Data-Logging Thermometer

// Data-Logging Thermometer
// DS18B20 (1-Wire, D2) -> Arduino Uno -> microSD Shield (SPI, CS on D8)
// Appends one CSV line per reading: millis,celsius

#include <OneWire.h>
#include <DallasTemperature.h>
#include <SPI.h>
#include <SD.h>

const int  ONE_WIRE_PIN = 2;      // DS18B20 data. Needs a 4.7k pull-up to 5V.
const int  SD_CS_PIN    = 8;      // Card select on the SparkFun microSD Shield
const unsigned long INTERVAL_MS = 10000;   // one sample every 10 s
const char LOG_FILE[]   = "templog.csv";

OneWire oneWire(ONE_WIRE_PIN);
DallasTemperature sensors(&oneWire);

void setup() {
  Serial.begin(9600);
  sensors.begin();

  if (!SD.begin(SD_CS_PIN)) {
    // Card absent, not FAT32, or CS on the wrong pin. Stop rather than
    // pretend to log -- a logger that silently writes nothing is worse
    // than one that refuses to start.
    Serial.println(F("SD init failed: card absent, not FAT32, or wrong CS pin"));
    while (true) { }
  }

  // Write the header once, only if the file does not exist yet.
  if (!SD.exists(LOG_FILE)) {
    File f = SD.open(LOG_FILE, FILE_WRITE);
    if (f) { f.println(F("millis,celsius")); f.close(); }
  }
  Serial.println(F("logging to templog.csv"));
}

void loop() {
  sensors.requestTemperatures();
  float c = sensors.getTempCByIndex(0);

  if (c == DEVICE_DISCONNECTED_C) {
    // -127 means the bus never went high: almost always the missing pull-up.
    Serial.println(F("no sensor -- check the 4.7k pull-up between D2 and 5V"));
  } else {
    File f = SD.open(LOG_FILE, FILE_WRITE);   // FILE_WRITE appends
    if (f) {
      f.print(millis());
      f.print(',');
      f.println(c, 2);
      f.close();          // close == flush; a power cut loses at most this sample
    } else {
      Serial.println(F("could not open log file"));
    }
    Serial.println(c, 2);
  }

  delay(INTERVAL_MS);
}