
  - Pin 15 (A) → 5V through a resistor
  - Pin 16 (K) → Ground
IR LED (Transmitter)
- Anode (longer lead) → Arduino digital pin 13
- Cathode (shorter lead) → Ground
- The IR LED remains ON continuously to emit an infrared beam toward the phototransistor.
IR Phototransistor (Receiver)
- Collector (shorter lead) → Arduino digital pin 2
- Emitter (longer lead) → Ground
- Position the phototransistor directly facing the IR LED so the beam is interrupted by the rotating object.
Final Checks
- Ensure all ground connections are common.
- Double-check pin numbers before powering the circuit.
- Adjust the potentiometer until text is clearly visible on the LCD.

Tip: Digital pin 2 is used because it supports hardware interrupts, allowing the Arduino to count beam interruptions accurately and calculate RPM reliably.

Arduino Code

Upload the following sketch to your Arduino board:

/*
 * Optical Tachometer
 *
 * Uses an IR LED and IR phototransistor to implement an optical tachometer.
 * The IR LED is connected to pin 13 and runs continuously.
 * Digital pin 2 (interrupt 0) is connected to the IR detector.
 */

#include <LiquidCrystal.h>

int ledPin = 13;                // IR LED connected to digital pin 13
volatile byte rpmcount;
unsigned int rpm;
unsigned long timeold;

// Initialize the LCD with the interface pins
LiquidCrystal lcd(7, 8, 9, 10, 11, 12);

void rpm_fun() {
  // This interrupt runs every time the IR beam is cut
  rpmcount++;
}

void setup() {
  lcd.begin(16, 2);             // Initialize the LCD

  // Attach interrupt to digital pin 2 (interrupt 0)
  attachInterrupt(0, rpm_fun, FALLING);

  // Turn on IR LED
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, HIGH);

  rpmcount = 0;
  rpm = 0;
  timeold = 0;
}

void loop() {
  // Update RPM every second
  delay(1000);

  // Temporarily stop interrupts during calculation
  detachInterrupt(0);

  rpm = 30 * 1000 / (millis() - timeold) * rpmcount;
  timeold = millis();
  rpmcount = 0;

  // Display RPM on LCD
  lcd.clear();
  lcd.print("RPM=");
  lcd.print(rpm);

  // Re-enable interrupt
  attachInterrupt(0, rpm_fun, FALLING);
}

Understanding the RPM Calculation

This project assumes two interruptions per revolution, such as when using a motor with a two-blade propeller.

That’s why the RPM calculation uses this formula:

rpm = 30 * 1000 / (millis() - timeold) * rpmcount;

Adjusting for Your Setup

One interruption per revolution:
Replace 30 with 60
More blades or markings:
Divide 60 by the number of interruptions per full rotation and update the formula accordingly.

This flexibility allows you to adapt the project to different motors and rotating objects.

Final Notes

Ensure the IR LED and phototransistor are properly aligned for reliable readings.
Use reflective tape or a slotted disk for more consistent beam interruption.
This project can be extended by logging RPM data or adding serial output.

Ready to Build?

This Arduino RPM counter is a great foundation for motor control projects, robotics, and mechanical diagnostics. Assemble the components, upload the code, and start measuring RPM with confidence.