Using multiple libraries together in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of combined sensor readings

What is the output of this Arduino sketch that uses two libraries to read temperature and humidity?

Arduino

#include <DHT.h>
#include <Wire.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();
  Serial.print("Humidity: ");
  Serial.print(h);
  Serial.print(" %");
  Serial.print(", Temperature: ");
  Serial.print(t);
  Serial.println(" *C");
  delay(2000);
}

A Humidity: 45.0 % , Temperature: 23.0 *C

B Humidity: nan % , Temperature: nan *C

C Humidity: 100 % , Temperature: 0 *C

D Compilation error due to missing Wire.begin()

💡 Hint

Check if the sensor is properly initialized and if the readings are valid.

Without a real sensor connected, the DHT library returns 'nan' for humidity and temperature. The Wire library is included but not used here, so no error occurs.

🧠 Conceptual

intermediate

Reason for including multiple libraries

Why do Arduino sketches often include multiple libraries like and together?

A To make the sketch run faster by default

B To reduce the size of the compiled program

C To avoid using any hardware pins

D To handle different hardware communication protocols simultaneously

💡 Hint

Think about what each library controls in hardware communication.

Wire.h is for I2C communication and SPI.h is for SPI communication. Including both allows the sketch to communicate with devices using different protocols.

🔧 Debug

advanced

Identify the error when using two libraries

What error will this Arduino code produce when compiling?

Arduino

#include <Wire.h>
#include <SPI.h>

void setup() {
  Wire.begin();
  SPI.begin();
}

void loop() {
  Wire.beginTransmission(0x3C);
  SPI.transfer(0xFF);
  Wire.endTransmission();
  delay(1000);
}

A Error: SPI.transfer() called outside SPI transaction

B Error: Wire.beginTransmission() missing address

C No error, code compiles and runs correctly

D Error: Cannot use Wire and SPI in the same sketch

💡 Hint

Check the correct usage of SPI.transfer() in SPI communication.

SPI.transfer() must be called between SPI.beginTransaction() and SPI.endTransaction() for proper operation. Calling it directly causes an error or undefined behavior.

📝 Syntax

advanced

Correct syntax for using two libraries

Which option shows the correct way to initialize both Wire and SPI libraries in Arduino?

A Wire.begin(); SPI.begin();

B Wire.start(); SPI.start();

C Wire.init(); SPI.init();

D Wire.beginTransmission(); SPI.beginTransmission();

💡 Hint

Check the official Arduino library functions for starting communication.

Wire.begin() and SPI.begin() are the correct functions to initialize I2C and SPI communication respectively.

🚀 Application

expert

Combining sensor data with display using multiple libraries

You want to read temperature from a DHT sensor and display it on an OLED screen using Wire and Adafruit_SSD1306 libraries. Which code snippet correctly initializes and uses both libraries together?

A

#include <Wire.h>
#include <Adafruit_SSD1306.h>
#include <DHT.h>
#define SCREEN_WIDTH 128
#define SCREEN_HEIGHT 64
#define DHTPIN 2
#define DHTTYPE DHT11
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &SPI);
DHT dht(DHTPIN, DHTTYPE);
void setup() {
  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  dht.begin();
}
void loop() {
  float temp = dht.readTemperature();
  display.clearDisplay();
  display.setTextSize(2);
  display.setCursor(0,0);
  display.print("Temp: ");
  display.print(temp);
  display.print(" C");
  display.display();
  delay(2000);
}

B

#include <Wire.h>
#include <Adafruit_SSD1306.h>
#include <DHT.h>
#define SCREEN_WIDTH 128
#define SCREEN_HEIGHT 64
#define DHTPIN 2
#define DHTTYPE DHT11
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT);
DHT dht(DHTPIN, DHTTYPE);
void setup() {
  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  dht.begin();
}
void loop() {
  float temp = dht.readTemperature();
  display.clearDisplay();
  display.setTextSize(2);
  display.setCursor(0,0);
  display.print("Temp: ");
  display.print(temp);
  display.print(" C");
  display.display();
  delay(2000);
}

C

#include <Wire.h>
#include <Adafruit_SSD1306.h>
#include <DHT.h>
#define SCREEN_WIDTH 128
#define SCREEN_HEIGHT 64
#define DHTPIN 2
#define DHTTYPE DHT11
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire);
DHT dht(DHTPIN, DHTTYPE);
void setup() {
  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  dht.begin();
}
void loop() {
  float temp = dht.readTemperature();
  display.clearDisplay();
  display.setTextSize(2);
  display.setCursor(0,0);
  display.print("Temp: ");
  display.print(temp);
  display.print(" C");
  display.display();
  delay(2000);
}

D

#include <Wire.h>
#include <Adafruit_SSD1306.h>
#include <DHT.h>
#define SCREEN_WIDTH 128
#define SCREEN_HEIGHT 64
#define DHTPIN 2
#define DHTTYPE DHT11
Adafruit_SSD1306 display;
DHT dht(DHTPIN, DHTTYPE);
void setup() {
  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  dht.begin();
}
void loop() {
  float temp = dht.readTemperature();
  display.clearDisplay();
  display.setTextSize(2);
  display.setCursor(0,0);
  display.print("Temp: ");
  display.print(temp);
  display.print(" C");
  display.display();
  delay(2000);
}

💡 Hint

Check how the display object is constructed and which communication protocol it uses.

The Adafruit_SSD1306 display must be constructed with the Wire object for I2C communication. Option C correctly passes &Wire and initializes both libraries properly.