Tutorial 16: DHT22 Temperature & Humidity Sensor Guide

Introduction to DHT22

The DHT22 (also known as AM2302) is a popular digital temperature and humidity sensor. It's known for its good accuracy, low cost, and ease of use with microcontrollers.

Pinout and Wiring

The DHT22 has 4 pins (some modules have 3 pins with built-in resistor):

DHT22 Pinout:
┌─────────────────┐
│   ┌─────────┐   │
│   │  DHT22  │   │
│   └─────────┘   │
│                 │
│  ┌───┐ ┌───┐   │
│  │ 1 │ │ 2 │   │  ← Pin 1: VCC, Pin 2: DATA
│  └───┘ └───┘   │
│  ┌───┐ ┌───┐   │
│  │ 3 │ │ 4 │   │  ← Pin 3: NC, Pin 4: GND
│  └───┘ └───┘   │
└─────────────────┘

Wiring to ESP8266 (D1 Mini)

ESP8266 D1 Mini + DHT22 Wiring:
┌─────────┐                    ┌──────────┐
│ D1 Mini │                    │  DHT22   │
├─────────┤                    ├──────────┤
│   3.3V  ├────────────────────┤ VCC (1)  │
│    D2   ├────────────────────┤ DATA (2) │
│   GND   ├────────────────────┤ GND (4)  │
└─────────┘                    └──────────┘

Optional: 4.7kΩ pull-up resistor between 3.3V and DATA

Wiring to ESP32

Wiring to Raspberry Pi Pico W

Installing the DHT Library

1. Open Arduino IDE
2. Go to Sketch → Include Library → Manage Libraries
3. Search for "DHT sensor library"
4. Find "DHT sensor library by Adafruit" and click Install
5. It will ask to install dependencies. Click Install All (includes "Adafruit Unified Sensor")

How OceanRemote Uses DHT22

When you select DHT22 in the device configuration, OceanRemote firmware automatically:

• Initializes the DHT sensor on the correct pin
• Reads temperature and humidity every 5 seconds (respects DHT22's 2-second minimum interval)
• Sends both values to the server with each poll
• Displays temperature and humidity on your dashboard
• Allows temperature calibration via offset (dashboard setting)

// How OceanRemote reads DHT22 (firmware snippet)
#include <DHT.h>

#define DHTPIN 4    // GPIO4 on ESP8266 (D2)
#define DHTTYPE DHT22

DHT dht(DHTPIN, DHTTYPE);

void readSensor() {
  static unsigned long lastDHTRead = 0;
  unsigned long now = millis();
  
  if (now - lastDHTRead >= 2000) {
    lastDHTRead = now;
    temperature = dht.readTemperature();
    humidity = dht.readHumidity();
    
    if (isnan(temperature) || isnan(humidity)) {
      temperature = -999;
      humidity = -999;
    }
  }
}

Testing Your DHT22

Before flashing the full OceanRemote firmware, test your DHT22 with this simple sketch:

#include <DHT.h>

#define DHTPIN 4    // Change to your GPIO pin
#define DHTTYPE DHT22

DHT dht(DHTPIN, DHTTYPE);

void setup() {
  Serial.begin(115200);
  dht.begin();
  Serial.println("DHT22 Test Started");
}

void loop() {
  delay(2000);
  
  float h = dht.readHumidity();
  float t = dht.readTemperature();
  
  if (isnan(h) || isnan(t)) {
    Serial.println("DHT22 read failed!");
  } else {
    Serial.print("Temperature: ");
    Serial.print(t);
    Serial.print("°C, Humidity: ");
    Serial.print(h);
    Serial.println("%");
  }
}

Calibrating DHT22 Readings

OceanRemote allows you to calibrate temperature readings using an offset. This is useful if your DHT22 consistently reads high or low.

How to Calibrate:

1. Place a known-accurate thermometer next to your DHT22
2. Note the difference between the two readings
3. In your OceanRemote dashboard, find your device's temperature card
4. Enter the offset value in the "Calibration Offset (±°C)" field
5. The formula is: Calibrated = Raw Reading + Offset

Example:
Raw reading: 23.5°C
Actual temperature: 22.8°C
Difference: -0.7°C
Enter offset: -0.7

Result: 23.5 + (-0.7) = 22.8°C ✓

Troubleshooting DHT22

Problem: No readings or "NaN" values

• Check wiring - DATA pin must be connected correctly
• Add a 4.7kΩ pull-up resistor between DATA and 3.3V
• Verify you selected the correct pin in configuration
• Try a different GPIO pin

Problem: Readings are 0 or -999

• Sensor may be damaged - try another DHT22
• Check power supply - DHT22 needs stable 3.3V or 5V
• Wiring may be loose - check connections

Problem: Temperature is inaccurate

• Use calibration offset in dashboard
• DHT22 has ±0.5°C accuracy - some variation is normal
• Avoid placing sensor near heat sources (ESP8266 gets warm!)
• Keep sensor away from direct sunlight

Problem: Humidity readings are inaccurate

• DHT22 humidity accuracy is ±2% - some variation is normal
• Sensor needs time to acclimate to environment (15-30 minutes)
• Avoid touching the sensor element with fingers (oils affect readings)

DHT22 Best Practices

• Mounting: Keep sensor away from the microcontroller (ESP8266/ESP32 generates heat)
• Ventilation: Ensure airflow around the sensor for accurate readings
• Cable Length: Keep wires under 20 meters for reliable communication
• Power: Use 3.3V or 5V consistently (don't switch between them)
• Sampling Rate: Don't read more than once every 2 seconds (OceanRemote respects this)
• Waterproof Version: Available for outdoor use (encapsulated in stainless steel probe)

Next Steps

Now that you understand the DHT22 sensor, continue with:

• Tutorial 17: DS18B20 Digital Temperature Sensor Guide
• Tutorial 18: NTC 10kΩ Thermistor Guide
• Tutorial 20: Calibrating Temperature Sensors with Offset
• Return to Tutorial 02: Generate firmware with DHT22 support!