Temperature data logger with an SD card on an Arduino Uno

// Temperature data logger: a DS18B20 thermometer, a DS3231 clock for the
// time stamps and a microSD card. Every five seconds one line goes into
// TEMPLOG.CSV, which opens in any spreadsheet.
//
//   DS18B20   DQ pin 2, with a 4.7k pull-up from DQ to 5V; VDD 5V, GND
//   DS3231    SDA A4, SCL A5, VCC 5V, GND
//   microSD   CS pin 10, MOSI 11, MISO 12, SCK 13, VCC 5V, GND
//   LED       pin 7 through 220 ohms: one blink for every line written
//
// THE FILE. The first time the card is used the sketch writes a header,
//
//   date,time,temp_c
//   2026-11-01,09:00:05,21.50
//
// and after that one line per reading. The file is opened, written and
// closed every time, so pulling the card (or the power) loses at most the
// line being written. Leave the card in for days and the file keeps
// growing: at one line every five seconds that is about 450 KB a day.
//
// THE CLOCK. millis() forgets the time at every reset, and a log without
// real dates is hard to use. The DS3231 keeps the time on its own coin cell,
// so it is set once and then left alone. To set a new module, upload once
// with the rtc.adjust() line in setup() uncommented (it takes the time the
// sketch was compiled), then comment it out again and upload, or every reset
// would put the clock back to that moment. In Mokxi the clock starts at the
// time in its start property.
//
// THE SERIAL MONITOR (115200) shows every line as it is logged. Type d and
// Enter to print the whole file back from the card, which is how you check
// what is on it without taking it out.
//
// NO CARD? The logger still reads and prints every five seconds, and says
// so once. Put a card in and press reset to start logging. A card pulled out
// while it runs is noticed at the next write, and the same applies.
//
// Change INTERVAL_MS to log more or less often.

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

const int SD_CS = 10;
const int LED = 7;
const unsigned long INTERVAL_MS = 5000;
const char *FILE_NAME = "TEMPLOG.CSV";

OneWire oneWire(2);
DallasTemperature sensors(&oneWire);
RTC_DS3231 rtc;

bool cardReady = false;
unsigned long lastLog = 0;

void print2(Print &out, int n) {
  if (n < 10) out.print('0');
  out.print(n);
}

// "2026-11-01,09:00:05,21.50" to wherever `out` points.
void writeLine(Print &out, const DateTime &t, float c) {
  out.print(t.year());
  out.print('-');
  print2(out, t.month());
  out.print('-');
  print2(out, t.day());
  out.print(',');
  print2(out, t.hour());
  out.print(':');
  print2(out, t.minute());
  out.print(':');
  print2(out, t.second());
  out.print(',');
  out.println(c, 2);
}

// Start the card, and write the header if the file is new.
bool startCard() {
  if (!SD.begin(SD_CS)) return false;
  if (!SD.exists(FILE_NAME)) {
    File file = SD.open(FILE_NAME, FILE_WRITE);
    if (!file) return false;
    file.println("date,time,temp_c");
    file.close();
  }
  return true;
}

void dump() {
  if (!cardReady) {
    Serial.println("no card to read");
    return;
  }
  File file = SD.open(FILE_NAME);
  if (!file) {
    Serial.println("could not open TEMPLOG.CSV");
    return;
  }
  Serial.println("--- TEMPLOG.CSV ---");
  while (file.available()) Serial.write(file.read());
  file.close();
  Serial.println("--- end ---");
}

void logReading() {
  sensors.requestTemperatures();
  float c = sensors.getTempCByIndex(0);
  DateTime t = rtc.now();
  if (c == DEVICE_DISCONNECTED_C) {
    Serial.println("no DS18B20: check DQ on pin 2 and the 4.7k pull-up");
    return;
  }
  writeLine(Serial, t, c);

  if (!cardReady) return;
  File file = SD.open(FILE_NAME, FILE_WRITE);
  if (!file) {
    Serial.println("could not write TEMPLOG.CSV: is the card still in? Press reset once it is.");
    cardReady = false;
    return;
  }
  writeLine(file, t, c);
  file.close();
  digitalWrite(LED, HIGH);
  delay(100);
  digitalWrite(LED, LOW);
}

void setup() {
  Serial.begin(115200);
  pinMode(LED, OUTPUT);
  sensors.begin();
  if (!rtc.begin()) Serial.println("no DS3231 at 0x68: check SDA and SCL");
  // rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));  // once, to set the clock

  cardReady = startCard();
  if (cardReady) {
    Serial.println("logging to TEMPLOG.CSV every 5 s (type d to print it)");
  } else {
    Serial.println("no SD card: check the card and CS on pin 10, then press reset. Readings go to the Serial Monitor only.");
  }
}

void loop() {
  while (Serial.available()) {
    char c = Serial.read();
    if (c == 'd' || c == 'D') dump();
  }
  if (millis() - lastLog >= INTERVAL_MS) {
    lastLog = millis();
    logReading();
  }
}