Using sensor libraries in Arduino - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a sensor library in Arduino?

A sensor library is a collection of pre-written code that helps you easily communicate with and get data from a sensor without writing all the low-level code yourself.

beginner

How do you include a sensor library in your Arduino sketch?

You include a sensor library by adding #include <LibraryName.h> at the top of your sketch. This tells the Arduino IDE to use the library's code.

beginner

Why is it helpful to use sensor libraries instead of writing sensor code from scratch?

Sensor libraries save time, reduce errors, and make your code easier to read because they handle complex sensor communication details for you.

intermediate

What is the typical first step after including a sensor library in your Arduino code?

You usually create an object (a variable) from the sensor library class to represent your sensor, so you can call its functions to read data.

intermediate

How do you usually start reading data from a sensor using its library?

You call a function like begin() or init() in the setup() to start the sensor, then use functions like read() or getValue() in the loop() to get sensor data.

What does the line #include <SensorLib.h> do in an Arduino sketch?

A Saves sensor data to memory

B Includes the sensor library code so you can use its functions

C Starts the sensor automatically

D Compiles the sketch without errors

Including a library with #include makes its code available to your sketch.

Why should you call sensor.begin() in the setup() function?

A To reset the Arduino board

B To read sensor data continuously

C To stop the sensor

D To initialize the sensor before reading data

begin() prepares the sensor to work properly.

What is the main advantage of using a sensor library?

A It simplifies programming by handling sensor details

B It makes the sensor cheaper

C It increases sensor power consumption

D It removes the need for wiring

Libraries simplify programming by managing sensor communication.

Which Arduino function is best for reading sensor data repeatedly?

A loop()

B main()

C init()

D setup()

loop() runs over and over, perfect for reading sensor data continuously.

If a sensor library provides a function called readTemperature(), what does it likely do?

A Calibrates the sensor

B Turns off the sensor

C Returns the current temperature reading

D Saves temperature data to a file

readTemperature() usually returns the sensor's temperature value.

Explain how to use a sensor library in an Arduino sketch from start to reading data.

Think about the order of steps in your code.

Why do sensor libraries make programming easier for beginners?

Consider what you avoid by using a library.

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