Temperature and humidity: your first DHT11 readings

What we will build

We will connect a DHT11 to the ESP32 and display air temperature and relative humidity in Serial Monitor. The program attempts a new reading every 2.5 seconds. A successful result includes both values and their units; an unsuccessful result produces a clear error message.

We will learn to handle numerical sensor data and unavailable measurements with equal care.

What you will learn

• Connect the power supply, DATA line and external pull-up resistor.
• Install the correct Arduino library and its dependency.
• Read temperature in °C and relative humidity as a percentage.
• Schedule readings with millis().
• Recognise NaN and report an error without inventing a zero value.

Prerequisites

Complete courses 01 and 07: sketch uploads, Serial Monitor and module power with common ground. We now move from a binary sensor output to a library for its data protocol.

Required equipment

Use a classic ESP32-WROOM-32 and a three-terminal DHT11 module confirmed to work at 3.3 V. Check the physical order of VCC, DATA and GND. A bare four-pin DHT11 has a different pin arrangement; do not substitute it based on this diagram's appearance.

Understanding the measurements

Temperature describes thermal conditions around the sensor. Relative humidity also depends on temperature; its percentage is not the percentage of air volume occupied by water. The DHT11 is useful for learning to acquire measurements, but displaying a decimal place does not establish that level of accuracy. The Adafruit DHT overview describes its limitations.

The DHT11 uses a digital protocol on one DATA line. This is neither an analogue output nor I²C. The library handles signal timing, decodes data and checks the transmission. The Adafruit library implementation shows its timing, checksum and failed-reading handling.

Wiring and assembly

Wiring diagram

Use a three-terminal DHT11 module verified for 3.3 V operation. R2 is an additional 10 kΩ pull-up to 3V3; DATA and GPIO26 share one node. Do not move a module with an onboard pull-up to 5 V without adapting the logic levels.

Connections · Wiring diagram

From	To	Connection
ESP32.3V3	DHT1.VCC	Use a DHT11 module that operates reliably at 3.3 V.
ESP32.GND	DHT1.GND	Common ground.
DHT1.DATA	ESP32.GPIO26	Bidirectional DHT11 data line.
ESP32.3V3	R2.1	R2 is a 10 kΩ pull-up to 3V3.
R2.2	DHT1.DATA	Connect the other end of R2 to the DATA/GPIO26 node.

1. Disconnect USB. Verify that your exact module's documentation permits a 3.3 V supply.
2. Connect module VCC to 3V3 and module GND to ESP32 GND.
3. Connect DATA to GPIO26.
4. Add R2, 10 kΩ between DATA and 3V3. It is not in series with the signal wire: one end joins the DATA/GPIO26 connection and the other joins 3V3.
5. If the module already has a pull-up, confirm that it also connects to 3.3 V. Our reference circuit retains the external R2 to make this connection explicit.
6. Inspect the short wires and terminals, then power the board through USB.

A pull-up returns the data line to a high level when communicating devices are not pulling it towards ground. The Adafruit wiring guide specifies a 10 kΩ resistor and notes that 3.3 V is insufficient for some units. If your module is unreliable at 3.3 V, verify its variant; do not simply power it at 5 V, because an onboard pull-up could then apply 5 V to GPIO26.

Arduino setup

In Arduino IDE 2.x, select ESP32 Dev Module, the correct port and esp32 by Espressif Systems 3.3.12. In Library Manager, install:

• DHT sensor library by Adafruit, version 1.4.7.
• Adafruit Unified Sensor, version 1.1.15, as the package dependency.

Check the author: similarly named libraries can expose different APIs. See the Adafruit Arduino guide. Our sketch uses DHT without the Unified API layer, but we install its declared dependency.

Extract the example, open dht11_monitor/dht11_monitor.ino and keep the matching folder name. Set Serial Monitor to 115200 baud.

