Build a Low-Voltage Greenhouse Vent Controller

// ESP32 low-voltage greenhouse controller demo
// DHT22 air readings control a relay with hysteresis.
#include <Wire.h>
#include <Adafruit_SSD1306.h>
#include <DHT.h>

const int DHT_PIN = 4;
const int FAN_RELAY = 26;
const float TEMP_ON_C = 30.0;
const float TEMP_OFF_C = 28.0;
const float HUM_ON_PERCENT = 85.0;
const float HUM_OFF_PERCENT = 78.0;

DHT dht(DHT_PIN, DHT22);
Adafruit_SSD1306 oled(128, 64, &Wire, -1);

bool fanOn = false;
unsigned long lastReadMs = 0;

void setFan(bool on) {
  fanOn = on;
  digitalWrite(FAN_RELAY, fanOn ? LOW : HIGH); // common active-LOW relay module
}

void setup() {
  Serial.begin(115200);
  Wire.begin(21, 22);
  dht.begin();
  oled.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  oled.setTextColor(WHITE);
  pinMode(FAN_RELAY, OUTPUT);
  setFan(false);
  Serial.println("Greenhouse controller ready. Low-voltage loads only.");
}

void loop() {
  if (millis() - lastReadMs < 2500) return;
  lastReadMs = millis();

  float tempC = dht.readTemperature();
  float humidity = dht.readHumidity();
  if (isnan(tempC) || isnan(humidity)) {
    setFan(false);
    Serial.println("DHT22 read failed. Fan set off until readings recover.");
    return;
  }

  if (!fanOn && (tempC >= TEMP_ON_C || humidity >= HUM_ON_PERCENT)) {
    setFan(true);
  } else if (fanOn && tempC <= TEMP_OFF_C && humidity <= HUM_OFF_PERCENT) {
    setFan(false);
  }

  Serial.printf("Temp %.1f C  Hum %.1f %%  Fan %s\n", tempC, humidity, fanOn ? "ON" : "off");

  oled.clearDisplay();
  oled.setTextSize(1);
  oled.setCursor(0, 0);
  oled.printf("Temp: %.1f C\n", tempC);
  oled.printf("Hum:  %.1f %%\n", humidity);
  oled.printf("Fan:  %s\n", fanOn ? "ON" : "off");
  oled.println("Hysteresis enabled");
  oled.display();
}