Unit C - Actuators & Displays

15. Stepper Motors & Drivers

Produce repeatable incremental motion.

Estimated time 4 hours

Learning outcomes

Compare stepper and brushed DC motor behaviour
Explain open-loop stepping and missed-step risk
Describe the 28BYJ-48 + ULN2003 beginner path
Use STEP/DIR on an A4988-style driver with safe current and power practice

Parts and preparation

Uno and either 28BYJ-48 + ULN2003 or NEMA17 + A4988/DRV8825 with motor supply.

Before power: inspect wiring, confirm supply voltage and ensure all connected circuits share GND.

Stepper Motors & Drivers instructional connection diagram

Stepper vs brushed DC

A brushed DC motor (lesson 14) 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 know the step angle / microstepping) and you can estimate shaft position - as long as every commanded step actually happened.

Comparison of continuous brushed DC rotation versus stepper incremental motion with discrete steps — DC: continuous spin. Stepper: counted increments from a coil sequence.

Stepping motion and open-loop control

A common NEMA17 full-step angle is 1.8 degrees, so 200 full steps make one revolution. Microstepping divides each full step into smaller moves for smoother motion (more pulses per revolution).

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

Diagram of counted steps along a path with a warning that missed steps break the open-loop position count — Open-loop: count pulses. Missed steps break the position estimate.

Item	Typical note
1.8 deg motor	200 full steps / revolution
Microstepping	More pulses per rev; smoother, less torque per microstep
Open-loop	No encoder; trust the step count
Closed-loop	Needs a sensor / encoder (beyond this lesson)

Beginner path: 28BYJ-48 + ULN2003

The 28BYJ-48 is a small geared unipolar stepper (five wires: four phases plus a common). A ULN2003 driver board accepts four Uno digital pins and sequences the coils. It is slow and soft, but excellent for learning step patterns and light mechanisms.

Pin order matters. Wrong phase order often vibrates or refuses to turn. Use the keyed cable orientation the board expects.

Flow from Uno through ULN2003 board to a 28BYJ-48 five-wire unipolar geared stepper — Simple unipolar path: four logic pins into ULN2003, then the geared motor.

A4988 path: STEP and DIR

Bipolar drivers such as A4988 or DRV8825 take STEP and DIR logic from the Uno. Set DIR for the desired direction, then pulse STEP. Each rising edge advances one full step or one microstep, depending on the MS1/MS2/MS3 mode jumpers.

The worked sketch on this page is written for that STEP/DIR style. Pulse rate controls speed; total pulse count controls distance.

Waveforms showing DIR level selecting direction and STEP pulses each advancing one increment — DIR first, then STEP pulses. Pulse rate = speed; count = distance.

const byte stepPin = 3;
const byte dirPin = 4;

void moveSteps(long steps, bool clockwise) {
  digitalWrite(dirPin, clockwise ? HIGH : LOW);
  for (long i = 0; i < steps; i++) {
    digitalWrite(stepPin, HIGH);
    delayMicroseconds(800);
    digitalWrite(stepPin, LOW);
    delayMicroseconds(800);
  }
}

Power, coils and current limit

Motor current comes from VMOT on an external supply, not from the Uno. Join grounds. Fit the capacitor recommended by the module close to VMOT.

Identify coil pairs (often by resistance / continuity) and wire them to 1A/1B and 2A/2B. Set the driver's current limit (Vref pot) before the first run to protect the motor and the module. Never connect or disconnect the motor while the driver is powered.

A4988 wiring overview with Uno STEP DIR, VMOT supply and capacitor, coil pairs, and a safety checklist including current limit — Logic from the Uno; current from VMOT. Set current limit before running.

Connection	Course practice
STEP	D3
DIR	D4
SLEEP / RESET	Held HIGH as module docs require
VMOT / GND	Motor supply + local capacitor
Logic VDD / GND	5 V and common GND
Coils	1A/1B and 2A/2B pairs

Torque, speed and acceleration

Available torque falls as step rate rises. Starting at a high pulse rate under load causes missed steps. For heavier mechanisms, start with longer delays and shorten them over the first many steps (acceleration). That is this lesson's extension challenge.

Curve showing torque decreasing as step rate increases, with practice notes to start slow and accelerate — Faster pulsing leaves less torque. Accelerate instead of jumping to full speed.

Wiring and safe build sequence

Breadboard wiring for lesson 15: Stepper Motors & Drivers — Breadboard layout for this lesson. Match colours and pins before powering the circuit. Click the image for a larger view.

A4988 STEP -> D3 and DIR -> D4
RESET and SLEEP held HIGH as module documentation specifies
VMOT and GND -> motor supply with local capacitor
Logic VDD -> 5 V, logic GND -> common GND
Motor coils -> 1A/1B and 2A/2B

Power rule: switch off before moving wires. Arduino I/O pins are control signals; high-current loads require a driver and suitable external supply.

Worked sketch

Download .ino sketch

const byte stepPin = 3;
const byte dirPin = 4;

void moveSteps(long steps, bool clockwise) {
  digitalWrite(dirPin, clockwise ? HIGH : LOW);
  for (long i = 0; i < steps; i++) {
    digitalWrite(stepPin, HIGH);
    delayMicroseconds(800);
    digitalWrite(stepPin, LOW);
    delayMicroseconds(800);
  }
}

void setup() {
  pinMode(stepPin, OUTPUT);
  pinMode(dirPin, OUTPUT);
}

void loop() {
  moveSteps(200, true);
  delay(500);
  moveSteps(200, false);
  delay(500);
}

How the code works

One pulse advances one full step only when microstep pins select full-step mode.
200 steps is one revolution for a common 1.8-degree motor, not for every motor/gearbox.
If you use a 28BYJ-48 instead, drive four phase pins in sequence - the STEP/DIR sketch will not match that wiring.

Test and record evidence

Expected result: The motor moves a fixed distance in each direction.

Practical evidence checklist

Labelled wiring or block diagram Uploaded sketch and observed result At least one measured/tested value Fault or improvement recorded in logbook

Common faults and checks

Identify coil pairs with resistance/continuity measurements.
Jerking without rotation suggests wrong coil pairing.
Never plug/unplug a motor while the driver is powered.
If it stalls or skips, slow the pulse rate and check the current limit.

Extension challenge: Add gradual acceleration by reducing the pulse delay over the first 50 steps.

Check your understanding

Q1. Why can a stepper lose position?

Show answer

Open-loop steps may be missed if load or speed exceeds available torque.

Q2. What must be set on an A4988?

Show answer

Its current limit, appropriate to the motor and module cooling.

Q3. What does one STEP pulse do on an A4988?

Show answer

Advances one full step or one microstep, depending on the MS mode.

Q4. Why is the 28BYJ-48 path useful for beginners?

Show answer

Simple ULN2003 wiring and a geared motor make sequences easy to learn, though motion is slow.