Complete Arduino sketch

#include <Arduino.h>
#include <DHT.h>
#include <math.h>

constexpr uint8_t DHT_PIN = 26;
constexpr uint32_t SAMPLE_MS = 2500;
DHT dht(DHT_PIN, DHT11);
uint32_t lastSampleAt = 0;

void setup() {
  Serial.begin(115200);
  dht.begin();
  lastSampleAt = millis();
  Serial.println("DHT11: first sample after 2.5 seconds; units are Celsius and percent RH.");
}

void loop() {
  const uint32_t now = millis();
  if (static_cast<uint32_t>(now - lastSampleAt) < SAMPLE_MS) return;
  lastSampleAt = now;
  const float humidity = dht.readHumidity();
  const float temperature = dht.readTemperature();
  if (isnan(humidity) || isnan(temperature)) {
    Serial.println("SENSOR ERROR: DHT11 read failed; check DATA, 3.3 V, GND and pull-up.");
    return;
  }
  Serial.print("T_C="); Serial.print(temperature, 1);
  Serial.print(" RH_pct="); Serial.println(humidity, 1);
}

Run the supplied sketch unchanged first. It uses DHT_PIN = 26, sensor type DHT11 and SAMPLE_MS = 2500.

Reading one sample

The object DHT dht(DHT_PIN, DHT11) associates the selected pin with the sensor type. dht.begin() prepares communication. The program stores lastSampleAt and makes its first attempt only after 2500 ms have passed from the schedule's starting point in setup().

Calls to readHumidity() and readTemperature() return humidity and temperature. Temperature defaults to Celsius. For consecutive calls, the library reuses the stored result of the same communication, so this pair should not be interpreted as two independent measurements at the same instant. Our chosen interval exceeds the library's internal two-second minimum.

Before printing, the program applies isnan() to both values. If either is invalid, it rejects the entire pair and reports SENSOR ERROR. Zero is a possible numerical value; it must not represent a failed transmission. The program continues and tries again at the next scheduled interval.

Running and interpreting the output

Click Verify, then Upload. Open Serial Monitor and wait for the first attempt. Successful messages use T_C and RH_pct; errors are labelled separately. Readings depend on your room, placement and sensor; no number here represents your experiment's actual outcome.

Do not expect every sample to change. Keep the sensor away from the warm ESP32 board, direct sunlight and contact with your fingers. Allow it time to respond to changed conditions. This arrangement is an indoor learning exercise; its enclosure and measurement setup are not prepared for outdoor weather exposure.

Experiment and observation log

Record ten consecutive attempts. For each, write down the message time, temperature, relative humidity and status. Leave the measurement fields blank when an error occurs.

For the final experiment, disconnect USB, remove the DATA connection, reconnect power and observe. Disconnect USB again before restoring the wire.

Troubleshooting

Independent challenge

Increase SAMPLE_MS to 5000 and observe the message interval. Then work through these fictional temperatures on paper: 22.0; invalid; 24.0. Calculate the mean of valid readings and compare it with the incorrect mean obtained by replacing the error with zero.

Check your understanding

1. Why does R2 connect to 3V3 rather than 5 V?
2. Does a communication error mean that the temperature is 0 °C?
3. Would more frequent printing make the DHT11 more accurate?

Worked guidance and answers

With an interval of 5000 ms, new attempts occur approximately every five seconds. The mean of valid readings 22.0 and 24.0 is 23.0 °C. Including an invented zero would produce approximately 15.3 °C and hide the missing data. This calculation is an exercise, not a measurement made on a board.

Pulling up to 3V3 maintains an appropriate DATA high level. NaN represents an invalid number, not a zero temperature. A higher printing rate cannot improve the sensor's measurement characteristics. Restore the interval to 2500 ms before the indoor station course.

Downloads

Primary references

• Adafruit: DHT sensor overview.
• Adafruit: power supply and pull-up connection.
• Adafruit: using DHT sensors with Arduino.
• Adafruit: DHT library releases.
• Adafruit: Unified Sensor releases.
• Adafruit: DHT protocol implementation.