The Peristaltic Pump: Two Places Closed at Once, and No Valve Touches the Fluid

/*
  peristaltic_drive.ino -- run a stepper-driven peristaltic pump at a set speed.

  Hardware: a NEMA 17 stepper on an A4988 (or DRV8825) driver board, a 10 k
  potentiometer for speed, and a push button to reverse.
  Wiring:   driver STEP -> D3, DIR -> D4, EN -> D8 (LOW = enabled)
            pot wiper -> A0, pot ends -> 5V and GND
            button    -> D2 and GND (internal pull-up)
  The driver's own VMOT must come from the 12 V supply, with a 100 uF capacitor
  across it at the board, and its current limit set BEFORE the motor is connected.
*/

const int PIN_STEP = 3;
const int PIN_DIR  = 4;
const int PIN_EN   = 8;
const int PIN_BTN  = 2;
const int PIN_POT  = A0;

const long STEPS_PER_REV = 200L * 16L;   // 1.8 deg motor, 1/16 microstepping
const float RPM_MIN = 2.0;
const float RPM_MAX = 150.0;

unsigned long lastStep = 0;
unsigned long interval = 0;              // microseconds between steps
bool forward = true;
bool lastBtn = HIGH;

void setup() {
  pinMode(PIN_STEP, OUTPUT);
  pinMode(PIN_DIR, OUTPUT);
  pinMode(PIN_EN, OUTPUT);
  pinMode(PIN_BTN, INPUT_PULLUP);
  digitalWrite(PIN_EN, LOW);
  digitalWrite(PIN_DIR, HIGH);
  Serial.begin(9600);
}

void loop() {
  // speed from the pot, re-read every loop so turning it takes effect at once
  float rpm = RPM_MIN + (RPM_MAX - RPM_MIN) * analogRead(PIN_POT) / 1023.0;
  interval = (unsigned long)(60.0e6 / (rpm * STEPS_PER_REV));

  // reverse on a button press (edge, not level), to clear an air lock
  bool b = digitalRead(PIN_BTN);
  if (b == LOW && lastBtn == HIGH) {
    forward = !forward;
    digitalWrite(PIN_DIR, forward ? HIGH : LOW);
    Serial.println(forward ? F("forward") : F("reverse"));
    delay(30);                           // crude debounce
  }
  lastBtn = b;

  unsigned long now = micros();
  if (now - lastStep >= interval) {
    lastStep = now;
    digitalWrite(PIN_STEP, HIGH);
    delayMicroseconds(2);
    digitalWrite(PIN_STEP, LOW);
  }
}

/*
  CALIBRATE, don't trust the arithmetic:
   1. Run into a graduated cylinder for exactly 100 rotor turns (count them on
      the serial print, or time them at a known rpm).
   2. Volume / 100 is your mL per turn. Compare it with the notebook's figure.
   3. Repeat with the outlet lifted 1 m higher. If the figure drops, the
      rollers are not closing the tube fully -- raise the occlusion.
*/