Why Humidity and temperature (DHT11/DHT22) in Arduino? - Purpose & Use Cases

The Big Idea

What if your project could 'feel' the air around it and act all by itself?

The Scenario

Imagine you want to measure the air temperature and humidity in your room using simple sensors. Without special tools, you might try guessing or using separate devices and writing down numbers by hand.

The Problem

Manually checking temperature and humidity is slow and can be inaccurate. Writing down numbers wastes time and can lead to mistakes. You can't easily track changes over time or react quickly to the environment.

The Solution

Using DHT11 or DHT22 sensors with Arduino lets you automatically read temperature and humidity values. The sensors send data directly to your program, which can display, log, or use it instantly without errors or delays.

Before vs After

✗ Before

// No code, just guessing or writing numbers by hand

✓ After

#include <DHT.h>
#define DHTPIN 2
#define DHTTYPE DHT11
DHT dht(DHTPIN, DHTTYPE);

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

void loop() {
  float h = dht.readHumidity();
  float t = dht.readTemperature();
  Serial.print("Humidity: " + String(h) + "% Temperature: " + String(t) + "C");
  delay(2000);
}

What It Enables

You can build smart projects that sense and respond to the environment automatically and accurately.

Real Life Example

For example, a plant watering system that checks soil humidity and air temperature to water plants only when needed, saving water and keeping plants healthy.

Key Takeaways

Manual tracking of temperature and humidity is slow and error-prone.

DHT11/DHT22 sensors automate reading these values with Arduino.

This enables smart, responsive projects that react to real-world conditions.

Practice

1. What does the DHT11 sensor measure in an Arduino project?

easy

A. Humidity and temperature

B. Light intensity

C. Sound levels

D. Distance

Solution

1. Step 1: Understand sensor purpose

   The DHT11 sensor is designed to measure environmental conditions.

2. Step 2: Identify measured values

   It specifically measures humidity and temperature, not light, sound, or distance.

3. Final Answer:

   Humidity and temperature -> Option A

4. Quick Check:

   DHT11 measures humidity and temperature [OK]

Hint: Remember DHT sensors measure air humidity and temperature [OK]

Common Mistakes:

• Confusing DHT11 with light or sound sensors
• Thinking it measures distance
• Assuming it measures pressure

2. Which of the following is the correct way to include the DHT library in an Arduino sketch?

easy

A. #include "dht11.h"

B. #include

C. #include <dht_sensor>

D. #include

Solution

1. Step 1: Recall library inclusion syntax

   Arduino libraries are included with #include and angle brackets for installed libraries.

2. Step 2: Identify correct library name

   The standard DHT library is included as #include <DHT.h>.

3. Final Answer:

   #include <DHT.h> -> Option D

4. Quick Check:

   Use #include <DHT.h> to include DHT library [OK]

Hint: Use angle brackets and correct case for library names [OK]

Common Mistakes:

• Using wrong file names
• Using quotes instead of angle brackets
• Incorrect capitalization

3. What will be the output of this Arduino code snippet if the sensor reading is successful?

float h = dht.readHumidity();
float t = dht.readTemperature();
Serial.print("Humidity: ");
Serial.print(h);
Serial.print(" %\t");
Serial.print("Temperature: ");
Serial.print(t);
Serial.println(" *C");

medium

A. Error reading sensor

B. Humidity: % Temperature: *C

C. Humidity: 45.00 % Temperature: 23.00 *C

D. Humidity: NaN % Temperature: NaN *C

Solution

1. Step 1: Understand sensor reading functions

   The functions readHumidity() and readTemperature() return float values if successful.

2. Step 2: Analyze Serial output

   The code prints humidity and temperature values with labels and units, separated by tab.

3. Final Answer:

   Humidity: 45.00 % Temperature: 23.00 *C -> Option C

4. Quick Check:

   Successful read prints values with labels [OK]

Hint: Successful reads print numbers, errors show NaN or messages [OK]

Common Mistakes:

• Expecting error message on success
• Ignoring units in output
• Confusing NaN with valid numbers

4. Identify the error in this Arduino code snippet for reading DHT sensor data:

float humidity = dht.readHumidity();
float temperature = dht.readTemperature();
if (humidity == NULL || temperature == NULL) {
  Serial.println("Failed to read from DHT sensor!");
}

medium

A. Using NULL to check float values is incorrect

B. Missing semicolon after readTemperature()

C. Serial.println syntax is wrong

D. Variables humidity and temperature should be int

Solution

1. Step 1: Understand data types and checks

   readHumidity() and readTemperature() return float, not pointers, so NULL is invalid.

2. Step 2: Correct way to check errors

   Use isnan() function to check if readings failed (NaN means not a number).

3. Final Answer:

   Using NULL to check float values is incorrect -> Option A

4. Quick Check:

   Check floats with isnan(), not NULL [OK]

Hint: Use isnan() to check float errors, not NULL [OK]

Common Mistakes:

• Checking floats against NULL
• Assuming int type for sensor data
• Syntax errors in Serial print

5. You want to display temperature in Fahrenheit using DHT22 sensor readings in Arduino. Which code snippet correctly converts Celsius to Fahrenheit after reading temperature?

float tempC = dht.readTemperature();
float tempF = ?;
Serial.print("Temperature: ");
Serial.print(tempF);
Serial.println(" *F");

hard

A. tempF = tempC * 5 / 9 + 32;

B. tempF = tempC * 9 / 5 + 32;

C. tempF = tempC / 9 * 5 + 32;

D. tempF = (tempC + 32) * 9 / 5;

Solution

1. Step 1: Recall Celsius to Fahrenheit formula

   The correct formula is Fahrenheit = Celsius * 9/5 + 32.

2. Step 2: Apply formula in code

   Assign tempF = tempC * 9 / 5 + 32; to convert temperature correctly.

3. Final Answer:

   tempF = tempC * 9 / 5 + 32; -> Option B

4. Quick Check:

   Celsius to Fahrenheit = C * 9/5 + 32 [OK]

Hint: Use C * 9/5 + 32 to convert Celsius to Fahrenheit [OK]

Common Mistakes:

• Mixing multiplication and division order
• Adding 32 before multiplying
• Using wrong formula