Humidity and temperature (DHT11/DHT22) in Arduino - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of the DHT11 and DHT22 sensors?

They measure humidity and temperature in the environment.

beginner

How do you connect a DHT sensor to an Arduino board?

Connect the sensor's power pin to 5V, ground pin to GND, and data pin to a digital input pin on the Arduino.

beginner

Which Arduino library is commonly used to read data from DHT11/DHT22 sensors?

The DHT sensor library by Adafruit is commonly used.

intermediate

What is the difference between DHT11 and DHT22 sensors?

DHT22 is more accurate and measures a wider range of humidity and temperature than DHT11.

intermediate

Why do you need to add a delay between readings when using DHT sensors?

Because the sensor needs time to stabilize and provide accurate readings; reading too fast can cause errors.

Which pin on the Arduino should the data pin of a DHT sensor be connected to?

A A digital input/output pin

B The 5V power pin

C The ground pin

D The analog input pin only

The data pin connects to a digital input/output pin to send and receive data.

What is the typical delay time recommended between DHT sensor readings?

A 2 seconds

B 10 seconds

C 1 millisecond

D No delay needed

A delay of about 2 seconds is recommended to allow the sensor to stabilize.

Which sensor is more accurate and supports a wider range: DHT11 or DHT22?

A Neither measures temperature

B DHT11

C Both are the same

D DHT22

DHT22 is more accurate and supports a wider range of humidity and temperature.

Which Arduino library is commonly used to read DHT sensors?

A Wire library

B Servo library

C DHT sensor library

D SPI library

The DHT sensor library by Adafruit is designed for these sensors.

What does the DHT sensor measure?

A Light intensity

B Humidity and temperature

C Sound levels

D Pressure

DHT sensors measure humidity and temperature.

Explain how to set up and read data from a DHT11 or DHT22 sensor using Arduino.

Think about wiring, library usage, and timing.

Describe the differences between the DHT11 and DHT22 sensors and when you might choose one over the other.

Consider accuracy, range, and cost.

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