The Fourier Transform Spectrometer

// Log a Michelson interferogram: one photodiode, one steadily moving mirror.
//
// The transform in the notebook assumes EVENLY SPACED samples, so the two things that
// matter are a constant sample interval and a constant mirror speed. Everything else is
// arithmetic done afterwards.
//
// Wiring: photodiode + 10k to GPIO34 through the transimpedance stage, or straight into
// the ADC with a 100k load if you only want the shape. Optional: a second photodiode on
// GPIO35 watching a reference laser, so fringe COUNT gives position without trusting the
// motor at all -- which is what a real instrument does.

const int PIN_SIG = 34;      // the interferogram
const int PIN_REF = 35;      // reference-laser fringes, optional but transformative

const unsigned long DT_US   = 500;      // 2 kHz sampling
const unsigned long N_SAMPS = 20000;    // 10 seconds of scan

void setup() {
  Serial.begin(115200);
  delay(300);
  analogReadResolution(12);
  analogSetPinAttenuation(PIN_SIG, ADC_11db);
  analogSetPinAttenuation(PIN_REF, ADC_11db);

  Serial.println("# Michelson interferogram log");
  Serial.printf("# %lu samples at %lu us -> %.1f s of scan\n",
                N_SAMPS, DT_US, N_SAMPS * DT_US / 1e6);
  Serial.println("# start the mirror moving BEFORE the countdown ends, and let it run past the end");
  for (int i = 3; i > 0; i--) { Serial.printf("# %d\n", i); delay(1000); }
  Serial.println("sample,signal,reference");

  unsigned long next = micros();
  long refCount = 0;
  int lastRef = 0;

  for (unsigned long k = 0; k < N_SAMPS; k++) {
    while ((long)(micros() - next) < 0) ;          // hold the interval, do not drift
    next += DT_US;

    int sig = analogRead(PIN_SIG);
    int ref = analogRead(PIN_REF);

    // Count reference fringes by threshold crossing: this is the position scale, and it
    // is immune to the motor speeding up or slowing down.
    int refHigh = ref > 2048;
    if (refHigh && !lastRef) refCount++;
    lastRef = refHigh;

    Serial.printf("%lu,%d,%ld\n", k, sig, refCount);
  }
  Serial.println("# done -- paste the two columns into the notebook");
  Serial.println("# resample the signal against REFERENCE COUNT, not against sample number,");
  Serial.println("# and an uneven scan stops mattering entirely.");
}

void loop() {}