19. Stepper Motors & the Stepper Library

Stepper vs brushed DC

A brushed DC motor (Lesson 18) spins continuously; an H-bridge sets direction and PWM sets speed. You do not get a free step count.

A stepper motor moves in fixed angular increments when its coils are energised in sequence. Count the steps and you know how far the shaft has turned - as long as every commanded step actually happened.

Stepping motion and open-loop control

The 28BYJ-48 has 32 steps per motor revolution and a gearbox of about 64:1, so its output shaft needs about 2048 steps for one full turn in the full-step sequence the Stepper library uses. The gearbox makes it slow but gives useful torque for light mechanisms.

Hobby setups are open-loop: the sketch assumes each step moved the shaft. If the load is too heavy or the speed too high, steps are missed and the software position drifts from reality until you home against a switch or sensor.

28BYJ-48 and the ULN2003 board

The 28BYJ-48 is a small unipolar stepper with five wires: four coil ends and a common red wire to +5 V. The ULN2003 board is a Darlington transistor array - seven NPN Darlington pairs with built-in flyback diodes - so each Uno pin only switches base current while the board sinks the coil current.

Four LEDs on the board show which inputs are HIGH, which makes the step sequence visible. Power the motor side from a separate 5 V supply and join its GND to the Uno GND.

The Stepper library

The built-in Stepper library sends the coil sequence for you. Create one Stepper object with the steps per revolution and the four pins, set the speed in revolutions per minute with setSpeed(), then call step(n). A positive n turns one way, a negative n the other.

Pin order matters: for the 28BYJ-48 on a ULN2003 board, give the pins as IN1, IN3, IN2, IN4 (8, 10, 9, 11). With the natural order the motor only buzzes or twitches.

Keep setSpeed() at 15 rpm or below for this motor. Faster than that it stalls and misses steps.

#include <Stepper.h>

const int stepsPerRev = 2048;
Stepper motor(stepsPerRev, 8, 10, 9, 11);   // IN1, IN3, IN2, IN4

void setup() {
  motor.setSpeed(10);        // rpm
}

void loop() {
  motor.step(512);           // quarter turn one way
  delay(1000);
  motor.step(-512);          // quarter turn back
  delay(1000);
}

step() blocks the sketch

step(n) does not return until every step has been sent. At 10 rpm a full turn of 2048 steps takes 6 seconds, and during that time loop() cannot read a button or update a display.

For a responsive sketch, move one step at a time from loop() using the millis pattern from Lesson 11, and keep a variable for how many steps are still to go.

long stepsToGo = 2048;
unsigned long lastStep = 0;
const unsigned long stepIntervalMs = 3;   // about 10 rpm

void loop() {
  if (stepsToGo > 0 && millis() - lastStep >= stepIntervalMs) {
    lastStep = millis();
    motor.step(1);
    stepsToGo--;
  }
  // buttons, Serial and displays still run here
}

Scope the coil sequence

Probe IN1 on channel 1 and IN2 on channel 2 (ground clip on GND), 2 V/div and 5 ms/div, while the motor turns at 10 rpm. The library energises two coils at a time in a four-step pattern, so each input is a square wave that is HIGH for two steps out of four.

At 10 rpm the motor takes 2048 × 10 / 60 ≈ 341 steps per second, about 2.9 ms per step. One four-step cycle is about 11.7 ms, so each input shows roughly 85 Hz at 50 % duty, with IN2 shifted by one step from IN1. Double the speed and the frequency doubles - check it.

Power, heat and station rules

Each energised coil of a 5 V 28BYJ-48 draws roughly 200 mA, and two are on at once - far more than the Uno 5 V pin should supply for long runs. Use the separate supply and a common GND.

The library leaves the last two coils energised when step() finishes, so a parked motor still draws current and warms up. Write all four pins LOW after a move if the shaft does not need holding torque.

There are two stepper stations in the class. Leave each station exactly as you found it for the next group.

Bigger steppers and STEP/DIR drivers

Larger bipolar steppers such as the NEMA17 need a current-limiting driver like the A4988 or DRV8825, controlled with STEP and DIR pins instead of four coil inputs. The optional appendix below covers that path for final projects that need more torque.

Wiring and safe build sequence

1. Power off. ULN2003 IN1 -> D8, IN2 -> D9, IN3 -> D10, IN4 -> D11
2. ULN2003 + -> separate 5 V supply +; ULN2003 - -> supply GND
3. Supply GND -> Uno GND (common ground)
4. Plug the 28BYJ-48 into the keyed socket on the ULN2003 board
5. Scope channel 1 on IN1, channel 2 on IN2, ground clip on GND