In this blog, we’ll learn how to read temperature and humidity from a DHT11 sensor using an ESP32, and display the values on a 128×32 OLED display using I2C.

This is a great beginner-friendly IoT project using Arduino IDE and minimal components!

🧰 Components Required

🔌 Circuit Diagram

DHT11 Wiring:

• VCC → 3.3V (or 5V on ESP32)
• GND → GND
• DATA → GPIO 4

OLED (I2C) Wiring:

• VCC → 3.3V
• GND → GND
• SDA → GPIO 21
• SCL → GPIO 22

⚠️ Note: GPIO 21 and 22 are default I2C pins on ESP32. Adjust if needed.

💽 Arduino Libraries Needed

Open Arduino IDE → Tools → Manage Libraries and install:

✅ Adafruit SSD1306
✅ Adafruit GFX Library
✅ DHT sensor library by Adafruit

🧾 Full Arduino Code

#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>
#include <DHT.h>

// === OLED Config ===
#define SCREEN_WIDTH 128
#define SCREEN_HEIGHT 32
#define OLED_RESET -1
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, OLED_RESET);

// === DHT11 Config ===
#define DHTPIN 4      // GPIO pin for DHT11 data
#define DHTTYPE DHT11
DHT dht(DHTPIN, DHTTYPE);

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

  // Initialize DHT
  dht.begin();

  // Initialize OLED
  if (!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) {
    Serial.println("SSD1306 init failed!");
    while (1);
  }

  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.println("Starting...");
  display.display();
  delay(1000);
}

void loop() {
  float temp = dht.readTemperature();
  float humi = dht.readHumidity();

  if (isnan(temp) || isnan(humi)) {
    Serial.println("Failed to read from DHT sensor!");
    display.clearDisplay();
    display.setCursor(0, 0);
    display.setTextSize(1);
    display.println("Sensor Error!");
    display.display();
    delay(2000);
    return;
  }

  // Display data
  display.clearDisplay();
  display.setTextSize(2);
  display.setCursor(0, 0);
  display.print("Temp: ");
  display.print((int)temp);
  display.print((char)247); // Degree symbol
  display.println("C");

  display.setCursor(0, 18);
  display.print("Humi: ");
  display.print((int)humi);
  display.println("%");

  display.display();
  delay(2000); // Update every 2 seconds
}

🖼️ OLED Output Preview

Temp: 29°C
Humi: 64%

🧪 Troubleshooting Tips

• If the display shows nothing, check I2C address (0x3C for most OLEDs).
• If the sensor reads “nan”, check DHT11 wiring and pin number.
• Use a breadboard for clean, secure connections.
• Make sure your display is 128×32, not 128×64. Adjust height if needed.

💡 Ideas to Expand

• Add a button to toggle °C and °F
• Log data to SD card or Firebase
• Send data over Wi-Fi to a web dashboard
• Add real-time clock to log timestamps

Here’s a detailed explanation of the code section by section for displaying DHT11 temperature and humidity on a 128×32 OLED with an ESP32:

✅ 1. Library Inclusions

#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>
#include <DHT.h>

These libraries are essential:

• Adafruit_GFX.h – Core graphics functions (text, lines, shapes).
• Adafruit_SSD1306.h – OLED driver for SSD1306 displays.
• DHT.h – Handles communication with DHT11 sensors.

✅ 2. OLED Display Configuration

#define SCREEN_WIDTH 128
#define SCREEN_HEIGHT 32
#define OLED_RESET -1
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, OLED_RESET);

• OLED has a resolution of 128×32 pixels.
• OLED_RESET -1 means we’re not using a dedicated reset pin.
• The display uses the Wire object (I2C).

✅ 3. DHT11 Sensor Configuration

#define DHTPIN 4
#define DHTTYPE DHT11
DHT dht(DHTPIN, DHTTYPE);

• We’re connecting the data pin of the DHT11 to GPIO 4.
• DHT11 tells the library which sensor type we are using (you can also use DHT22 if you upgrade the sensor).

✅ 4. Setup Function

void setup() {
  Serial.begin(115200);
  dht.begin();

• Starts serial communication (for debugging).
• Initializes the DHT sensor.

  if (!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) {
    Serial.println("SSD1306 allocation failed");
    while (1);
  }

• Initializes the OLED at I2C address 0x3C. If the display isn’t found, the loop stops.

  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.println("Starting...");
  display.display();
  delay(1000);

• Clears the display.
• Prints “Starting…” message as a confirmation that setup is working.

✅ 5. Loop Function – Reading & Displaying Data

a. Read Sensor Data

  float temp = dht.readTemperature(); // in Celsius
  float humi = dht.readHumidity();    // in Percent

• Reads the temperature and humidity from DHT11.
• Returns NAN (not a number) if there’s a problem.

b. Handle Read Failure

  if (isnan(temp) || isnan(humi)) {
    Serial.println("Failed to read from DHT sensor!");
    display.clearDisplay();
    display.setTextSize(1);
    display.setCursor(0, 0);
    display.println("Sensor Error!");
    display.display();
    delay(2000);
    return;
  }

• Checks if readings are valid.
• Displays “Sensor Error!” if something goes wrong.

c. Serial Output for Debugging

  Serial.print("Temp: ");
  Serial.print(temp);
  Serial.print(" °C  |  Humi: ");
  Serial.print(humi);
  Serial.println(" %");

• Shows readings in the Serial Monitor for testing.

d. Display on OLED

  display.clearDisplay();           // Erase old display
  display.setTextSize(2);          // Set large font
  display.setCursor(0, 0);
  display.print("Temp: ");
  display.print((int)temp);        // Integer only
  display.print((char)247);        // Degree symbol
  display.println("C");

• Displays temperature like: Temp: 28°C.

  display.setCursor(0, 18);
  display.print("Humi: ");
  display.print((int)humi);        // Integer only
  display.println("%");

• Displays humidity like: Humi: 60%.

  display.display();               // Push to screen
  delay(2000);               // Update every 2 seconds