Humidity and temperature (DHT11/DHT22) in Arduino - Step-by-Step Execution

Concept Flow - Humidity and temperature (DHT11/DHT22)

Start

↓

Initialize DHT sensor

↓

Read humidity

↓

Read temperature

↓

Check if readings are valid

↓

Print values

↓

Wait before next read

↓

Repeat loop

The program initializes the sensor, reads humidity and temperature, checks if readings are valid, prints them or an error, then waits before repeating.

Execution Sample

Arduino

#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();
  if (isnan(h) || isnan(t)) {
    Serial.println("Failed to read from DHT sensor!");
    return;
  }
  Serial.print("Humidity: ");
  Serial.print(h);
  Serial.print(" %\t");
  Serial.print("Temperature: ");
  Serial.print(t);
  Serial.println(" *C");
  delay(2000);
}

This code reads humidity and temperature from a DHT11 sensor and prints the values every 2 seconds, or prints an error if reading fails.

Execution Table

Step	Action	Humidity (h)	Temperature (t)	Condition	Output
1	Start loop	N/A	N/A	N/A	N/A
2	Read humidity	45.3	N/A	N/A	N/A
3	Read temperature	45.3	23.7	N/A	N/A
4	Check if h or t is NaN	45.3	23.7	False	Proceed
5	Print humidity	45.3	23.7	N/A	Humidity: 45.3 %
6	Print temperature	45.3	23.7	N/A	Temperature: 23.7 *C
7	Delay 2000ms	45.3	23.7	N/A	Wait 2 seconds
8	Repeat loop	N/A	N/A	N/A	Back to step 1
9	Read humidity (error case)	NaN	N/A	N/A	N/A
10	Read temperature (error case)	NaN	NaN	N/A	N/A
11	Check if h or t is NaN	NaN	NaN	True	Print error
12	Print error	NaN	NaN	N/A	Failed to read from DHT sensor!
13	Return early	NaN	NaN	N/A	Skip printing values

💡 Loop repeats indefinitely; error case returns early to avoid printing invalid data.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 9 (error)	After Step 10 (error)
h (humidity)	N/A	45.3	45.3	45.3	NaN	NaN
t (temperature)	N/A	N/A	23.7	23.7	N/A	NaN

Key Moments - 3 Insights

Why do we check if humidity or temperature is NaN before printing?

Because the sensor might fail to read data sometimes, resulting in NaN (not a number). Checking prevents printing invalid values, as shown in steps 4 and 11 of the execution_table.

What happens if the sensor reading fails?

The program prints an error message and returns early from the loop iteration, skipping the printing of humidity and temperature values. This is shown in steps 11 to 13.

Why is there a delay after printing the values?

The delay of 2000 milliseconds (2 seconds) slows down the loop so readings are spaced out and the serial output is readable, as shown in step 7.

Visual Quiz - 3 Questions

Look at the execution_table, what is the humidity value after step 2?

A NaN

B 23.7

C 45.3

D N/A

💡 Hint

Check the 'Humidity (h)' column at step 2 in the execution_table.

At which step does the program detect a failed sensor reading?

A Step 11

B Step 4

C Step 7

D Step 2

💡 Hint

Look for the step where the condition 'h or t is NaN' is True in the execution_table.

If the delay(2000) was removed, how would the execution_table change?

A Temperature would not be read

B Step 7 would be missing and loop would run faster

C Humidity values would become NaN

D Error message would print every time

💡 Hint

Refer to step 7 where delay is applied before repeating the loop.

Concept Snapshot

Use DHT library to read humidity and temperature.
Check if readings are valid (not NaN) before printing.
Print values to Serial Monitor.
Use delay to space out readings.
Loop repeats indefinitely for continuous monitoring.

Full Transcript

This Arduino program reads humidity and temperature from a DHT11 or DHT22 sensor. It starts by initializing the sensor and serial communication. In each loop, it reads humidity and temperature values. It checks if either reading failed (NaN). If so, it prints an error message and skips printing values. Otherwise, it prints humidity and temperature to the Serial Monitor. Then it waits 2 seconds before repeating. This cycle continues indefinitely, providing updated sensor readings every 2 seconds.

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