Yún Datalogger

This example shows how to log data from three analog sensors to an SD card mounted on the Arduino Yún using the Bridge library. The SD card is not connected to the 32U4, but the AR9331, which is why Bridge must be used.

Prepare your SD card by creating an empty folder in the root directory named "arduino". When OpenWrt-Yun finds this folder on an attached storage device, it creates a link to the SD to the "/mnt/sd" path.

You can remove the SD card while Linux and the sketch are running but be careful not to remove it while data is writing to the card.

Hardware Required

  • Arduino Yún
  • micro-SD card
  • analog sensors attached to analog input pins 0, 1, and 2


There is no circuit for this example.

image developed using Fritzing. For more circuit examples, see the Fritzing project page


Include the FileIO header, for communicating with the SD card.
#include <FileIO.h>

in setup(), initialize Bridge, Serial communication, and FileSystem (for communicating with the OpenWrt-Yun file system). Wait for an active serial connection before starting the remainder of the sketch.

void setup() {

  Serial.println("Filesystem datalogger\n");

In loop(), create a string that starts with a timestamp to organize the data to be logged. You'll create the getTimeStamp() function below.

void loop () {
  String dataString;
  dataString += getTimeStamp();
  dataString += " = ";

Read the data from the sensors and append them to the string, separating the values with a comma :

for (int analogPin = 0; analogPin < 3; analogPin++) {
    int sensor = analogRead(analogPin);
    dataString += String(sensor);
    if (analogPin < 2) {
      dataString += ",";  

Open the file you'll be writing the data to using a File object and FileSystem.open(). With the modifier FILE_APPEND, you can write information to the end of the file. If the file doesn't already exist, it will be created. In this case, you'll be creating and writing to a file at the root directory of the SD card named "datalog.txt".

File dataFile = FileSystem.open("/mnt/sd/datalog.txt", FILE_APPEND);

If the file opens successfully, write the string to it, close the file, and print the information to the Serial monitor.

if (dataFile) {

If there is a problem opening the file, send an error to the Serial monitor :

else {
    Serial.println("error opening datalog.txt");

Last, you'll write the function getTimeStamp() to retrieve the time the information was read. It will be returning a string. First, create a string to hold the current time. You'll also create an instance of Process called "time". start the process and call the "date" application. "date" is a command line utility that returns the current date and the time. Using time.addParameter(), you'll specify the parameters D and T, which will return the date (mm/dd/yy), and the current time (hh:mm:ss). Run the process and read the result into the string.

String getTimeStamp() {
  String result;
  Process time;

  while(time.available()>0) {
    char c = time.read();
    if(c != '\n')
      result += c;

  return result;

The complete sketch is below :

SORRY, There is an error at our code repository, please inform to web@arduino.cc

See Also

  • Bridge Library - Your reference to the Bridge Library
  • Bridge – Simple REST style calls to access analog and digital pins
  • Consolle Ascii Table – A complete ASCII table printed to the Console
  • Console Pixel – Turn an LED on and off through the Console
  • Console Read - Read data coming from bridge using the Console.read() function
  • File Write - How to write file into the Yún filesystem.
  • Http Client - A basic HTTP client that connects to the internet and downloads content.
  • Mailbox Read Message - How to read the messages queue, called Mailbox, using the Bridge library.
  • Process - How to run linux processes using an Arduino Yún.
  • Shell Commands - How to run linux shell commands using an Arduino Yún.
  • Temperature Web Panel - How to serve data from an analog input via the Arduino Yún's built-in webserver.
  • Time check - Gets the time from Linux via Bridge then parses out hours, minutes and seconds.
  • WiFi Status - Prints information about the status of your wifi connection.
  • Serial Terminal - Use the Yún's 32U4 processor as a serial terminal for the Linux side on the Yún.

Last revision 2015/08/12 by SM