Stepper Motors

A stepper motor moves in discrete steps rather than a continuous sweep. Each pulse to the driver advances the motor by one step, so position is tracked by counting steps — no position feedback sensor is needed for open-loop control.

The 28BYJ-48 is the most common starter stepper motor. It runs on 5 V, has an internal gear reduction of ~64:1, and comes bundled with a ULN2003A driver board that accepts 3.3 V logic directly from the ESP32.

// 28BYJ-48 with ULN2003 — Library Manager → "AccelStepper" by Mike McCauley
#include <AccelStepper.h>

// HALF4WIRE (8-step) mode: IN1=23, IN3=21, IN2=22, IN4=19
// Note: AccelStepper expects (pin1, pin2, pin3, pin4) = (IN1, IN3, IN2, IN4)
AccelStepper stepper(AccelStepper::HALF4WIRE, 23, 21, 22, 19);

// 28BYJ-48: 64 electrical steps × gear ratio ≈ 64 = 4096 steps/rev (half-step)
const int STEPS_PER_REV = 4096;

void setup() {
    stepper.setMaxSpeed(1000);       // steps/second
    stepper.setAcceleration(500);    // steps/second²
}

void loop() {
    stepper.moveTo(STEPS_PER_REV);   // one full clockwise revolution
    while (stepper.distanceToGo() != 0) {
        stepper.run();
    }
    delay(500);
    stepper.moveTo(0);               // return to home position
    while (stepper.distanceToGo() != 0) {
        stepper.run();
    }
    delay(500);
}

NEMA 17 is the industry-standard bipolar stepper motor for 3D printers and CNC machines. It delivers significantly more torque than the 28BYJ-48 and supports up to 1/16 microstepping for smooth motion. The A4988 driver requires a separate high-voltage motor supply (8–35 V) and a 3.3 V logic supply.

// NEMA 17 with A4988 driver — STEP / DIR interface
#include <AccelStepper.h>

const int STEP_PIN = 23;
const int DIR_PIN  = 22;
const int EN_PIN   = 21;  // active LOW — pull LOW to enable

AccelStepper stepper(AccelStepper::DRIVER, STEP_PIN, DIR_PIN);

// NEMA 17 standard: 200 full steps/rev (1.8° per step)
// With 1/16 microstepping on A4988: 3200 steps/rev
const int STEPS_PER_REV = 200;

void setup() {
    pinMode(EN_PIN, OUTPUT);
    digitalWrite(EN_PIN, LOW);       // enable the driver

    stepper.setMaxSpeed(800);
    stepper.setAcceleration(400);
}

void loop() {
    stepper.moveTo(STEPS_PER_REV);
    while (stepper.distanceToGo() != 0) { stepper.run(); }
    delay(500);
    stepper.moveTo(0);
    while (stepper.distanceToGo() != 0) { stepper.run(); }
    delay(500);
}

A gauge stepper with a stop, such as the X27.168, finds its zero by driving onto that stop at every power-up. A compass card, a heading or ADF card, or an altimeter hand turns all the way round and has no stop, so it uses a stepper without one (an X27.589, for example). In CockpitConnect, set the Gauge Stepper's rotation to Continuous and wire an Index Sensor that the card passes once per turn. At every power-up the controller turns the card forward until the sensor triggers, then takes the shorter way round to the simulator's value (359° → 1° is 2° forward). If the sensor hasn't triggered within 1¼ turns, the controller stops the motor and the gauge shows as failed in the Flow Builder. It never guesses where the card is. On four coil pins the coils are switched off too; a driver chip keeps the motor energised.

Measuring the index angle. The index angle is where the card is when it passes the sensor, read off the face. The easiest option is to mount the sensor where north passes and leave the angle at 0. Otherwise, turn the card by hand until the sensor triggers and enter the heading that sits under the lubber line. Then check it with the Pin Test's 0–360° slider: send the card to 90° and it should show east.

An X27.589 driven directly takes 1 080 steps a turn, the default. On a driver chip, enter the steps per turn yourself. An altimeter hand works the same way: map the altitude × 0.36 to the needle angle and the controller winds the hand round as many times as it takes.