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Built-in project · Arduino Uno R3

DHT11 temperature and humidity on pin 10, on an Arduino Uno

A DHT11 on pin 10, with a 10k pull-up on its data line, reports the humidity and the temperature to the serial monitor once a second. The whole build, an Arduino Uno and 1 more part, runs here in your browser on the firmware below; open it in the editor to change the wiring or the code and run it again.

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Beginner About 10 minutes to build and run. Runs in your browser. Free, and no account needed.

The parts retail for about $34. On Mokxi they're free.

The circuit runs here once the page has loaded.

What you'll learn

  • Using a library with #include
  • Reading a one-wire digital sensor
  • Checking for bad readings with isnan
  • Printing labeled values to the serial monitor
  • Difficulty: Beginner
  • Time: About 10 minutes
  • Board: Arduino Uno R3
  • Parts: 3

How it works

The sensor has three connections: 5V, GND, and a DATA line to pin 10. The sketch asks for a reading. The sensor answers with 40 bits of timed pulses that hold humidity, temperature and a checksum.

The DHT library does the timing. If the checksum fails, the library hands back NaN, which means "not a number". The sketch checks for that and prints "Read error". Otherwise it prints the humidity and the temperature in Celsius once a second. Click the sensor in the simulator to change the weather.

Parts list

3 parts, plus the jumper wires. They retail for about $34; on Mokxi they're free.

Kits that can build it

These 8 starter kits have every part in the circuit. Each kit's page lists what else it can build.

Wiring table

Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.

Part and pinConnects to
Temperature and humidity, DHT22 pin GNDArduino Uno R3 pin GND
Temperature and humidity, DHT22 pin DATAArduino Uno R3 pin 10
Temperature and humidity, DHT22 pin VCCArduino Uno R3 pin 5V

The code

The firmware exactly as the editor opens it. Change a line there and press Run: it compiles in the browser.

sketch.ino · Arduino Uno R3
// Temperature and humidity: a DHT11 on pin 10.
//
//   DHT11 VCC -> 5V   DATA -> pin 10 (10k from DATA to 5V)   GND -> GND
//
// The DHT11 answers a start signal with 40 bits of pulses: humidity,
// temperature and a checksum. The DHT library times them; a reading that
// fails its checksum comes back as NaN, and the sketch says so.
//
// Click the sensor to change the weather.

#include <DHT.h>

DHT dht(10, DHT11);

void setup() {
  Serial.begin(9600);
  dht.begin();
  Serial.println("UNO is ready!");
}

void loop() {
  float humidity = dht.readHumidity();
  float temperature = dht.readTemperature();
  if (isnan(humidity) || isnan(temperature)) {
    Serial.println("Read error");
  } else {
    Serial.print("Humidity: ");
    Serial.print(humidity);
    Serial.print("%, Temperature: ");
    Serial.print(temperature);
    Serial.println("C");
  }
  delay(1000);
}

Code walkthrough

The key functions of the sketch, and what each one does.

setup sketch.ino

Opens serial at 9600 baud and calls dht.begin() to get the sensor ready. The sensor object dht was made at the top with pin 10 and the type DHT11. It then prints "UNO is ready!".

void setup() {
  Serial.begin(9600);
  dht.begin();
  Serial.println("UNO is ready!");
}

loop sketch.ino

Calls readHumidity and readTemperature. If either one is NaN, it prints "Read error". Otherwise it prints both values with labels. Then it waits 1000 ms, because these sensors are slow and need a pause between readings.

void loop() {
  float humidity = dht.readHumidity();
  float temperature = dht.readTemperature();
  if (isnan(humidity) || isnan(temperature)) {
    Serial.println("Read error");
  } else {
    Serial.print("Humidity: ");
    Serial.print(humidity);
    Serial.print("%, Temperature: ");
    Serial.print(temperature);
    Serial.println("C");
  }
  delay(1000);
}

Troubleshooting

The usual mistakes with this circuit, and the fix for each.

  • "Read error" keeps printing This project
    Check the three wires: VCC to 5V, GND to GND, and DATA to pin 10.
  • The readings do not match the sensor type This project
    The sketch creates DHT dht(10, DHT11). If you use a different sensor type, change that type in the code.
  • Readings are NaN or fail, or never appear. Temperature and humidity
    Put a 10 k ohm pull-up between DATA and VCC (bare sensors need it). A 4-pin DHT22 has pin 3 unused. Check the data pin number in your code.
  • Values freeze or errors show up when you read often. Temperature and humidity
    The DHT22 can only be read about once every 2 seconds. Wait between reads, and check for NaN before using the numbers.
  • Upload fails or nothing runs after wiring pins 0 and 1 to a module. Arduino Uno R3
    Pins 0 and 1 are the USB serial port. Move the module to other pins, or unplug it while uploading. Also check the right board is selected.
  • Pins misbehave or the board resets when you power motors or servos from it. Arduino Uno R3
    Each I/O pin gives about 20 mA, 40 mA max. Power motors and servos from their own supply and join its ground to the Uno's GND.

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