Humidity and temperature (DHT11/DHT22) in Arduino - Time & Space Complexity

Time Complexity: Humidity and temperature (DHT11/DHT22)

O(n)

Understanding Time Complexity

When reading humidity and temperature from sensors like DHT11 or DHT22, it's important to understand how the time to get data changes as we read more times or handle more sensors.

We want to know how the program's running time grows when we increase the number of readings.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

#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();
  delay(2000);
}
    

This code reads humidity and temperature once every 2 seconds from a single DHT11 sensor.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Reading humidity and temperature from the sensor.
• How many times: Once every 2 seconds, repeated indefinitely in the loop.

How Execution Grows With Input

Each reading takes roughly the same time, so if we take more readings, the total time grows directly with the number of readings.

Input Size (n readings)	Approx. Operations
10	10 sensor reads
100	100 sensor reads
1000	1000 sensor reads

Pattern observation: The time grows linearly as we increase the number of readings.

Final Time Complexity

Time Complexity: O(n)

This means the total time to read data grows directly in proportion to how many times we read the sensor.

Common Mistake

[X] Wrong: "Reading from the sensor is instant and does not affect program time."

[OK] Correct: Each sensor read takes a fixed amount of time (usually around 250 milliseconds to 1 second), so reading many times adds up and affects total running time.

Interview Connect

Understanding how sensor reading time adds up helps you design efficient programs that handle data smoothly without delays piling up.

Self-Check

"What if we read from multiple DHT sensors in the same loop? How would the time complexity change?"

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