Using sensor libraries in Arduino - Mini Project: Build & Apply

Using sensor libraries

📖 Scenario: You have a temperature sensor connected to your Arduino. You want to read the temperature using a sensor library and display it on the Serial Monitor.

🎯 Goal: Learn how to include a sensor library, initialize the sensor, read temperature data, and print it to the Serial Monitor.

📋 What You'll Learn

Include the sensor library DHT.h

Create a DHT sensor object with pin 2 and type DHT11

Initialize the sensor in setup()

Read temperature in Celsius using sensor.readTemperature()

Print the temperature value to the Serial Monitor

💡 Why This Matters

🌍 Real World

Reading sensor data is common in home automation, weather stations, and robotics.

💼 Career

Understanding how to use sensor libraries is important for embedded systems and IoT developer roles.

1

Include the sensor library and create sensor object

Write #include <DHT.h> at the top. Then create a DHT object called sensor using pin 2 and type DHT11.

Arduino

# Your code here

Hint

Use #include <DHT.h> to add the library. Then create the sensor object with DHT sensor(2, DHT11);.

2

Initialize Serial and sensor in setup

Write void setup() function. Inside it, start Serial communication at 9600 baud with Serial.begin(9600);. Then initialize the sensor with sensor.begin();.

Arduino

#include 

DHT sensor(2, DHT11);

// Your code here

Hint

Use void setup() { Serial.begin(9600); sensor.begin(); } to prepare the sensor and Serial Monitor.

3

Read temperature in loop

Write void loop() function. Inside it, read temperature in Celsius using float temp = sensor.readTemperature();. Then add a delay of 2000 milliseconds with delay(2000);.

Arduino

#include 

DHT sensor(2, DHT11);

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

// Your code here

Hint

Use float temp = sensor.readTemperature(); to get temperature and delay(2000); to wait 2 seconds.

4

Print temperature to Serial Monitor

Inside loop(), after reading temperature, print it using Serial.print("Temperature: "); and Serial.println(temp);.

Arduino

#include 

DHT sensor(2, DHT11);

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

void loop() {
  float temp = sensor.readTemperature();
  // Your code here
  delay(2000);
}

Hint

Use Serial.print("Temperature: "); and Serial.println(temp); to show the temperature value.

Practice

1. What is the first step when using a sensor library in an Arduino sketch?

easy

A. Include the sensor library at the top of the sketch

B. Read sensor values in the loop() function

C. Create a sensor object inside the loop()

D. Start the sensor in the loop() function

Solution

1. Step 1: Understand the library usage

   Before using a sensor, you must include its library so Arduino knows how to communicate with it.

2. Step 2: Identify correct placement

   Including the library is done at the top of the sketch, before setup() and loop().

3. Final Answer:

   Include the sensor library at the top of the sketch -> Option A

4. Quick Check:

   Include library first = A [OK]

Hint: Always include libraries before setup() [OK]

Common Mistakes:

• Trying to create sensor object before including library
• Starting sensor in loop() instead of setup()
• Reading sensor before initializing it

2. Which of the following is the correct way to create a sensor object for a DHT sensor using its library?

easy

A. DHT sensor = DHT(2);

B. DHT sensor = new DHT(2, DHT11);

C. DHT sensor(2, DHT11);

D. DHT sensor = DHT11(2);

Solution

1. Step 1: Recall object creation syntax in Arduino C++

   Objects are created by calling the constructor directly, like ClassName objectName(parameters); without '=' or 'new' keyword.

2. Step 2: Match correct syntax for DHT library

   The DHT library uses: DHT sensor(pin, type); so DHT sensor(2, DHT11); matches this exactly.

3. Final Answer:

   DHT sensor(2, DHT11); -> Option C

4. Quick Check:

   Object creation = ClassName object(params); [OK]

Hint: Create objects without '=' or 'new' in Arduino [OK]

Common Mistakes:

• Using '=' sign when creating objects
• Using 'new' keyword which is not typical in Arduino sketches
• Confusing sensor type and pin order

3. Given the code below, what will be printed if the sensor reading fails?

#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();
  if (isnan(h)) {
    Serial.println("Failed to read humidity!");
  } else {
    Serial.print("Humidity: ");
    Serial.println(h);
  }
  delay(2000);
}

medium

A. Failed to read humidity!

B. Humidity: 0.00

C. Humidity: NaN

D. No output

Solution

1. Step 1: Understand the isnan() check

   The code checks if the humidity reading is 'NaN' (not a number), which means the sensor failed to read.

2. Step 2: Determine output when reading fails

   If isnan(h) is true, the code prints "Failed to read humidity!" to Serial.

