How DHT11 DHT22 Sensors Work & Interface With Arduino

Its summer, you’re sweating heavily and all you really want to know is why the weatherman predicted this morning that today’s relative temperature would max out to 20°C while it feels more like 25°C. Enter the DHT11, DHT22/AM2302 Digital Temperature & Humidity Sensor from AOSONG – the best way to prove the weatherman wrong!

These sensors are fairly simple to use, low cost and are great for hobbyists who want to sense the world around them! These sensors are pre-calibrated and don’t require extra components so you can start measuring relative humidity and temperature right away.

One of the greatest features they provide is that both temperature & humidity are measured to the nearest tenth; that is, to one decimal place. The only downside of this sensor is you can get new data from it once every second or two. But, considering its performance and price, you can’t complain.

DHT11 Vs DHT22/AM2302

We have two versions of the DHTxx sensor series. They look a bit similar and have the same pinout, but have different characteristics. Here are the details:

The DHT22 is the more expensive version which obviously has better specifications. Its temperature measuring range is from -40°C to +125°C with +-0.5 degrees accuracy, while the DHT11 temperature range is from 0°C to 50°C with +-2 degrees accuracy. Also the DHT22 sensor has better humidity measuring range, from 0 to 100% with 2-5% accuracy, while the DHT11 humidity range is from 20 to 80% with 5% accuracy.

Though DHT22/AM2302 is more precise, more accurate and works in a bigger range of temperature & humidity; there are three things where the DHT11 beats the hell out of DHT22. It’s less expensive, smaller in size and has higher sampling rate. The sampling rate of the DHT11 is 1Hz i.e. one reading every second, while the sampling rate of DHT22 is 0.5Hz i.e. one reading every two seconds.

However, the operating voltage of both sensors is from 3 to 5 volts, while the max current used during conversion (while requesting data) is 2.5mA. And the best thing is that DHT11 and DHT22/AM2302 sensors are ‘swappable’ – meaning, if you build your project with one you can just unplug it and use another. Your code may have to adjust a little but at least the wiring is the same!

For a little more insight, please refer below datasheets for DHT11 and DHT22/AM2302 sensor.DHT11 DatasheetDHT22 Datasheet

Hardware Overview

Now let’s move on to the interesting stuff. Let’s teardown both the DHT11 and DHT22/AM2302 sensors and see what’s inside.

The casing is in two parts so to get inside it is just a matter of getting a sharp knife and splitting the case apart. Inside the case, on the sensing side, there is a humidity sensing component along with a NTC temperature sensor (or thermistor)

Humidity sensing component is used, of course to measure humidity, which has two electrodes with moisture holding substrate (usually a salt or conductive plastic polymer) sandwiched between them. The ions are released by the substrate as water vapor is absorbed by it, which in turn increases the conductivity between the electrodes. The change in resistance between the two electrodes is proportional to the relative humidity. Higher relative humidity decreases the resistance between the electrodes, while lower relative humidity increases the resistance between the electrodes.

Besides, they consist of a NTC temperature sensor/Thermistor to measure temperature. A thermistor is a thermal resistor – a resistor that changes its resistance with temperature. Technically, all resistors are thermistors – their resistance changes slightly with temperature – but the change is usually very very small and difficult to measure.

Thermistors are made so that the resistance changes drastically with temperature so that it can be 100 ohms or more of change per degree! The term “NTC” means “Negative Temperature Coefficient”, which means that the resistance decreases with increase of the temperature.

On the other side, there is a small PCB with an 8-bit SOIC-14 packaged IC. This IC measures and processes the analog signal with stored calibration coefficients, does analog to digital conversion and spits out a digital signal with the temperature and humidity.

DHT11 And DHT22/AM2302 Pinout

The DHT11, DHT22/AM2302 sensors are fairly easy to connect. They have four pins:

VCC pin supplies power for the sensor. Although supply voltage ranges from 3.3V to 5.5V, 5V supply is recommended. In case of 5V power supply, you can keep the sensor as long as 20 meters. However, with 3.3V supply voltage, cable length shall not be greater than 1 meter. Otherwise, the line voltage drop will lead to errors in measurement.

Data pin is used to communication between the sensor and the microcontroller.

NC Not connected

GND should be connected to the ground of Arduino.

Wiring – Connecting DHT11 And DHT22/AM2302 To Arduino UNO

Now that we have a complete understanding of how DHT sensors work, we can begin hooking it up to our Arduino!

Luckily, it is trivial to connect DHT11, DHT22/AM2302 sensors to Arduino. They have fairly long 0.1″-pitch pins so you can easily plug them into any breadboard. Power the sensor with 5V and connect ground to ground. Finally, connect the Data pin to a digital pin #2.

Remember, as discussed earlier in this tutorial, we need to place a pull-up resistor of 10KΩ between VCC and data line to keep it HIGH for proper communication between sensor and MCU. If you happen to have a breakout board of the sensor, you need not add any external pull-up. It comes with a built-in pull-up resistor.

With that, you’re now ready to upload some code and get it working.

Arduino Code – Printing Values On Serial Monitor

As discussed earlier in this tutorial, the DHT11, DHT22/AM2302 sensors have their own single wire protocol used for transferring the data. This protocol requires precise timing. Fortunately, we don’t have to worry much about this because we are going to use the DHT Library which takes care of almost everything.

