The Selenium Photoconductive Cell

// Photoconductor sweep — resistance vs illuminance across four decades.
//
// The LDR sits in a divider with a known fixed resistor. The ESP32 drives a white LED
// with a GEOMETRIC series of duty cycles, because a linear ramp puts nine of ten points
// in the bright end and tells you nothing about the dark end, which is where the
// interesting physics is.
//
// Sweeps UP then DOWN on purpose. The two curves will not match. That gap is light
// history, and it is the subject of the last step.
//
// Wiring
//   LDR      3V3 ---- LDR ---- GPIO34 ---- 10k ---- GND
//   LED      GPIO25 ---- 220R ---- LED ---- GND
//   Put the LED 50 mm from the LDR inside a CLOSED box. Room light swamps the dark end.

const int PIN_LDR   = 34;      // ADC1, safe with WiFi active
const int PIN_LED   = 25;
const float R_FIXED = 10000.0; // ohm, measure yours and put the real value here
const float VCC     = 3.3;

const int  LEDC_CH   = 0;
const int  LEDC_BITS = 8;      // 0..255
const int  LEDC_FREQ = 20000;  // 20 kHz, well above any photoconductor response

// Geometric ladder: each step doubles. Nine points, four decades of relative light.
const int DUTY[] = {1, 2, 4, 8, 16, 32, 64, 128, 255};
const int NDUTY  = sizeof(DUTY) / sizeof(DUTY[0]);

// A photoconductor is SLOW. Settle generously or you measure the transient, not the level.
const unsigned long SETTLE_MS = 4000;

float readCellResistance() {
  // Average heavily: the ESP32 ADC is noisy and the LDR is high impedance.
  const int N = 256;
  uint32_t acc = 0;
  for (int i = 0; i < N; i++) { acc += analogRead(PIN_LDR); delay(2); }
  float counts = (float)acc / N;

  float v = counts * (VCC / 4095.0);
  if (v <= 0.001)      return -1;   // pegged dark
  if (v >= VCC - 0.01) return -2;   // pegged bright

  // Divider: 3V3 - LDR - node - R_FIXED - GND, so V(node) = VCC * Rf/(Rldr+Rf)
  return R_FIXED * (VCC - v) / v;
}

void sweep(bool ascending) {
  Serial.println(ascending ? "\n# direction: UP (dark -> bright)"
                           : "\n# direction: DOWN (bright -> dark)");
  Serial.println("# duty\trel_E\tR_ohm\tlux_if_you_have_a_meter");

  for (int k = 0; k < NDUTY; k++) {
    int i = ascending ? k : (NDUTY - 1 - k);
    ledcWrite(LEDC_CH, DUTY[i]);
    delay(SETTLE_MS);

    float r = readCellResistance();
    Serial.print(DUTY[i]);   Serial.print('\t');
    Serial.print(DUTY[i]);   Serial.print('\t');       // duty doubles as relative illuminance
    if      (r == -1) Serial.print("OPEN");
    else if (r == -2) Serial.print("SHORT");
    else              Serial.print(r, 0);
    Serial.println("\t-");
  }
}

void setup() {
  Serial.begin(115200);
  delay(500);

  ledcSetup(LEDC_CH, LEDC_FREQ, LEDC_BITS);
  ledcAttachPin(PIN_LED, LEDC_CH);

  analogReadResolution(12);
  analogSetPinAttenuation(PIN_LDR, ADC_11db);   // full 0..3.3 V range

  Serial.println("# Photoconductor sweep");
  Serial.println("# Close the box. Wait for the dark reading to stop falling before trusting anything.");

  // Dark baseline first, and give it a real amount of time — a cell that has just been
  // in room light is nowhere near its dark resistance for several minutes.
  ledcWrite(LEDC_CH, 0);
  Serial.println("# settling in the dark for 120 s...");
  for (int t = 0; t < 12; t++) {
    delay(10000);
    Serial.print("#   t="); Serial.print((t + 1) * 10);
    Serial.print(" s  R="); Serial.println(readCellResistance(), 0);
  }

  sweep(true);
  sweep(false);

  ledcWrite(LEDC_CH, 0);
  Serial.println("\n# done. Paste both tables into the notebook.");
  Serial.println("# If UP and DOWN differ by more than a few percent, that is not error.");
  Serial.println("# That is the cell remembering.");
}

void loop() { }