Seed Drying Box Controller

#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>
#include <Adafruit_SHT31.h>

#define I2C_SDA_PIN 21
#define I2C_SCL_PIN 22
#define FAN_PWM_PIN 25

constexpr uint8_t SCREEN_WIDTH = 128;
constexpr uint8_t SCREEN_HEIGHT = 64;
constexpr uint8_t FAN_PWM_CHANNEL = 0;
constexpr uint16_t FAN_PWM_FREQUENCY = 25000;
constexpr uint8_t FAN_PWM_RESOLUTION = 8;
constexpr uint8_t FAN_MIN_PERCENT = 30;
constexpr float DRY_TARGET_RH = 45.0f;
constexpr float HIGH_RH = 65.0f;
constexpr unsigned long SAMPLE_INTERVAL_MS = 2000;

Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, -1);
Adafruit_SHT31 sht31 = Adafruit_SHT31();

float shownTemperature = NAN;
float shownHumidity = NAN;
int shownFanPercent = -1;
bool shownSensorFault = false;
unsigned long lastSampleMs = 0;

int fanPercentForHumidity(float humidity) {
  if (humidity <= DRY_TARGET_RH) {
    return FAN_MIN_PERCENT;
  }
  if (humidity >= HIGH_RH) {
    return 100;
  }
  float fraction = (humidity - DRY_TARGET_RH) / (HIGH_RH - DRY_TARGET_RH);
  return FAN_MIN_PERCENT + (int)((100 - FAN_MIN_PERCENT) * fraction + 0.5f);
}

void setFanPercent(int percent) {
  percent = constrain(percent, FAN_MIN_PERCENT, 100);
  uint32_t maxDuty = (1UL << FAN_PWM_RESOLUTION) - 1;
  // A 4-pin fan internally pulls PWM high. The MOSFET makes a GPIO HIGH pull it LOW,
  // so invert the ESP32 duty cycle to give the fan the requested high-time percentage.
  uint32_t mosfetDuty = (maxDuty * (100 - percent)) / 100;
  ledcWrite(FAN_PWM_PIN, mosfetDuty);
}

void drawReadings(float temperature, float humidity, int fanPercent) {
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.println(F("SEED DRYER"));
  display.drawFastHLine(0, 11, SCREEN_WIDTH, SSD1306_WHITE);

  display.setTextSize(2);
  display.setCursor(0, 18);
  display.print(humidity, 1);
  display.println(F("%RH"));

  display.setTextSize(1);
  display.setCursor(0, 43);
  display.print(F("Temp: "));
  display.print(temperature, 1);
  display.println(F(" C"));
  display.setCursor(0, 54);
  display.print(F("Fan:  "));
  display.print(fanPercent);
  display.print(F("%  Target: "));
  display.print(DRY_TARGET_RH, 0);
  display.print(F("%"));
  display.display();
}

void drawSensorFault() {
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.println(F("SEED DRYER"));
  display.drawFastHLine(0, 11, SCREEN_WIDTH, SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 25);
  display.println(F("SHT31 not found"));
  display.setCursor(0, 40);
  display.println(F("Check SDA / SCL"));
  display.display();
}

void takeSample() {
  float temperature = sht31.readTemperature();
  float humidity = sht31.readHumidity();

  if (isnan(temperature) || isnan(humidity)) {
    setFanPercent(100);
    if (!shownSensorFault) {
      drawSensorFault();
      shownSensorFault = true;
    }
    return;
  }

  int fanPercent = fanPercentForHumidity(humidity);
  setFanPercent(fanPercent);

  bool changed = shownSensorFault || isnan(shownTemperature) ||
                 fabsf(temperature - shownTemperature) >= 0.1f ||
                 fabsf(humidity - shownHumidity) >= 0.1f ||
                 fanPercent != shownFanPercent;
  if (changed) {
    drawReadings(temperature, humidity, fanPercent);
    shownTemperature = temperature;
    shownHumidity = humidity;
    shownFanPercent = fanPercent;
    shownSensorFault = false;
  }
}

void setup() {
  Wire.begin(I2C_SDA_PIN, I2C_SCL_PIN);

  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  display.clearDisplay();
  display.display();

  ledcAttach(FAN_PWM_PIN, FAN_PWM_FREQUENCY, FAN_PWM_RESOLUTION);
  setFanPercent(100);

  sht31.begin(0x44);
  takeSample();
}

void loop() {
  unsigned long now = millis();
  if (now - lastSampleMs >= SAMPLE_INTERVAL_MS) {
    lastSampleMs = now;
    takeSample();
  }

  unsigned long elapsed = millis() - lastSampleMs;
  unsigned long remaining = elapsed < SAMPLE_INTERVAL_MS ? SAMPLE_INTERVAL_MS - elapsed : 1;
  delay(remaining > 20 ? 20 : remaining);
}