Rotary Gravity Slide Projector

// Rotary gravity slide tray - batch 99, after US 3,276,314
// EasyDriver: STEP D3, DIR D4, motor on A/B, 12 V to PWR IN. Lift servo D9. Advance button D2 -> GND.
#include <Servo.h>

const int STEP_PIN = 3, DIR_PIN = 4, BTN = 2;
const long STEPS_PER_REV = 3200;     // 400-step motor x 8 microsteps (EasyDriver default)
const int SLOTS = 12;
const int LIFT_DOWN = 10, LIFT_UP = 100;   // servo angles: arm clear / slide pushed back into tray

Servo lift;
int slot = 0;
long position = 0;                   // absolute microsteps from the start slot

void stepTo(long target) {
  digitalWrite(DIR_PIN, target > position ? HIGH : LOW);
  while (position != target) {
    digitalWrite(STEP_PIN, HIGH); delayMicroseconds(400);
    digitalWrite(STEP_PIN, LOW);  delayMicroseconds(400);
    position += (target > position) ? 1 : -1;
  }
}

void setup() {
  pinMode(STEP_PIN, OUTPUT); pinMode(DIR_PIN, OUTPUT); pinMode(BTN, INPUT_PULLUP);
  lift.attach(9); lift.write(LIFT_DOWN);
  Serial.begin(115200);
  Serial.println(F("Slot 0 in the gate. Press to advance."));
}

void loop() {
  if (digitalRead(BTN) == HIGH) return;
  lift.write(LIFT_UP);  delay(600);            // push the shown slide back up into its slot
  slot = (slot + 1) % SLOTS;
  // target from the ABSOLUTE slot number, so the 266.67-step fraction never accumulates
  long target = (long)slot * STEPS_PER_REV / SLOTS;
  if (slot == 0) target = STEPS_PER_REV;        // finish the turn forwards...
  stepTo(target);
  if (slot == 0) position = 0;                   // ...then call it zero again
  lift.write(LIFT_DOWN); delay(600);           // the next slide drops into the gate by gravity
  Serial.print(F("showing slot ")); Serial.println(slot);
  while (digitalRead(BTN) == LOW) {}
}