DIY Hall Effect Tachometer: Arduino RPM & Speedometer

How Magnetic Speed Tracking Works

To track rotational speed (RPM) or linear speed (speedometer), we mount a small permanent magnet (typically a strong neodymium disc) onto the rotating component (wheel, shaft, or motor gear). A digital unipolar Hall sensor (like the A3144) is mounted nearby on a stationary bracket.

Each time the magnet completes one rotation and passes the sensor face, it triggers the sensor. This pulls the digital output pin LOW. When the magnet passes, the output floats back to HIGH via a pull-up resistor. Counting these high-to-low transitions provides the rotational rate.

Why Hardware Interrupts are Essential

Consider a shaft spinning at 3,000 RPM. This equates to 50 rotations per second. The magnet passes the sensor in just a few hundred microseconds. If the microcontroller is busy writing to an LCD or running a delay function in the main loop, it will miss the passing magnet.

Using Hardware Interrupts (e.g. attachInterrupt on Arduino pins D2 or D3) fixes this problem. An interrupt monitors the hardware pin directly. The instant the voltage drops from 5V to 0V (a Falling edge), the MCU suspends its main routine, executes a tiny Interrupt Service Routine (ISR) to increment a pulse counter, and returns to main code. This guarantees zero missed cycles.

Calculating RPM and Linear Velocity

To translate raw pulse counts into Revolutions Per Minute, we count pulses over a fixed sample period (e.g., 1000 ms) and calculate:

To calculate linear vehicle speed in km/h, compute the circumference and factor in diameter (in mm):

Tachometer Wiring & Code

Connect the A3144 Hall sensor to the Arduino: 1. **VCC:** 5V 2. **GND:** GND 3. **OUT:** Pin 2 (Interrupt Pin 0) — add a 10kΩ pull-up resistor between OUT and VCC.

// Arduino Tachometer (RPM Counter) Code
const int INTERRUPT_PIN = 2; // Interrupt D2
volatile unsigned long pulseCount = 0;
volatile unsigned long lastPulseTime = 0;
volatile unsigned long minPeriod = 2; // Software debounce: 2ms (30,000 RPM max)

unsigned long lastUpdate = 0;
const unsigned long UPDATE_INTERVAL = 1000; // 1 second update

void IRAM_ATTR countPulse() {
  unsigned long currentTime = millis();
  // Simple software debounce to filter noise/multiple triggers
  if (currentTime - lastPulseTime > minPeriod) {
    pulseCount++;
    lastPulseTime = currentTime;
  }
}

void setup() {
  Serial.begin(115200);
  pinMode(INTERRUPT_PIN, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(INTERRUPT_PIN), countPulse, FALLING);
  Serial.println("RPM Meter Initiated.");
}

void loop() {
  unsigned long now = millis();
  if (now - lastUpdate >= UPDATE_INTERVAL) {
    noInterrupts(); // Temporarily disable interrupts for atomic read
    unsigned long pulses = pulseCount;
    pulseCount = 0;
    interrupts(); // Re-enable interrupts
    
    // Calculate RPM (1 magnet per rotation)
    float rpm = (float(pulses) / 1.0) * (60000.0 / UPDATE_INTERVAL);
    
    Serial.print("RPM: ");
    Serial.println(rpm, 0);
    
    lastUpdate = now;
  }
}

Tachometers, Speed Sensors & Neodymium Magnets Sourcing in India

Sourcing A3144 Hall switches, speed sensor breakout boards, or tiny neodymium disc magnets in India is very convenient:

Frequently Asked Questions

Conclusion

Building a DIY tachometer connects physical motion, electromagnetism, and micro-controller software. By choosing the unipolar A3144 digital sensor, routing signals through hardware interrupt channels, and writing efficient pulse aggregation math, you can accurately capture and compute RPM for robotics, vehicle speedometers, and engine monitoring projects.

Ready to expand your speedometer code? Learn to output your speed parameters on a display using our OLED screen integration guide, adjust speeds in closed loop using the PID calculator guide, or read the core physics in the General Hall Effect overview.