How to Use a Hall Effect Sensor for Direction Sensing in Motors

If you want to use a hall effect sensor for direction sensing on a rotating shaft, a single digital switch won't cut it. You need a quadrature array: two linear analog Hall sensors mounted 90° apart, reading a diametrically magnetized ring. This setup outputs two sine waves phase-shifted by 90 degrees, allowing your microcontroller to calculate absolute angle, rotational speed, and exact direction of travel without the mechanical wear of a traditional potentiometer or optical encoder.

Below is the exact wiring, the raw-to-unit math, and the microcontroller code to make this work on 5V Arduinos and 3.3V ESP32 boards.

The Physics: How Linear Hall Sensors Measure Flux

When a current flows through a semiconductor and a magnetic field is applied perpendicular to that current, the Lorentz force deflects the charge carriers to one side of the material. This accumulation of charge creates a measurable transverse voltage known as the Hall voltage. In raw silicon, this voltage is in the microvolt range and highly susceptible to temperature drift.

Modern linear Hall ICs, like the Honeywell SS49E or TI DRV5055, integrate a precision amplifier and temperature compensation circuitry directly on the die. Instead of acting as a simple on/off switch, these ICs output a ratiometric analog voltage proportional to the magnetic flux density (measured in Gauss or milliTesla). At zero magnetic field, the output sits at exactly half the supply voltage (Vcc/2). A north magnetic pole pushes the voltage toward Vcc, while a south pole pulls it toward GND.

Wiring the Quadrature Hall Array

To detect direction, we treat the two sensors as the X and Y axes of a Cartesian plane. As the diametric magnet rotates, the X sensor reads the cosine of the magnetic field, and the Y sensor reads the sine.

1. Mount the Magnet: Press a diametrically magnetized ring magnet (e.g., 6mm x 2.5mm x 2mm) onto your motor shaft. Ensure it is seated flush to minimize axial runout.
2. Position Sensor X: Mount the first SS49E as close to the magnet face as possible (1-2mm air gap) without physical contact. Wire Vout to your microcontroller's Analog Pin 0.
3. Position Sensor Y: Mount the second SS49E exactly 90° mechanically from the first sensor around the magnet's circumference. Wire Vout to Analog Pin 1.
4. Power and Ground: Wire both Vcc pins to a clean, regulated 5V (or 3.3V for ESP32) rail. Tie both GND pins to the microcontroller's common ground. Do not share this ground return path with high-current motor drivers.

The Math: Converting Raw ADC to Angle and Direction

The raw ADC reading is useless on its own. We must convert it to a voltage, subtract the zero-field offset, and divide by the sensor's sensitivity to get the magnetic flux density in Gauss. Once we have the X and Y flux vectors, we use the atan2() function to find the absolute angle.

Here is the exact math for an Arduino Uno (10-bit ADC, 5V reference) using the SS49E (1.4 mV/G sensitivity):

• Voltage: V = ADC_raw * (5.0 / 1023.0)
• Zero Offset: V_offset = 2.5V (Calibrate this! See below)
• Flux (Gauss): B = (V - V_offset) / 0.0014
• Angle (Radians): theta = atan2(B_y, B_x)

To determine direction, we calculate the change in angle over time (delta theta). Because atan2 wraps around from +π to -π, we must mathematically handle the discontinuity so the microcontroller doesn't think the motor instantly spun 360° backward when crossing the zero-degree mark.

// Direction Sensing Core Logic (Arduino C++)
float prev_theta = 0.0;
int direction = 0; // 1 = CW, -1 = CCW, 0 = Stopped

void calculateDirection(float Bx, float By) {
  float current_theta = atan2(By, Bx);
  float delta_theta = current_theta - prev_theta;
  
  // Handle the +/- Pi wrap-around
  if (delta_theta > PI) {
    delta_theta -= 2 * PI;
  } else if (delta_theta < -PI) {
    delta_theta += 2 * PI;
  }
  
  // Determine direction based on delta threshold
  // Threshold prevents noise jitter from registering as movement
  if (delta_theta > 0.02) {
    direction = 1;  // Clockwise
  } else if (delta_theta < -0.02) {
    direction = -1; // Counter-Clockwise
  } else {
    direction = 0;  // Stopped or below noise floor
  }
  
  prev_theta = current_theta;
}

Calibration and Beating Magnetic Interference

Theoretical math assumes your 5V USB rail is exactly 5.00V and your sensor's internal resistor divider is perfectly centered. In reality, a cheap USB hub might output 4.8V, shifting your zero-field offset from 2.5V down to 2.4V. If you don't calibrate, your atan2 circle will be offset, causing angular distortion.

The Calibration Routine: On bootup, before the motor spins, take 100 rapid ADC samples from both X and Y sensors with the magnet in a static position (or removed entirely). Average these samples to establish your true V_offset_x and V_offset_y. Use these dynamic offsets in your math instead of the hardcoded 2.5V.

For deeper technical specifications on ratiometric Hall sensor behavior and thermal drift compensation, refer to the Texas Instruments DRV5055 Datasheet. If you are debugging ESP32 ADC non-linearity issues that affect your offset calculations, consult the official Espressif ADC Oneshot Driver Documentation.