3. Final Answer:

   Failed to read humidity! -> Option A

4. Quick Check:

   isnan true prints error message [OK]

Hint: Check isnan() to detect sensor read failure [OK]

Common Mistakes:

• Assuming sensor returns 0 on failure
• Printing raw value without checking for NaN
• Not calling dht.begin() in setup()

4. Identify the error in this Arduino sketch snippet using a sensor library:

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

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

void loop() {
  dht.begin();
  float t = dht.readTemperature();
  Serial.println(t);
  delay(1000);
}

medium

A. readTemperature() returns int, not float

B. dht.begin() should be called in setup(), not loop()

C. Missing #define for sensor pin

D. Serial.begin() should be called in loop()

Solution

1. Step 1: Locate sensor initialization

   dht.begin() initializes the sensor and should be called once in setup() to avoid repeated initialization.

2. Step 2: Identify why calling dht.begin() in loop() is wrong

   Calling dht.begin() repeatedly in loop() can cause sensor communication issues and slow down the program.

3. Final Answer:

   dht.begin() should be called in setup(), not loop() -> Option B

4. Quick Check:

   Initialize sensor once in setup() [OK]

Hint: Initialize sensors once in setup() [OK]

Common Mistakes:

• Calling sensor start functions repeatedly in loop()
• Confusing Serial.begin() placement
• Assuming readTemperature() returns int

5. You want to read temperature and humidity from a DHT11 sensor and turn on an LED if humidity is above 70%. Which code snippet correctly uses the sensor library and controls the LED?

hard

A. #include <DHT.h>
#define DHTPIN 2
#define DHTTYPE DHT11
#define LEDPIN 13
DHT dht(DHTPIN, DHTTYPE);

void setup() {
Serial.begin(9600);
dht.begin();
pinMode(LEDPIN, OUTPUT);
}

void loop() {
float h = dht.readHumidity();
if (!isnan(h) || h > 70) {
digitalWrite(LEDPIN, HIGH);
} else {
digitalWrite(LEDPIN, LOW);
}
delay(2000);
}

B. #include <DHT.h>
#define DHTPIN 2
#define DHTTYPE DHT11
#define LEDPIN 13
DHT dht(DHTPIN, DHTTYPE);

void setup() {
Serial.begin(9600);
pinMode(LEDPIN, OUTPUT);
}

void loop() {
float h = dht.readHumidity();
if (h > 70) {
digitalWrite(LEDPIN, HIGH);
} else {
digitalWrite(LEDPIN, LOW);
}
delay(2000);
}

C. #include <DHT.h>
#define DHTPIN 2
#define DHTTYPE DHT11
#define LEDPIN 13
DHT dht(DHTPIN, DHTTYPE);

void setup() {
Serial.begin(9600);
dht.begin();
pinMode(LEDPIN, OUTPUT);
}

void loop() {
float h = dht.readHumidity();
if (h < 70) {
digitalWrite(LEDPIN, HIGH);
} else {
digitalWrite(LEDPIN, LOW);
}
delay(2000);
}

D. #include <DHT.h>
#define DHTPIN 2
#define DHTTYPE DHT11
#define LEDPIN 13
DHT dht(DHTPIN, DHTTYPE);

void setup() {
Serial.begin(9600);
dht.begin();
pinMode(LEDPIN, OUTPUT);
}

void loop() {
float h = dht.readHumidity();
if (!isnan(h) && h > 70) {
digitalWrite(LEDPIN, HIGH);
} else {
digitalWrite(LEDPIN, LOW);
}
delay(2000);
}

Solution

1. Step 1: Check sensor initialization and pin setup

   #include <DHT.h>
   #define DHTPIN 2
   #define DHTTYPE DHT11
   #define LEDPIN 13
   DHT dht(DHTPIN, DHTTYPE);
   
   void setup() {
     Serial.begin(9600);
     dht.begin();
     pinMode(LEDPIN, OUTPUT);
   }
   
   void loop() {
     float h = dht.readHumidity();
     if (!isnan(h) && h > 70) {
       digitalWrite(LEDPIN, HIGH);
     } else {
       digitalWrite(LEDPIN, LOW);
     }
     delay(2000);
   }
   

   correctly calls dht.begin() in setup() and sets LED pin as output.

2. Step 2: Verify humidity check and LED control logic

   #include <DHT.h>
   #define DHTPIN 2
   #define DHTTYPE DHT11
   #define LEDPIN 13
   DHT dht(DHTPIN, DHTTYPE);
   
   void setup() {
     Serial.begin(9600);
     dht.begin();
     pinMode(LEDPIN, OUTPUT);
   }
   
   void loop() {
     float h = dht.readHumidity();
     if (!isnan(h) && h > 70) {
       digitalWrite(LEDPIN, HIGH);
     } else {
       digitalWrite(LEDPIN, LOW);
     }
     delay(2000);
   }
   

   checks if humidity is not NaN and greater than 70 before turning LED on, which prevents errors from failed reads.

3. Final Answer:

   The code snippet with !isnan(h) check and && h > 70 -> Option D

4. Quick Check:

   Initialize sensor + check isnan + control LED = A [OK]

Hint: Always check isnan() before using sensor values [OK]

Common Mistakes:

• Not calling dht.begin() in setup()
• Missing isnan() check causing wrong LED behavior
• Using || instead of && in humidity condition