Read a DHT22 on Arduino Uno

A DHT22 measures two things at once: air temperature and relative humidity. It is one of the first sensors people add to an Arduino, because it needs only three wires and a short sketch. In this post you will read a DHT22 on an Arduino Uno in your browser, print both numbers to the serial monitor, and learn what to do when the sensor answers "NaN".

What you need

An Arduino Uno
A DHT22 sensor (Mokxi draws the white DHT22 and the blue DHT11 from one part, and a model property picks which one)
Three jumper wires

Nothing here needs a resistor on the breadboard. The three-pin DHT22 module that kits ship already carries a pull-up resistor on its data line, and the Mokxi part models that. More on the pull-up below.

Wire up the DHT22

The sensor has three pins: VCC, DATA and GND. Connect them like this.

DHT22 pin	Uno pin	What it does
VCC	5V	Power. The DHT22 accepts 3.3 to 6 volts.
DATA	2	One wire that carries both readings.
GND	GND	Ground.

Drag the Uno and the DHT onto the canvas, draw the three wires, and click the sensor. You will see two sliders on the part: one for temperature and one for humidity. In a real room you cannot slide the weather, but here the slider is the world, so you can test your code at any value.

The library

The sketch uses the DHT sensor library, which talks to the sensor for you. It sends the start signal, times the pulses the sensor sends back and turns them into numbers.

In Mokxi the header DHT.h is already built in under its usual name, so the include line below compiles as it stands. If you ever need a different library, the Arduino Library Manager search under the code editor finds any library in the official index by name, and installs it in one tap. The help page on compile errors explains how.

The complete sketch

#include <DHT.h>

const int DHT_PIN = 2;

DHT dht(DHT_PIN, DHT22);

void setup() {
  Serial.begin(9600);
  dht.begin();
  Serial.println("DHT22 ready");
}

void loop() {
  delay(2000);

  float humidity = dht.readHumidity();
  float celsius = dht.readTemperature();

  if (isnan(humidity) || isnan(celsius)) {
    Serial.println("Read failed (NaN). Check the DATA wire and the pull-up.");
    return;
  }

  float fahrenheit = celsius * 9.0 / 5.0 + 32.0;

  Serial.print("Humidity: ");
  Serial.print(humidity, 1);
  Serial.print(" %  Temperature: ");
  Serial.print(celsius, 1);
  Serial.print(" C (");
  Serial.print(fahrenheit, 1);
  Serial.println(" F)");
}

I compiled this sketch for the Uno in Mokxi's own build path before writing this post. Press Run, open the serial monitor, and with the sliders at 22 C and 45 percent you should see Humidity: 45.0 % Temperature: 22.0 C (71.6 F) every two seconds.

Line by line

#include <DHT.h> brings in the library.
const int DHT_PIN = 2; names the pin once, so you change it in one place.
DHT dht(DHT_PIN, DHT22); makes a sensor object. The second argument says which model you have. Use DHT11 for the blue one.
Serial.begin(9600); opens the serial monitor at 9600 baud.
dht.begin(); gets the sensor ready.
delay(2000); waits two seconds. A DHT22 can only produce a fresh reading about every two seconds, so asking more often gains nothing. The Mokxi part behaves the same way: ask again inside two seconds and it hands back the reading it already had.
readHumidity() and readTemperature() return a float. Temperature is in Celsius unless you pass true.
isnan(...) checks for "not a number", which is how the library says a read failed.
return; skips the printing when a read fails, so you never print nonsense.
The last block converts Celsius to Fahrenheit and prints one tidy line. The 1 in print(humidity, 1) means one digit after the decimal point.

Why you see NaN

NaN stands for "not a number". The library returns it when the sensor did not answer properly. Three causes cover almost every case.

No pull-up on the data line. The DHT22 only ever pulls the DATA wire low and lets go. Something has to pull it back up to a high level, and that something is a pull-up resistor, usually 5.1 k or 10 k ohms. A pull-up resistor keeps a wire at a known high level when nothing is driving it. The three-pin module has one built in. A bare four-pin sensor does not, so you add one between DATA and VCC. The Mokxi part has a pullup property that is on by default. Turn it off and the line floats, and your sketch prints the NaN message. It is a safe way to see the failure.
Reading too soon after power-up. Both datasheets say to leave the sensor alone for about a second after power arrives. Mokxi models that, so a read in the first second goes unanswered. The delay(2000) at the top of loop() keeps you clear of it.
The wrong pin or a loose wire. Check that DATA goes to the pin in DHT_PIN.

Try next

Change DHT22 to DHT11 in the sketch and set the part's model property to match. Notice that the DHT11 range is narrower: 0 to 50 C and 20 to 90 percent, according to the part's datasheet figures.
Add an LED on pin 8 behind a 220 ohm resistor and light it when humidity goes above a number you choose.
Print only when the reading changes by half a degree, so the monitor stays quiet in a steady room.
Read the Arduino DHT22 guide for a second wiring and a longer walkthrough, and the DHT part page for the pins.

Try it on Mokxi

Open the DHT sensor part to see the pins, and the Arduino Uno board page for the Uno's header.

Ready to build it yourself? Create your free Mokxi account and open the editor in your browser. No installs, no parts to buy.