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How Stepper Motors Work: Arduino Control with 28BYJ-48, ULN2003 and A4988

Probot Studio

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What we'll do

  1. Watch the coils inside a stepper motor switch on in order.
  2. Learn the difference between full steps, half steps and microsteps.
  3. Wire a 28BYJ-48 with a ULN2003, and a bigger motor with an A4988, to an Arduino.
  4. Use the calculator to find steps per revolution and the pulse rate you need.

Why it's useful: a stepper is the easiest way to say "turn exactly 90 degrees" or "move 5 mm" without any sensor. That's why 3D printers and CNC machines use them.

Schematic cross-section of a stepper motor: four coils A, B, C, D light up orange in order, and the arrow on the magnet rotor turns to the lit coil each time; a table beside it shows which coil is on at each step

What is a stepper motor used for?

A stepper motor turns by a fixed angle (one step) for every electrical pulse. Count the pulses and you know where the shaft is, with no extra sensor.

Where you'll find them: 3D printer and CNC axes, laser cutters, camera sliders, turntables, scanners.

How does a stepper motor work?

The rotor is a magnet surrounded by coils. A coil with current through it becomes a magnet and pulls the rotor toward it. Switch the coils on and off in order and the rotor turns step by step.

In a real motor the rotor and stator have fine teeth, so the steps are tiny: on common motors one step is 1.8°, or 200 steps per revolution.

Full step, half step, microstepping

Three rows of motor drawings: wave drive has four positions, full step has four positions halfway between with two neighboring coils on, half step has eight positions; the arrow moves 45 degrees per half step

Try it yourself

Pick a drive mode and press 1 step or Play: watch which coil lights up and where the rotor turns. Then work out your own mechanism below.

This is a simulator where you fire the coils in order and watch the rotor step, plus a calculator for steps per revolution, pulse frequency and steps per mm; it needs JavaScript. If it didn't load, the pictures above show the same sequence.

Try this: in half step, press eight times and the model rotor makes one full turn. Then pick a 1.8° motor, 1/16 and a 20-tooth GT2 pulley in the calculator: one step is 0.0125 mm, so 1 mm takes 80 pulses.

Unipolar vs bipolar stepper motors

Unipolar Bipolar
Wires 5 or 6 (coils have a center tap) 4
Driver Simple: a transistor array like the ULN2003 One H-bridge per coil: A4988, L298N
Example 28BYJ-48 (5 wires) 1.8° motors (the NEMA 17 size in 3D printers)

A bipolar motor needs the current in each coil to reverse, so every coil needs its own H-bridge, like the one in the DC motor guide.

What is a stepper driver? ULN2003 and A4988

The coils draw more current than an Arduino pin can supply; the driver switches that current on the Arduino's command.

How to wire a 28BYJ-48 to a ULN2003

Wiring diagram: Arduino pins 8, 9, 10, 11 go in order to ULN2003 IN1-IN4, 5V and GND to the board plus and minus; the 28BYJ-48 plugs into the board with a five-wire plug, red is the common

// 28BYJ-48 + ULN2003: one turn forward, one back, with the Stepper library
#include <Stepper.h>

const int STEPS_PER_REV = 2048;  // full steps per turn (catalog value)
// Order matters: IN1, IN3, IN2, IN4 -> pins 8, 10, 9, 11
Stepper motor(STEPS_PER_REV, 8, 10, 9, 11);

void setup() {
  motor.setSpeed(10);            // rpm; 10-15 suits the 28BYJ-48
}

void loop() {
  motor.step(STEPS_PER_REV);     // one turn forward (waits until done)
  delay(1000);
  motor.step(-STEPS_PER_REV);    // one turn back
  delay(1000);
}

How to wire a stepper to an A4988

A4988 wiring: Arduino pin 3 to STEP, pin 2 to DIR; RST tied to SLP; MS pins left open; VMOT and GND to an 8-35 V supply with at least 47 µF across them; motor coils to 1A-1B and 2A-2B; VDD and GND to Arduino 5V and GND

// A4988 + 1.8° bipolar stepper: turns with STEP/DIR
const int STEP_PIN = 3;
const int DIR_PIN = 2;
const int STEPS_PER_REV = 200;  // MS1-MS3 unconnected: full step

void setup() {
  pinMode(STEP_PIN, OUTPUT);
  pinMode(DIR_PIN, OUTPUT);
}

// send `steps` pulses; gapUs: time from one pulse to the next
void move(int steps, bool forward, unsigned int gapUs) {
  digitalWrite(DIR_PIN, forward ? HIGH : LOW);
  for (int i = 0; i < steps; i++) {
    digitalWrite(STEP_PIN, HIGH);
    delayMicroseconds(gapUs / 2);  // datasheet: at least 1 µs
    digitalWrite(STEP_PIN, LOW);
    delayMicroseconds(gapUs / 2);
  }
}

void loop() {
  move(STEPS_PER_REV, true, 5000);   // 200 Hz = 60 rpm
  delay(1000);
  move(STEPS_PER_REV, false, 5000);  // back
  delay(1000);
}

What voltage does a stepper motor need?

A stepper draws current even when standing still, to hold its position, so it gets warm while idle too.

How to identify stepper motor wires

Set your multimeter to resistance (ohms):

  1. 4 wires (bipolar): if you read a resistance between two wires, they're the same coil; if not, they're different coils. You'll find two pairs: one goes to 1A-1B, the other to 2A-2B.
  2. 5 wires (28BYJ-48): red is the common; you'll read the same resistance from red to each of the other wires.
  3. Turns the wrong way? Swap the two ends of one coil, or flip DIR in code.

Stepper vs servo motor

DC motor Servo motor Stepper motor
Motion Spins continuously Goes to an angle, most 0-180° Step by step, can also spin
How it knows position It doesn't, needs an encoder Measures it internally Counts pulses, no measurement
Driving it H-bridge + PWM One signal wire Driver + pulse train
Weak spot Needs a sensor and PID for position Limited angle Skips steps under overload and never notices

More detail: what is a servo motor?

Common mistakes

Symptom Cause Fix
28BYJ-48 buzzes but won't turn Pins written as 8, 9, 10, 11 Stepper motor(2048, 8, 10, 9, 11)
Bipolar motor jitters Coil pairs mixed up Find the pairs with a multimeter
A4988 does nothing RST left floating Tie RST to SLP
Position drifts over time Too fast, too much load, or current limit too low Slow down, accelerate gently, check Vref
Motor and driver run very hot Current limit too high Set Vref for the motor's rated current
28BYJ-48 is a bit off after a full turn Real gear ratio is 63.68:1 Use your measured value instead of 2048

Sources

Accessed: 17 October 2026