Reading data from SD card in Arduino - Mini Project: Build & Apply

Reading data from SD card

📖 Scenario: You have a small Arduino project that needs to read text data stored on an SD card. This is common in projects like data loggers or music players where the Arduino reads files from the SD card.

🎯 Goal: Build a simple Arduino program that reads the contents of a text file called data.txt from an SD card and prints it to the Serial Monitor.

📋 What You'll Learn

Use the SD library to access the SD card

Initialize the SD card on pin 10

Open the file data.txt for reading

Read the file content line by line

Print each line to the Serial Monitor

💡 Why This Matters

🌍 Real World

Reading data from an SD card is useful in projects like data loggers, audio players, or any device that needs to access stored files.

💼 Career

Understanding how to interface with SD cards is important for embedded systems developers and IoT engineers working with external storage.

1

Set up SD card and Serial communication

Write code to include the SD and SPI libraries, create a File variable called myFile, and start Serial communication at 9600 baud inside setup(). Also, initialize the SD card on pin 10 using SD.begin(10) and print "SD card initialized." if successful.

Arduino

# Include SD and SPI libraries
#include 
#include 

File myFile;

void setup() {
  Serial.begin(9600);
  // Initialize SD card on pin 10
  // Your code here
}

void loop() {
  // Empty loop
}

Hint

Remember to include both SD.h and SPI.h libraries. Use SD.begin(10) to start the SD card on pin 10.

2

Open the file for reading

Inside setup(), after initializing the SD card, open the file data.txt for reading using myFile = SD.open("data.txt"). Then check if myFile is valid (opened successfully) and print "File opened successfully." if it is.

Arduino

#include 
#include 

File myFile;

void setup() {
  Serial.begin(9600);
  if (SD.begin(10)) {
    Serial.println("SD card initialized.");
  } else {
    Serial.println("SD card initialization failed.");
  }
  
  // Open the file data.txt for reading
  // Your code here
}

void loop() {
  // Empty loop
}

Hint

Use SD.open("data.txt") to open the file. Check if myFile is true to confirm it opened.

3

Read the file content line by line

Inside setup(), after opening the file, use a while loop with myFile.available() to read each character from the file. Use myFile.read() to get each character and print it to Serial using Serial.write().

Arduino

#include 
#include 

File myFile;

void setup() {
  Serial.begin(9600);
  if (SD.begin(10)) {
    Serial.println("SD card initialized.");
  } else {
    Serial.println("SD card initialization failed.");
  }

  myFile = SD.open("data.txt");
  if (myFile) {
    Serial.println("File opened successfully.");
    // Read and print file content
    // Your code here
  } else {
    Serial.println("Failed to open file.");
  }
}

void loop() {
  // Empty loop
}

Hint

Use while (myFile.available()) to loop through the file. Read each character with myFile.read() and print it with Serial.write(). Don't forget to close the file.

4

Print the file content to Serial Monitor

Add a Serial.println("End of file."); statement after closing the file to indicate the file reading is complete.

Arduino

#include 
#include 

File myFile;

void setup() {
  Serial.begin(9600);
  if (SD.begin(10)) {
    Serial.println("SD card initialized.");
  } else {
    Serial.println("SD card initialization failed.");
  }

  myFile = SD.open("data.txt");
  if (myFile) {
    Serial.println("File opened successfully.");
    while (myFile.available()) {
      char c = myFile.read();
      Serial.write(c);
    }
    myFile.close();
    // Your code here
  } else {
    Serial.println("Failed to open file.");
  }
}

void loop() {
  // Empty loop
}

Hint

Print "End of file." after closing the file to show reading is done.

Practice

1. What is the correct function to open a file for reading from an SD card in Arduino?

easy

A. SD.open(filename, FILE_READ)

B. SD.read(filename)

C. SD.begin(filename)

D. SD.load(filename)

Solution

1. Step 1: Identify the function to open files on SD card

   The Arduino SD library uses SD.open() to open files.

2. Step 2: Specify the mode for reading

   To read a file, the mode FILE_READ must be passed as the second argument.

