From Zero to Hero: Masterclass on How to Make a Tachometer Using Arduino

Are you looking for a way to measure the speed of a rotating object using your Arduino? Building a tachometer with an Arduino is a fun and rewarding project that can be applied to various applications, from measuring engine speed to monitoring the rotation of a motor. In this blog post, we’ll guide you through the steps involved in creating your own Arduino tachometer, explaining the concepts, components, and code required.

Understanding Tachometers and RPM

A tachometer is a device used to measure the rotational speed of an object, typically expressed in revolutions per minute (RPM). It’s a crucial tool in various fields, including automotive, industrial, and hobbyist applications.

Components for Your Arduino Tachometer

Before we dive into the code and circuitry, let’s gather the necessary components:

• Arduino Board: Any Arduino board will work, but a Uno or Mega is recommended for their ease of use and availability.
• Rotary Encoder: A rotary encoder is a sensor that outputs a series of pulses proportional to the rotation of a shaft. It’s the heart of our tachometer, providing the input signal for the Arduino.
• LCD Display (Optional): An LCD display will allow you to visualize the RPM readings clearly.
• Resistors: A few resistors will be needed to connect the rotary encoder to the Arduino.
• Jumper Wires: These are used to connect components together.
• Breadboard (Optional): A breadboard is useful for prototyping and experimenting with the circuit.

Choosing the Right Rotary Encoder

Rotary encoders come in various types, but for our Arduino tachometer, we’ll focus on two common types:

• Incremental Encoder: This type of encoder generates a pulse train with a specific number of pulses per revolution. The Arduino can count these pulses to determine the RPM.
• Absolute Encoder: This encoder provides a unique digital code for each rotational position. It offers more precise positioning but is typically more expensive.

For our project, an incremental encoder is sufficient. You can find various rotary encoders online, with different resolutions (pulses per revolution) and specifications.

Connecting the Rotary Encoder to the Arduino

The rotary encoder has three pins:

• A Pin: This pin outputs a pulse signal with each rotation step.
• B Pin: This pin outputs a second pulse signal, often 90 degrees out of phase with the A pin, allowing for direction detection.
• VCC Pin: This pin connects to the positive voltage supply (5V).
• GND Pin: This pin connects to the ground.

Connect the A and B pins of the rotary encoder to two digital input pins on your Arduino. Connect the VCC pin to the 5V pin on the Arduino and the GND pin to the ground pin.

Arduino Code for Tachometer

Now, let’s write the Arduino code to read the pulses from the rotary encoder and calculate the RPM. Here’s a basic example:
“`c++
const int encoderA = 2; // Digital pin for encoder A
const int encoderB = 3; // Digital pin for encoder B
volatile long encoderCount = 0; // Variable to store the pulse count
unsigned long lastTime = 0; // Variable to store the last time the count was updated
void setup() {
pinMode(encoderA, INPUT_PULLUP);
pinMode(encoderB, INPUT_PULLUP);
Serial.begin(9600);
}
void loop() {
unsigned long currentTime = millis();
if (currentTime – lastTime >= 1000) { // Update every second
float rpm = (encoderCount * 60.0) / (currentTime – lastTime);
Serial.print(“RPM: “);
Serial.println(rpm);
encoderCount = 0;
lastTime = currentTime;
}
}
// Interrupt routine to increment the encoder count
void encoderISR() {
// Determine the direction of rotation based on the state of the A and B pins
if (digitalRead(encoderA) == LOW && digitalRead(encoderB) == HIGH) {
encoderCount++;
} else if (digitalRead(encoderA) == HIGH && digitalRead(encoderB) == LOW) {
encoderCount–;
}
}
// Attach interrupt routines to the encoder pins
void setup() {
// … (previous setup code)
attachInterrupt(digitalPinToInterrupt(encoderA), encoderISR, CHANGE);
attachInterrupt(digitalPinToInterrupt(encoderB), encoderISR, CHANGE);
}
“`
Explanation of the Code:
1. Pin Definitions: Define the digital pins used for the encoder’s A and B signals.
2. Variables: Declare variables to store the encoder count, the last time the count was updated, and the calculated RPM.
3. Setup: Set the encoder pins as input with pull-up resistors and initialize the serial communication.
4. Loop: This function runs continuously and calculates the RPM every second.
5. RPM Calculation: The RPM is calculated by multiplying the encoder count by 60 (seconds per minute) and dividing by the time elapsed since the last count update.
6. Interrupt Routine: The `encoderISR` function is called every time there’s a change in the state of either encoder pin. It increments or decrements the `encoderCount` based on the direction of rotation.
7. Attach Interrupts: The `attachInterrupt` function connects the `encoderISR` to the encoder pins, triggering the routine whenever there’s a change in the input signal.

