Stepper dial on the analyzer, on an Arduino Uno

// Stepper dial: a knob on A0, the shaft follows it.
//
//   A0            the potentiometer's wiper; its ends to 5 V and GND
//   pins 8..11    IN1 to IN4 on the ULN2003 board
//
// A stepper does not go at a speed, it goes to a *position*, and this is the
// shortest circuit that makes that obvious: turn the knob and the shaft goes
// there and stays there.
//
// # What a driver actually is
//
// A table and a delay. The motor is four coils; switching them in order pulls
// the rotor round one point of a magnetic circle at a time. Nothing in
// firmware/lib/mokxi_stepper.h is cleverer than that, and nothing in the part
// is either; Mokxi's model works out where the field is pointing and moves
// the rotor there, so half-step, wave drive and two-phase-on all fall out of
// one table rather than being three special cases.
//
// # 4096
//
// The rotor turns 5.625 degrees per half-step and the gearbox divides by 64, so
// the output shaft moves 5.625/64 of a degree at a time and a whole turn is
// 4096 half-steps. That number is in every 28BYJ-48 sketch ever written and
// this is where it comes from. Half a turn (2048) is the sweep here, which
// is about as far as a dial wants to go.
//
// # The rate ceiling is real
//
// The datasheet's idle out-traction frequency is over 1000 Hz, so steps closer
// together than a millisecond are steps the rotor cannot follow. Mokxi's part
// misses them exactly as a real motor does. Turn the knob quickly and the
// shaft follows at its own top speed rather than teleporting, which is what a
// real one does too.
//
// # It does not know where it is
//
// Nothing here reads the shaft. The sketch counts the steps it has sent and
// assumes the motor made them, which is true until it is not (a stall, a
// jam, or a rate past the ceiling), and is why real positioning systems have
// a limit switch to home against. Press nothing and watch: if the count and
// the shaft ever disagree, the sketch will never find out.

#include "mokxi_stepper.h"

const int KNOB = A0;
const int IN1 = 8;
const int IN2 = 9;
const int IN3 = 10;
const int IN4 = 11;

// Half a turn of the output shaft across the knob's whole sweep.
const long SWEEP_STEPS = STEPS_PER_TURN_HALF / 2;
// The knob has to move this far before the motor is asked to, so the last bit
// of the ADC does not make it hum.
const int DEADBAND = 8;

Stepper28byj motor(IN1, IN2, IN3, IN4);

long at = 0;      // where the sketch believes the shaft is, in half-steps
int lastKnob = -1000;

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Uno: 28BYJ-48 following a knob on A0");
  motor.begin();
}

void loop() {
  int knob = analogRead(KNOB);
  if (knob > lastKnob - DEADBAND && knob < lastKnob + DEADBAND) {
    return;
  }
  lastKnob = knob;

  // Where the knob says the shaft should be, in half-steps from the start.
  long want = ((long)knob * SWEEP_STEPS) / 1023;
  long move = want - at;
  if (move == 0) {
    return;
  }

  motor.stepHalf(move, 1500);
  at = want;

  // Tenths of a degree, so there is no floating point anywhere:
  //   degrees = half-steps * 5.625 / 64, and ten times that in tenths.
  long tenths = (at * 5625L) / 6400L;
  Serial.print("knob ");
  Serial.print(knob);
  Serial.print(" -> ");
  Serial.print(at);
  Serial.print(" half-steps, ");
  Serial.print(tenths / 10);
  Serial.print(".");
  Serial.print(tenths % 10);
  Serial.println(" degrees");

  // Let go once it is there: nothing holds the shaft, nothing draws current,
  // and a real one stops getting warm.
  motor.release();
}