3. Final Answer:

   SD.open(filename, FILE_READ) -> Option A

4. Quick Check:

   Open file for reading = SD.open(filename, FILE_READ) [OK]

Hint: Use SD.open() with FILE_READ to open files for reading [OK]

Common Mistakes:

• Using SD.read() to open files
• Forgetting FILE_READ mode
• Using SD.begin() to open files
• Using SD.load() which doesn't exist

2. Which of the following is the correct syntax to check if a file opened successfully before reading?

easy

A. if (file != NULL) { /* read file */ }

B. if (file.isOpen()) { /* read file */ }

C. if (file) { /* read file */ }

D. if (file.exists()) { /* read file */ }

Solution

1. Step 1: Understand file object truthiness in Arduino

   In Arduino, File objects can be checked directly in if statements to see if they opened successfully.

2. Step 2: Recognize correct syntax

   Using if (file) is the standard way to check if the file opened.

3. Final Answer:

   if (file) { /* read file */ } -> Option C

4. Quick Check:

   Check file open with if (file) [OK]

Hint: Use if (file) to check if file opened successfully [OK]

Common Mistakes:

• Comparing file to NULL (not Arduino style)
• Using file.isOpen() which doesn't exist
• Using file.exists() which checks SD, not file open

3. What will be the output of this Arduino code snippet reading from an SD card file named "data.txt" containing the text "Hello"?

File file = SD.open("data.txt", FILE_READ);
if (file) {
  char c = file.read();
  Serial.println(c);
  file.close();
} else {
  Serial.println("Error opening file");
}

medium

A. H

B. Hello

C. Error opening file

D. 104

Solution

1. Step 1: Understand file.read() returns a byte

   The read() method returns the ASCII value of the first character, but when assigned to a char and printed with Serial.println(char), it prints the character itself.

2. Step 2: Check code carefully

   Since c is a char and Serial.println(c) prints the character, the output is 'H'.

3. Final Answer:

   H -> Option A

4. Quick Check:

   file.read() returns char 'H' printed as H [OK]

Hint: file.read() returns one char; Serial.println prints char [OK]

Common Mistakes:

• Thinking read() returns whole string
• Expecting ASCII number printed
• Confusing file.read() with readStringUntil()

4. Identify the error in this Arduino code snippet that reads a line from an SD card file:

File file = SD.open("log.txt", FILE_READ);
if (file) {
  String line = file.readStringUntil('\n');
  Serial.println(line);
}
file.close();

medium

A. readStringUntil() is not a valid File method

B. file.close() is called outside the if block

C. Missing SD.begin() before opening file

D. Using single quotes instead of double quotes in readStringUntil

Solution

1. Step 1: Check file.close() placement

   file.close() should be called only if file opened successfully, so it must be inside the if block.

2. Step 2: Understand consequences

   If file failed to open, calling file.close() causes error because file is invalid.

3. Final Answer:

   file.close() is called outside the if block -> Option B

4. Quick Check:

   Close file only if opened [OK]

Hint: Call file.close() only inside if (file) block [OK]

Common Mistakes:

• Calling file.close() unconditionally
• Assuming readStringUntil() is invalid
• Forgetting SD.begin() but not shown here
• Confusing single vs double quotes in char argument

5. You want to read all lines from a file "config.txt" on an SD card and print each line to Serial. Which code snippet correctly does this?

hard

A. File file = SD.open("config.txt", FILE_READ);
while (file) {
Serial.println(file.readStringUntil('\n'));
}
file.close();

B. File file = SD.open("config.txt", FILE_READ);
while (!file.available()) {
Serial.println(file.readStringUntil('\n'));
}
file.close();

C. File file = SD.open("config.txt", FILE_READ);
while (file.read()) {
Serial.println(file.readStringUntil('\n'));
}
file.close();

D. File file = SD.open("config.txt", FILE_READ);
while (file.available()) {
Serial.println(file.readStringUntil('\n'));
}
file.close();

Solution

1. Step 1: Use file.available() to check for data

   file.available() returns true if there is data left to read, so loop while available() is true.

2. Step 2: Read each line with readStringUntil('\n')

   This method reads until newline, perfect for line-by-line reading.

3. Step 3: Close file after reading

   Call file.close() after finishing reading to free resources.

4. Final Answer:

   Use while (file.available()) with readStringUntil('\n') -> Option D

5. Quick Check:

   Loop with file.available() to read lines [OK]

Hint: Loop while file.available() to read all lines [OK]

Common Mistakes:

• Using while (!file.available()) which loops incorrectly
• Using while (file.read()) which reads bytes, not lines
• Using while (file) which doesn't check data availability