Arduino SD data logger with time stamp

I've been learning about powering my ham shack with solar power and have been working on logging battery voltage to get information about how my power budget is going. At first I used an Arduino's A/D to read the voltage and periodically send it out the USB serial port, but as the shed has no mains power the laptop was a major drain on the battery itself. Yes, measuring was really altering the experiment.

An SD Shield Plus, purchased via eBay, has all the things you need for stand alone data logging. An SD slot, a battery backed clock, and even a bit of non-volatile utility ram.

My Arduino is a slightly older model, the Diecimila and I found that as soon as I called SD.begin() the device would reset. It turns out that the SD library code uses more ram than is available in the older models.

The new Arduino Uno looks like a good improvement. Aside from a more spacious Atmel chip, the USB interface is a processor itself and can be set as a HID device which could be handy. I ordered a couple of Uno's from Little Bird Electronics, and they shipped within minutes, but I still couldn't wait and went to Jaycar to pick up an interesting Uno clone called "Eleven" made by Freetronics.

I hacked together the sample code to write a log file to the SD card with a date time stamp from the clock chip in a format that a spreadsheet will parse. It reads three of the analog inputs and writes a line to the file every ten seconds. Here's the code for future reference:


/*
  SD card datalogger
  by Tom Igoe
  hacked by Peter Marks to write time stamps

 This example code is in the public domain.
 */

#include <SD.h>
// For the SD Shield Plus
const int chipSelect = 10;

// This is to talk to the real time clock
#include "Wire.h"
#define DS1307_I2C_ADDRESS 0x68  // This is the I2C address

// Global Variables
int i;
byte second, minute, hour, dayOfWeek, dayOfMonth, month, year;

void setup()
{
  Wire.begin();
  Serial.begin(9600);
  Serial.print("Initializing SD card...\n");
  // make sure that the default chip select pin is set to
  // output, even if you don't use it:
  pinMode(chipSelect, OUTPUT);
  Serial.print("chipSelect set to output\n");
  
  // see if the card is present and can be initialized:
  if (!SD.begin(chipSelect)) 
  {
    Serial.println("Card failed, or not present\n");
    // don't do anything more:
    return;
  }
  Serial.println("card initialized.\n");
}

void loop()
{
  // make a string for assembling the data to log:
  String dataString = getDateDs1307();
  dataString += String(",");
  
  // read three sensors and append to the string:
  for (int analogPin = 0; analogPin < 3; analogPin++) 
  {
    int sensor = analogRead(analogPin);
    dataString += String(sensor);
    if (analogPin < 2) 
    {
      dataString += ","; 
    }
  }

  // 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 = SD.open("datalog.csv", FILE_WRITE);

  // if the file is available, write to it:
  if (dataFile) 
  {
    dataFile.println(dataString);
    dataFile.close();
    // print to the serial port too:
    Serial.println(dataString);
  }  
  // if the file isn't open, pop up an error:
  else 
  {
    Serial.println("error opening datalog.csv");
  } 
  delay(1000 * 10);
}


// Gets the date and time from the ds1307 and return
// result in a format a spreadsheet can parse: 06/10/11 15:10:00
String getDateDs1307()
{
  // Reset the register pointer
  Wire.beginTransmission(DS1307_I2C_ADDRESS);
  Wire.send(0x00);
  Wire.endTransmission();

  Wire.requestFrom(DS1307_I2C_ADDRESS, 7);

  // A few of these need masks because certain bits are control bits
  second     = bcdToDec(Wire.receive() & 0x7F);
  minute     = bcdToDec(Wire.receive());
  hour       = bcdToDec(Wire.receive() & 0x3F);  
  dayOfWeek  = bcdToDec(Wire.receive());
  dayOfMonth = bcdToDec(Wire.receive());
  month      = bcdToDec(Wire.receive());
  year       = bcdToDec(Wire.receive());
  
  String dataString = "";
  
  dataString += Print2Digit(dayOfMonth); 
  dataString += String("/");
  dataString += Print2Digit(bcdToDec(month)); 
  dataString += String("/"); // Y2k1 bug!
  dataString += Print2Digit(bcdToDec(year));
  dataString += String(" ");
  dataString += Print2Digit(hour);
  dataString += String(":");
  dataString += Print2Digit(minute);
  dataString += String(":");
  dataString += Print2Digit(second);

  return dataString;
}

// Convert binary coded decimal to normal decimal numbers
byte bcdToDec(byte val)
{
  return ( (val/16*10) + (val%16) );
}

String Print2Digit(byte Val)
{
  String dataString = "";
  if (Val < 10)
  {
    dataString = "0";
  }  
  dataString += String(Val, DEC);
  return dataString;
}

The output file looks like this:


06/10/11 15:36:16,555,489,479
06/10/11 15:36:18,461,427,438
06/10/11 15:36:19,419,401,414
06/10/11 15:36:21,397,387,404
06/10/11 15:36:22,394,389,403
06/10/11 15:36:23,380,375,393
06/10/11 15:36:25,386,383,399
06/10/11 15:36:26,382,380,396
06/10/11 15:36:28,380,378,395
06/10/11 15:36:29,387,385,400

Here's a few days of data. Today was partially cloudy but the battery was re-charged in the end.

Plotted with python and matplotlib with this:


#!/usr/local/bin/python

import datetime
import matplotlib
matplotlib.use('Agg')

import matplotlib.pyplot as plt
import csv

INFILENAME = 'datalog.csv'
VOLTSCALE = 665.0 / 13.7

def main():
    datareader = csv.reader(open(INFILENAME,'r'), delimiter=',')
    dates = []
    points = []
    for row in datareader:
        if len(row) > 1:
            #Converts str into a datetime object. 08/10/11 12:23:12
            dates.append(datetime.datetime.strptime(row[0],'%d/%m/%y %H:%M:%S'))
            volts = float(row[1]) / VOLTSCALE
            points.append(volts)
    
    fig = plt.figure()
    plt.plot(dates, points)
    plt.title("Battery Voltage over several days")
    plt.ylabel('Battery Volts')
    fig.autofmt_xdate()
    plt.ylim(ymin=8)
    plt.show()
    plt.savefig('voltgraph')

if __name__ == "__main__":
    main()