Here's an update on the datalogger, the line that sets the RTC was twice in...

Here's an update on the datalogger, the line that sets the RTC was twice in the code, but should have been there just once.

CO2_Sensor_RTC_Clock.ino

+#include "RTClib.h"; // For clock module
+#include "Wire.h"; // For communication with clock & T,h sensor via I2C communication
+#include "Adafruit_Si7021.h"; // For the T and h sensor
+Adafruit_Si7021 sensor = Adafruit_Si7021();
+#include "SPI.h"; // Library for SPI bus
+#include "SD.h"; // Library for SD card module
+
+RTC_DS1307 rtc;
+
+int year;
+int month;
+int day;
+int hour;
+int minute;
+int second;
+int lightIntensity;
+float relativeHumidity;
+float temperature;
+int ppmCO2;
+String filename;
+
+const int chipSelect = 10; // Variable as read only, when declared as constant
+
+// CO2-Messung mit Sensor Typ MHZ19B
+// Messwerterfassung durch PWM-Signal
+// Quelle: https://www.blikk.it
+// Implement this code into your code and write the CO2 concentration into the file on the SD card
+
+// The sensor signal (pulse width modulation) is coming into PIN 7
+const int pwmpin = 7;
+// The measurement range is (0-5000 ppm CO2)
+const int range = 5000;
+
+int photozellePin = 0;
+
+// Setup
+void setup() {
+  // Set PIN7 on input
+  pinMode(pwmpin, INPUT);
+  // Start serial communication
+  Serial.begin(9600);
+  // Clear console screen
+  Serial.println("");
+  Serial.println("*****************************");
+  Serial.println("* Weather station           *");
+  Serial.println("*****************************");
+  while (!Serial) {
+    // Waits for the serial port to be ready
+  }
+  if ( !SD.begin(chipSelect) )
+  {
+    Serial.println("-> SD card missing or defect.");
+    return;
+  }
+  Serial.println("-> SD card is now correctly initialized.");
+  // Start clock
+  rtc.begin();
+  rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
+  // Start T and h sensor module
+  sensor.begin();
+  // Create the filename
+  DateTime now = rtc.now();
+  year = now.year();
+  month = now.month();
+  day = now.day();
+  hour = now.hour();
+  minute = now.minute();
+  second = now.second();
+  if ( month < 10 )
+  {
+    filename += "0";
+  }
+  filename += month;
+  if ( day < 10 )
+  {
+    filename += "0";
+  }
+  filename += day;
+  if ( hour < 10 )
+  {
+    filename += "0";
+  }
+  filename += hour;
+  if ( minute < 10 )
+  {
+    filename += "0";
+  }
+  filename += minute;
+  filename += ".txt";
+  // filename = "hallo.txt";
+  // Now write the header
+  File datafile = SD.open(filename, FILE_WRITE);
+  // If file does not exists, a new with the given filename is created
+  if (datafile) // Check if the file exists
+  {
+    Serial.print("-> A file with filename ");
+    Serial.print(filename);
+    Serial.println(" has been created.");
+    // Write the first line (header) into the text file
+    datafile.println("dateTime,lightIntensity,temperature,relativeHumidity,ppmCO2");
+    Serial.println("-> A header line has been written into the file.");
+    Serial.println("");
+  }
+  datafile.close();
+}
+
+// The main program
+void loop() {
+  lightIntensity = analogRead( photozellePin ); // Get the signal from the phototransistor
+  temperature = sensor.readTemperature();
+  relativeHumidity = sensor.readHumidity();
+  // Measure the CO2 concentration width with the function shown below the loop function
+  ppmCO2 = readCO2PWM();
+  DateTime now = rtc.now();
+  year = now.year();
+  month = now.month();
+  day = now.day();
+  hour = now.hour();
+  minute = now.minute();
+  second = now.second();
+
+  // Print date and time to serial port
+  Serial.print("-> ");
+  Serial.print(year, DEC);
+  Serial.print("-");
+  if ( month < 10 )
+  {
+    Serial.print("0");
+  }
+  Serial.print(month, DEC);
+  Serial.print("-");
+  if ( day < 10 )
+  {
+    Serial.print("0");
+  }
+  Serial.print(day, DEC);
+  Serial.print(" ");
+  if ( hour < 10 )
+  {
+    Serial.print("0");
+  }
+  Serial.print(hour, DEC);
+  Serial.print(":");
+  if ( minute < 10 )
+  {
+    Serial.print("0");
+  }
+  Serial.print(minute, DEC);
+  Serial.print(":");
+  if ( second < 10 )
+  {
+    Serial.print("0");
+  }
+  Serial.print(second, DEC);
+  Serial.print(",");
+  Serial.print( lightIntensity );
+  Serial.print(",");
+  Serial.print( temperature, 2 );
+  Serial.print(",");
+  Serial.print( relativeHumidity, 2 );
+  Serial.print(",");
+  Serial.print(ppmCO2);
+
+  // Now write things onto the SD card
+  File datafile = SD.open(filename, FILE_WRITE);
+  if (datafile) // Check if the file exists
+  {
+    Serial.println(", written to file.");
+    datafile.print(year, DEC);
+    datafile.print("-");
+    if ( month < 10 )
+    {
+      datafile.print("0");
+    }
+    datafile.print(month, DEC);
+    datafile.print("-");
+    if ( day < 10 )
+    {
+      datafile.print("0");
+    }
+    datafile.print(day, DEC);
+    datafile.print(" ");
+    if ( hour < 10 )
+    {
+      datafile.print("0");
+    }
+    datafile.print(hour, DEC);
+    datafile.print(":");
+    if ( minute < 10 )
+    {
+      datafile.print("0");
+    }
+    datafile.print(minute, DEC);
+    datafile.print(":");
+    if ( second < 10 )
+    {
+      datafile.print("0");
+    }
+    datafile.print(second, DEC);
+    datafile.print(",");
+    datafile.print( lightIntensity );
+    datafile.print(",");
+    datafile.print( temperature, 2 );
+    datafile.print(",");
+    datafile.print( relativeHumidity, 2 );
+    datafile.print(",");
+    datafile.println(ppmCO2);
+  }
+  datafile.close();
+  delay(100);
+}
+
+
+// This function reads the PWM signal width on PIN7
+int readCO2PWM() {
+  unsigned long th; // This is the width of the PWM signal in microseconds, maximum length is 1004 ms = 1004000 us
+  int ppm_pwm = 0;
+  float pulsepercent;
+  // Wait for the measurement value
+  do {
+    th = pulseIn(pwmpin, HIGH, 2500000) / 1000; // Gives the witdh in milliseconds (division by 1000)
+    // Pulse length in %
+    float pulsepercent = th / 1004.0;
+    // CO2 concentration in ppm
+    ppm_pwm = range * pulsepercent;
+  } while (th == 0);
+  // Return the measured value to the main loop
+  return ppm_pwm;
+}