Stepper follows a knob (pins 2 to 5) on an Arduino Uno

// A stepper that follows a knob: the 28BYJ-48 on pins 2 to 5, the knob on A0.
//
//   ULN2003 IN1 -> pin 2   IN2 -> pin 3   IN3 -> pin 4   IN4 -> pin 5
//   ULN2003 + -> 5V   - -> GND
//   potentiometer: ends to 5V and GND, middle -> A0
//
// A stepper turns one step each time its coils are switched in order, so the
// sketch always knows where the shaft is. It reads the knob, works out where
// the shaft should be (one full turn across the knob's travel), and steps the
// difference. The Stepper library takes the pins as IN1, IN3, IN2, IN4.

#include <Stepper.h>

const int STEPS_PER_TURN = 2048;
const int KNOB = A0;

Stepper motor(STEPS_PER_TURN, 2, 4, 3, 5);
long at = 0;

void setup() {
  motor.setSpeed(12);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: stepper on pins 2 to 5 follows the knob on A0");
}

void loop() {
  long target = (long)analogRead(KNOB) * STEPS_PER_TURN / 1024;
  long move = target - at;
  if (move > 8 || move < -8) {
    motor.step(move);
    at = target;
    Serial.print("shaft at step ");
    Serial.println(at);
  }
  delay(20);
}