I have designed and wrote a program to monitor temperature and humidity 2 months ago. It took me another a month to publish it. Why? Because I can not find any suitable circuit sketch for Arduino. And lazy. Hehhe.

So, what I can give to the other people about my latest project is only in short description. Any problem/questions, just ask me in comment down below using Facebook.

Objective: Build a system using Arduino to monitor temperature and humidity and save it in memory card.

Part:

1.Arduino (whatever you have. Mine, not the latest one).

2.Arduino RTC (real time clock) v1.1. To read date/time.

3.Humidity & temperature sensor DHT11

4.SD card reader/write from LC studio, www.lcsoft.net

5.SD card

6.Some wire jumpers, if needed. I need a lot of them.

Design:

Just view it from here.

Program?

#include <SD.h>
#include "RTClib.h"
#include <Wire.h>
#include <string.h>
#include <dht11.h>
#include <stdlib.h>
RTC_DS1307 RTC;
int delaytime= 3000;
dht11 DHT11;
#define DHT11PIN 2
File myFile;
void setup()
{
  Serial.begin(9600);
  Wire.begin(); //Important for RTClib.h
  RTC.begin();
  
  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(__DATE__, __TIME__));
    return;
  }
  
 Serial.print("Initializing SD card...");
  // On the Ethernet Shield, CS is pin 4. It's set as an output by default.
  // Note that even if it's not used as the CS pin, the hardware SS pin 
  // (10 on most Arduino boards, 53 on the Mega) must be left as an output 
  // or the SD library functions will not work. 
  pinMode(10, OUTPUT);
  if (!SD.begin(4)) {
    Serial.println("initialization failed!");
    return;
  }
  Serial.println("initialization done.");
} 
void loop()
{
  openFile();
  writeFile();
  closeFile();
  delay(delaytime);
}
void openFile(){
  char filename[] = "00000000.CSV";
  DateTime now = RTC.now();
  filename[0] = (now.year()/1000)%10 + '0'; //To get 1st digit from year()
  filename[1] = (now.year()/100)%10 + '0'; //To get 2nd digit from year()
  filename[2] = (now.year()/10)%10 + '0'; //To get 3rd digit from year()
  filename[3] = now.year()%10 + '0'; //To get 4th digit from year()
  filename[4] = now.month()/10 + '0'; //To get 1st digit from month()
  filename[5] = now.month()%10 + '0'; //To get 2nd digit from month()
  filename[6] = now.day()/10 + '0'; //To get 1st digit from day()
  filename[7] = now.day()%10 + '0'; //To get 2nd digit from day()
  
  //Check file name exist?
  if (SD.exists(filename)) {
    Serial.println("exists.");
    myFile = SD.open(filename, FILE_WRITE);
  }
  else {
    Serial.println("doesn't exist.");
    Serial.println("Creating new file");
    myFile = SD.open(filename, FILE_WRITE); 
  }
}
void writeFile(){
  DateTime now = RTC.now();
  myFile.print(now.year(),DEC);
  myFile.print("/");
  myFile.print(now.month(),DEC);
  myFile.print("/");
  myFile.print(now.day(),DEC);
  myFile.print(" ");
  myFile.print(now.hour(),DEC);
  myFile.print(":");
  myFile.print(now.minute(),DEC);
  myFile.print(":");
  myFile.print(now.second(),DEC);
  myFile.print(",");
  DHT11.read(DHT11PIN);
  myFile.print((float)DHT11.temperature, 2);
  Serial.println((float)DHT11.temperature, 2);
  myFile.println((float)DHT11.humidity, 2);
  Serial.println((float)DHT11.humidity, 2);
}
void closeFile(){
    myFile.close(); 
}

The output in SD card must Date/Time; Temperature ; Humidity.