Download the library first, by visiting the GitHub Repo or, just click this button to download the zip:DHTlib.Zip

To install it, open the Arduino IDE, go to Sketch > Include Library > Add .ZIP Library, and then select the DHTlib ZIP file that you just downloaded.  If you need more details on installing a library, visit this Installing An Arduino Library tutorial.

Once you have the library installed, you can copy this sketch into the Arduino IDE. The following test sketch will print the temperature and relative humidity values on the serial monitor. Try the sketch out; and then we will explain it in some detail.

#include <dht.h>
#define dataPin 8 // Defines pin number to which the sensor is connected
dht DHT; // Creats a DHT object

void setup() 
{
	Serial.begin(9600);
}
void loop() 
{
	//Uncomment whatever type you're using!
	int readData = DHT.read22(dataPin); // DHT22/AM2302
	//int readData = DHT.read11(dataPin); // DHT11

	float t = DHT.temperature; // Gets the values of the temperature
	float h = DHT.humidity; // Gets the values of the humidity

	// Printing the results on the serial monitor
	Serial.print("Temperature = ");
	Serial.print(t);
	Serial.print(" ");
	Serial.print((char)176);//shows degrees character
	Serial.print("C | ");
	Serial.print((t * 9.0) / 5.0 + 32.0);//print the temperature in Fahrenheit
	Serial.print(" ");
	Serial.print((char)176);//shows degrees character
	Serial.println("F ");
	Serial.print("Humidity = ");
	Serial.print(h);
	Serial.println(" % ");
	Serial.println("");

	delay(2000); // Delays 2 secods
}

The sketch starts by including DHT library. Next, we need to define the Arduino pin number to which our sensor’s Data pin is connected and create a DHT object. So, that we can access special functions related to the library.

#include <dht.h>
#define dataPin 8 // Defines pin number to which the sensor is connected
dht DHT; // Creats a DHT object

In the ‘setup’ function; we need to initiate the serial communication as we will use the serial monitor to print the results.

void setup() {
  Serial.begin(9600);
}

In the ‘loop’ function; we will use the read22() function which reads the data from the DHT22/AM2302. It takes sensor’s Data pin number as a parameter. If you are tinkering with DHT11, you need to use read11() function. You can do that by uncommenting second line.

//Uncomment whatever type you're using!
int readData = DHT.read22(dataPin); // DHT22/AM2302
//int readData = DHT.read11(dataPin); // DHT11

Once the humidity and temperature values are calculated, we can access them by:

float t = DHT.temperature; // Gets the values of the temperature
float h = DHT.humidity; // Gets the values of the humidity

The DHT object returns temperature value in Celsius (°C). It can be converted in to Fahrenheit (°F) using a simple formula:

T_(°F) = T_(°C) × 9/5 + 32

//print the temperature in Fahrenheit
Serial.print((t * 9.0) / 5.0 + 32.0);

At the end we will print the temperature and the humidity values on the serial monitor.

Arduino Project

Arduino Code – DHT11 And DHT22/AM2302 With LCD

Sometimes you come up with an idea where you want to monitor temperature and humidity levels in your DIY incubator. Then you’ll probably need 16×2 character LCD to display prevailing conditions in your incubator, instead of a serial monitor. So, in this example, we’ll hook the LCD up to the Arduino along with the DHT11, DHT22/AM2302 sensor.

In case you are not familiar with 16×2 character LCDs, consider reading (at least skimming) below tutorial.

Interfacing 16×2 Character LCD Module With ArduinoWant your Arduino projects to display status messages, sensor readings, or you just want to create your own endless runner game? These LCD displays might be the…

Next, we need to make connections to the LCD as shown below.

The following sketch will print the temperature and relative humidity values on the 16×2 character LCD. It uses the same code except we print values on LCD.

#include <LiquidCrystal.h> // includes the LiquidCrystal Library
#include <dht.h>
#define dataPin 8

LiquidCrystal lcd(12, 11, 5, 4, 3, 2); // Creates an LCD object. Parameters: (rs, enable, d4, d5, d6, d7)
dht DHT;
bool showcelciusorfarenheit = false;

void setup() 
{
	lcd.begin(16,2); // Initializes the interface to the LCD screen, and specifies the dimensions (width and height) of the display
}

void loop() 
{
	int readData = DHT.read22(dataPin);
	float t = DHT.temperature;
	float h = DHT.humidity;
	lcd.setCursor(0,0); // Sets the location at which subsequent text written to the LCD will be displayed
	lcd.print("Temp.: "); // Prints string "Temp." on the LCD

	//Print temperature value in Celcius and Fahrenheit every alternate cycle
	if(showcelciusorfarenheit)
	{
		lcd.print(t); // Prints the temperature value from the sensor
		lcd.print(" ");
		lcd.print((char)223);//shows degrees character
		lcd.print("C");
		showcelciusorfarenheit = false;
	}
	else
	{
		lcd.print((t * 9.0) / 5.0 + 32.0); // print the temperature in Fahrenheit
		lcd.print(" ");
		lcd.print((char)223);//shows degrees character
		lcd.print("F");
		showcelciusorfarenheit = true;
	}
	
	lcd.setCursor(0,1);
	lcd.print("Humi.: ");
	lcd.print(h);
	lcd.print(" %");
	delay(5000);
}