Displaying the RPM Readings

To display the RPM readings, you can use an LCD display or simply print the values to the serial monitor.
Using an LCD Display:
1. Connect the LCD: Connect the LCD to the Arduino using the appropriate pins. You’ll need to use a library to control the LCD (e.g., LiquidCrystal).
2. Display the RPM: Modify the `loop()` function to display the RPM on the LCD using the library functions.
Using the Serial Monitor:
1. Open the Serial Monitor: Open the Serial Monitor in the Arduino IDE.
2. Print the RPM: The `Serial.println(rpm);` line in the `loop()` function will print the RPM values to the Serial Monitor.

Calibration and Testing

Once you have the code running, you need to calibrate and test the tachometer.
1. Calibration: Rotate the shaft connected to the rotary encoder and observe the RPM readings on the display or Serial Monitor. You may need to adjust the code’s calculation if the readings are inaccurate.
2. Testing: Test the tachometer with different speeds of rotation to ensure accurate readings across a range.

Applications of Arduino Tachometer

Here are some practical applications of an Arduino tachometer:

• Engine Speed Measurement: Monitor the RPM of an engine to diagnose potential issues or optimize performance.
• Motor Speed Control: Use the tachometer feedback to control the speed of a DC motor or other rotating equipment.
• Robotics: Measure the speed of robotic joints or wheels for precise control.
• Hobby Projects: Build a tachometer for your RC car, model airplane, or other hobby projects.

Final Thoughts: Your Arduino Tachometer is Ready!

Building an Arduino tachometer is a great way to learn about electronics, sensors, and programming. By following the steps outlined in this blog post, you can create a versatile tool for measuring RPM in various applications. Remember to experiment with different rotary encoders, displays, and code variations to customize your tachometer to your specific needs.

Information You Need to Know

Q: What kind of rotary encoder should I use for my project?
A: The best rotary encoder for your project depends on the specific requirements. Consider the resolution (pulses per revolution), the operating voltage, and the shaft size. For basic applications, a low-resolution incremental encoder is usually sufficient.
Q: How do I determine the direction of rotation with a rotary encoder?
A: You can determine the direction of rotation by analyzing the phase difference between the A and B signals from the encoder. If the A signal leads the B signal, the shaft is rotating in one direction. If the B signal leads the A signal, the shaft is rotating in the opposite direction.
Q: Can I use a tachometer to measure the speed of a linear motion?
A: While a tachometer is designed to measure rotational speed, you can use it indirectly to measure linear speed. You can attach a wheel with a known circumference to the object undergoing linear motion. The RPM of the wheel will be proportional to the linear speed of the object.
Q: What are some common errors to watch out for when building a tachometer?
A: Some common errors include incorrect wiring, faulty components, and code errors. It’s essential to double-check your connections, ensure all components are working correctly, and carefully review your code for any logical errors.
Q: How can I improve the accuracy of my tachometer?
A: To improve accuracy, use a high-resolution rotary encoder, calibrate the tachometer carefully, and consider using a timer interrupt to measure the time between pulses more precisely.