Overview

The A3144 (also sold under part numbers 3144, A3144E, A3144L) is a popular unipolar digital Hall effect switch IC manufactured by Allegro MicroSystems and generic semiconductor vendors. Integrated into a 3-pin TO-92 package or mounted on a 3-pin / 4-pin breakout board, it detects the presence of a magnetic field and switches its output state digitally.

Operating as an active-low unipolar switch, the A3144 turns ON (pulling its open-collector output to GND) when exposed to a sufficiently strong South magnetic pole perpendicular to its branded face. When the magnetic field is removed, the output turns OFF (returning to High impedance). It is universally used for motor RPM tachometers, wheel speed sensing, magnetic door proximity switches, and contactless limit switches.

Quick reference

Pinout (TO-92 Package & Breakout Board)

TO-92 Package (Front Branded Face)

        ┌─────────┐
        │  A3144  │
        └────┬────┘
             │ │ │
             1 2 3

Breakout Module Header (3-Pin Header)

Specifications

Operating Principle & Magnetic Orientation

• No Magnetic Field: Output transitors are OFF; the OUT pin floats High via the pull-up resistor ($V_{OUT} = V_{CC}$).
• South Magnetic Pole Brought Near Branded Face: When magnetic flux density exceeds $B_{OP}$, the NPN transistor conducts, pulling OUT Low ($V_{OUT} \approx 0.175\text{ V}$).
• North Magnetic Pole: The unipolar A3144 ignores North magnetic fields.
• Hysteresis: Built-in magnetic hysteresis ($B_{HYS} \approx 55\text{ Gauss}$) prevents chatter and false triggering when magnets pass near the threshold distance.

Wiring

Example (RPM Tachometer Counter)

const int hallPin = 2; // Interrupt 0 on Arduino Uno
volatile unsigned int pulseCount = 0;

void countPulse() {
  pulseCount++;
}

void setup() {
  Serial.begin(9600);
  pinMode(hallPin, INPUT_PULLUP); // Enable internal pull-up
  attachInterrupt(digitalPinToInterrupt(hallPin), countPulse, FALLING);
}

void loop() {
  pulseCount = 0;
  delay(1000); // Measure pulses in 1 second
  
  // Calculate RPM assuming 1 magnet on rotating shaft
  unsigned int rpm = pulseCount * 60;

  Serial.print("Pulse Count: "); Serial.print(pulseCount);
  Serial.print(" | Motor Speed: "); Serial.print(rpm); Serial.println(" RPM");
}

Common mistakes

• Forgetting the open-collector pull-up resistor: Raw TO-92 A3144 ICs have open-collector outputs. Leaving OUT without a $10\text{ k}\Omega$ pull-up resistor (or enabling INPUT_PULLUP in software) results in floating readings that never return High.
• Presenting the North pole of the magnet: The A3144 is unipolar and only responds to South magnetic poles presented to the front face. Flip the magnet over if no pulses are detected.

Notes

• A3144 vs US1881: A3144 is a unipolar switch (turns ON with South, turns OFF when magnet leaves); US1881 is a bipolar latching sensor (turns ON with South, stays ON until a North pole arrives).