Problem with writing to datalog.txt and timestamps

Hey, I’ve been having problems with some code that i cannibalized from the example datalogger file. the purpose of the code is to read a voltage and then store the value in a file (datalog.txt) with the time stamp of when it was read. the problem I’m having is that whenever it goes to log the value, i get on the serial monitor “error opening datalog.txt” and I cant figure out why it keeps doing that, it did work but the times were all screwy and giving me 165/165/165 as a date and time. I’m pretty new to Arduinos and the coding so as much info would be much appreciated :slight_smile:

#include <SPI.h>
#include <SD.h>
#include <Wire.h>
#include "RTClib.h"

const int chipSelect = 10;

RTC_DS1307 rtc;

void setup() {
  // Open serial communications and wait for port to open:
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only
  if (! rtc.begin()) {
    Serial.println("Couldn't find RTC");
    while (1);

    if (! rtc.isrunning()) {
      Serial.println("RTC is NOT running!");
      // following line sets the RTC to the date & time this sketch was compiled
      rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));

    Serial.print("Initializing SD card...");

    // see if the card is present and can be initialized:
    if (!SD.begin(chipSelect)) {
      Serial.println("Card failed, or not present");
      // don't do anything more:
    Serial.println("card initialized.");


void loop() {
  // make a string for assembling the data to log:
  String dataString = "";

  int sensorValue = analogRead(A0);
  float voltage = sensorValue * (5.0 / 1023.0);
  String voltages = String(voltage, 3);

  DateTime now =;

  String theyear = String(now.year(), DEC);
  String mon = String(now.month(), DEC);
  String theday = String(, DEC);
  String thehour = String(now.hour(), DEC);
  String themin = String(now.minute(), DEC);

  //Put all the time and date strings into one String
  dataString = String("voltages, " + theyear + "/" + mon + "/" + theday + ", " + thehour + ":" + themin);

  // open the file. note that only one file can be open at a time,
  // so you have to close this one before opening another.
  File dataFile ="datalog.txt", FILE_WRITE);

  // if the file is available, write to it:
  if (dataFile) {
    // print to the serial port too:
  // if the file isn't open, pop up an error:
  else {
    Serial.println("error opening datalog.txt");

DataloggerVoltage.ino (2.17 KB)

It is absolutely pointless to be pissing away resources using the String class when the File class inherits from Print, which KNOWS how to convert an int to text.

Post ALL of your Serial output.

Hi Allenjn,

  1. I’d start by learning how to use the file system. I assume you’re using an SD card of some sort. Load the SD library examples and make sure you can open/close/reopen, … files before you start logging data.

You should be able to use the logger example as a starting point and modify it with your log data.

  1. But before you start writing log data to your log file, start by writing it to the screen. It may take some tweaking to make sure your outputs are correct before you start filling a file.

It’ll also be much easier to debug your code using the serial output than writing to a file, pulling the SD card, putting in your PC, opening the file and reviewing it…


PS - rather than attaching the file like you did, you should post your code in your comment. Click on the </> button above the text window and paste your code into the block. Folks here rarely download and review files. I don’t.