SD Card Data Logger Build

Hardware

Arduino Uno/Nano/Mega
SD card module (SPI interface — cheap, widely available)
A real-time clock module (DS3231 — much more accurate than the cheaper DS1307) if you want timestamped logs
Whatever sensor you're logging (temp/humidity, light, voltage, etc.)

Wiring (SPI)

SD module CS → pin 10 (or any digital pin, configurable in code)
SD module MOSI → pin 11
SD module MISO → pin 12
SD module SCK → pin 13
SD module VCC/GND → 5V/GND (check your module — some need 3.3V, frying a 5V-intolerant module is a common mistake)

Basic Logging Code

#include <SPI.h> #include <SD.h> const int chipSelect = 10; void setup() { Serial.begin(9600); if (!SD.begin(chipSelect)) { Serial.println("SD init failed"); while (1); // halt — no point continuing without storage } } void loop() { File dataFile = SD.open("log.csv", FILE_WRITE); if (dataFile) { float sensorValue = analogRead(A0) * (5.0 / 1023.0); dataFile.print(millis()); dataFile.print(","); dataFile.println(sensorValue); dataFile.close(); } delay(1000); }

Adding Real Timestamps (DS3231)

#include <RTClib.h> RTC_DS3231 rtc; void setup() { rtc.begin(); // rtc.adjust(DateTime(F(__DATE__), F(__TIME__))); // uncomment once to set clock } // in loop(), before writing: DateTime now = rtc.now(); dataFile.print(now.timestamp()); dataFile.print(",");

Run the adjust() line once with your computer connected to set the clock, then comment it back out — the DS3231's onboard coin cell battery keeps time afterward even with the Arduino powered off.

Power Considerations for Long-Term Logging

If this is running unattended for days/weeks, a few things matter: use SD.open/close per write rather than holding the file open (reduces data loss risk on unexpected power loss), and consider putting the Arduino into sleep mode between readings if you're on battery power (the LowPower library handles this cleanly) to extend runtime significantly.

Reading the Data Back

Pull the SD card, the resulting log.csv opens directly in Excel/Sheets/any CSV tool — no special software needed, which is the whole appeal of this approach over something requiring live network connectivity.

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