DHT11 Temperature and Humidity Sensor: Arduino, ESP32 and Pico Guide

The DHT11 is a cheap digital sensor in a blue plastic case that reads both temperature and relative humidity over a single data wire. You'll find it in almost every Arduino weather station, greenhouse and plant monitor, nursery thermometer and smart home project that switches on a fan when humidity rises. Unlike an analog sensor, you don't measure a voltage and fiddle with formulas: the sensor sends its reading as a ready-made number.

What is the DHT11 and where is it used?

The humidity it measures is relative humidity: the percentage of the maximum water vapour the air can hold at that temperature. 40–60% is considered comfortable; above 70% means damp and mould, below 30% means dry air. The DHT11 measures this value and the ambient temperature about once per second.

• Home weather station: show temperature and humidity on an LCD or OLED display.
• Greenhouse and plant monitoring: warn when humidity drops, and water together with a soil moisture sensor.
• Damp and mould alert: sound an alarm when a bathroom, basement or wardrobe goes above 70% humidity.
• Automatic fan or dehumidifier: switch the device on with a relay when a humidity threshold is crossed.
• Data logger: write readings to an SD card and plot a daily chart.
• Remote monitoring: use an ESP32 to publish the values on a web page you can open on your phone.

How does the DHT11 work?

There are two sensing elements inside the case. The humidity element is a moisture-absorbing material between two electrodes; as the humidity in the air rises, its resistance drops. Temperature is measured with an NTC thermistor: a small resistor whose resistance falls as it gets warmer. A tiny microcontroller inside the sensor measures both resistances, corrects them with calibration values stored at the factory and sends the result digitally.

Communication happens over a single wire. The Arduino wakes the sensor by holding the data line LOW for at least 18 milliseconds, and the sensor replies with a 40-bit packet: 16 bits of humidity, 16 bits of temperature and an 8-bit checksum to catch bad readings. You don't have to write this timing yourself; the library we'll install below handles all of it.

Specifications: DHT11 vs DHT22

In short: if you want a rough reading of indoor temperature and humidity, the DHT11 is enough and cheaper. Outdoors, below freezing, or wherever half a degree matters, choose the DHT22. Both are wired and coded the same way; you only change the sensor type in the code.

DHT11 pinout and wiring

The DHT11 is sold in two forms: a 3-pin module with a resistor and LED on the board, or a 4-leg bare sensor. On the bare sensor, count the legs from left to right with the grilled face towards you.

Installing the DHT library

1. Open the Library Manager in the Arduino IDE (the book icon in the left sidebar of Arduino IDE 2).
2. Type DHT sensor library in the search box and install the library by Adafruit.
3. When it asks about dependencies, choose Install all so that Adafruit Unified Sensor is installed too.
4. If you like, try the ready-made example under File > Examples > DHT sensor library > DHTtester.

Arduino example code: temperature, humidity and heat index

The code below takes a reading every two seconds, reports when a reading fails and combines temperature and humidity to calculate the heat index (how hot it feels). Open the Serial Monitor at 9600 baud to see the results.

1#include <DHT.h>
2
3#define DHT_PIN 2          // veri pini / data pin
4#define DHT_TYPE DHT11     // DHT22 kullanıyorsan DHT22 yaz / write DHT22 for a DHT22
5
6DHT dht(DHT_PIN, DHT_TYPE);
7
8void setup() {
9  Serial.begin(9600);
10  dht.begin();
11}
12
13void loop() {
14  delay(2000);                                // ölçümler arası bekleme / wait between readings
15
16  float humidity = dht.readHumidity();        // % bağıl nem / % relative humidity
17  float temperature = dht.readTemperature();  // °C
18
19  if (isnan(humidity) || isnan(temperature)) {
20    Serial.println("Okuma hatası / read error");
21    return;
22  }
23
24  // hissedilen sıcaklık / heat index (°C)
25  float heatIndex = dht.computeHeatIndex(temperature, humidity, false);
26
27  Serial.print("Sicaklik / Temp: ");
28  Serial.print(temperature);
29  Serial.print(" C  Nem / Humidity: %");
30  Serial.print(humidity);
31  Serial.print("  Hissedilen / Feels like: ");
32  Serial.print(heatIndex);
33  Serial.println(" C");
34}

Using the DHT11 with the ESP32

The same code runs on the ESP32; just set DHT_PIN to, for example, 4. Power the sensor from the ESP32's 3V3 pin so the data line also stays at 3.3 V; the DHT11 works comfortably at this voltage. GPIO 34–39 are input-only, so they're not suitable for a DHT, because the data line is bidirectional. When picking a pin, check which ESP32 pins are safe in the Pinout tool. You can show the readings on a web page over Wi-Fi and monitor them from your phone.

DHT11 with the Raspberry Pi Pico (MicroPython)

MicroPython has a built-in dht module for DHT sensors, so there's no library to install. Connect the sensor to the Pico's 3V3 pin, GND and pin GP15:

1import dht
2from machine import Pin
3import time
4
5sensor = dht.DHT11(Pin(15))   # GP15 veri pini / data pin (DHT22 için dht.DHT22)
6
7while True:
8    try:
9        sensor.measure()
10        t = sensor.temperature()   # °C
11        h = sensor.humidity()      # %
12        print("Sicaklik:", t, "C  Nem: %", h)
13    except OSError:
14        print("Okuma hatası / read error")
15    time.sleep(2)

DHT11 project ideas

• Digital thermometer with an LCD: write the values to a 16x2 display; see how in Using an I2C LCD Display.
• Humidity-controlled fan: run a fan through a relay when humidity passes 65%; for the wiring see What Is a Relay Module?
• Damp alarm: sound a buzzer when the threshold is crossed; What Is a Buzzer? covers audible alerts.
• Temperature logger: save a reading every 10 minutes with a clock module and an SD card.
• Wi-Fi weather station: publish the readings with an ESP32 on a page any device on your home network can open.

Common mistakes and how to fix them

• Constant NaN or "Failed to read": the data wire may be on a different pin from the one in the code, the pull-up resistor may be missing, or the code may use the wrong sensor type (DHT22 instead of DHT11).
• Reading too often: a sensor read at intervals shorter than 2 seconds returns the old value or an error.
• Temperature higher than it should be: the sensor may be too close to the Arduino's voltage regulator or in the sun; place it a few centimetres away from the board.
• Dropped readings on long cables: the signal weakens after a few metres; lower the pull-up to 4.7 kΩ, power the sensor at 5 V and use good-quality cable.
• Unstable on the ESP32: if the sensor is powered at 5 V, the data line is pulled up to 5 V; power it from 3V3 and use a GPIO that can be both input and output (for example 4).
• Expecting an instant response: it takes a few seconds to register a change in humidity; the DHT11 is designed for monitoring a room, not for fast measurements.

If you're wondering what other sensors you can use, take a look at the Arduino Sensors Guide, and to measure distance, see HC-SR04 Distance Sensor. In the BlueGrays app you'll find step-by-step DHT11 and DHT22 projects with circuit diagrams and code.