In this project, we are going to make a temperature and relative humidity data logger. Arduino is the brain of this project. DHT22 sensor is used for sensing temperature and relative humidity. Arduino Uno is programmed to read temperature, humidity values from DHT22 sensor and save it to a file in an SD Card. So whenever required we can take the SD Card for viewing data. Here we will take data from SD card and import it to excel to plot graphs.

Components Required

• Arduino Uno
• SD Card Module
• DHT22 Temperature and Humidity Sensor
• SD Card (Higher than 8GB)
• Breadboard
• Connecting wires

Circuit Diagram

Explanation

First of all, we will connect the SD Card module to the Arduino. Arduino can do read write operations on SD Card via SPI protocol. So we need to connect the SD card module to SPI pins of Arduino, which are pins 13, 12, 11 and 10. Since it is using SPI protocol, it won’t work if we connect SD card module to normal digital IO pins. So make connections as below.

• SD Card Module Ground to Arduino Ground
• VCC of the module to 5V output of Arduino
• MISO of module to pin 12 of Arduino
• MOSI of module to pin 11 of Arduino
• SCK of module to pin 13 of Arduino
• CS of module to pin 10 of Arduino

Then we can connect the DHT22 to the Arduino.

• Connect VCC of DHT22 to 5V output of Arduino
• Ground to ground of Arduino
• Data pin of DHT22 to pin 8 of Arduino

Program

#include <SD.h>
#include <SPI.h>
#include "DHT.h"

#define DHTPIN 8
#define DHTTYPE DHT22

long seconds=00;
long minutes=00;
long hours=00;

int CS_pin = 10;

DHT dht(DHTPIN, DHTTYPE);
File sd_file;

void setup()  {
  Serial.begin(9600);
  pinMode(CS_pin, OUTPUT);
  dht.begin();
  // SD Card Initialization
  if (SD.begin())  {
    Serial.println("SD card is initialized. Ready to go");
  } 
  else  {
    Serial.println("Failed");
    return;
  }

  sd_file = SD.open("data.txt", FILE_WRITE);

  if (sd_file)  {
    Serial.print("Time");
    Serial.print(",");
    Serial.print("Humidity");
    Serial.print(",");
    Serial.print("Temperature_C");
    Serial.print(",");
    Serial.print("Temperature_F");
    Serial.print(",");
    Serial.println("Heat_index");

    sd_file.print("Time");
    sd_file.print(",");
    sd_file.print("Humidity");
    sd_file.print(",");
    sd_file.print("Temperature_C");
    sd_file.print(",");
    sd_file.print("Temperature_F");
    sd_file.print(",");
    sd_file.println("Heat_index");
  }
  sd_file.close(); //closing the file
} 

void loop()  {
  sd_file = SD.open("data.txt", FILE_WRITE);
  if (sd_file)  {
    senddata();
  }
  // if the file didn't open, print an error:
  else  {
    Serial.println("error opening file");
  }
  delay(1000);
}

void senddata()  {
  for(long seconds = 00; seconds < 60; seconds=seconds+2)  {
    float temp = dht.readTemperature(); //Reading the temperature as Celsius and storing in temp
    float hum = dht.readHumidity();     //Reading the humidity and storing in hum
    float fah = dht.readTemperature(true);
    float heat_index = dht.computeHeatIndex(fah, hum);

    sd_file.print(hours);
    sd_file.print(":");
    sd_file.print(minutes);
    sd_file.print(":");
    sd_file.print(seconds);
    sd_file.print(",  ");
    sd_file.print(hum);
    sd_file.print(",    ");
    sd_file.print(temp);
    sd_file.print(",      ");
    sd_file.print(fah);
    sd_file.print(",      ");
    sd_file.println(heat_index);

    Serial.print(hours);
    Serial.print(":");
    Serial.print(minutes);
    Serial.print(":");
    Serial.print(seconds);
    Serial.print(",  ");
    Serial.print(hum);
    Serial.print(",    ");
    Serial.print(temp);
    Serial.print(",       ");
    Serial.print(fah);
    Serial.print(",      ");
    Serial.println(heat_index);

    if(seconds>=58)  {
      minutes= minutes + 1;
    }

    if (minutes>59)  {
      hours = hours + 1;
      minutes = 0;
    }

    sd_file.flush(); //saving the file

    delay(2000);
  }
  sd_file.close();   //closing the file
}

DHT22 Library

You need to manually add DHT library to Arduino IDE as it is not included by default. You can ignore it if you already added it. Otherwise you can do following steps for that.

• Download DHT Library from here : DHT Sensor Library.
• Open Arduino IDE.
• Go to Sketch >> Include Library >> Add .ZIP Library
• Select the downloaded ZIP file and press open.

Explanation

First of all, we have added libraries required for this project. Then we defined the DHT sensor type and pin to which it is connected.

#include <SD.h>       // Including SD Card Library
#include <SPI.h>      // Including SPI Library
#include "DHT.h"      // Including DHT Sensor Library

#define DHTPIN 8      // Declaring the pin to which DHT data pin is connected
#define DHTTYPE DHT22 // Selecting the type of DHT sensor used in this project

long seconds=00;      // For storing second
long minutes=00;      // For storing minute
long hours=00;        // For storing hour 

int CS_pin = 10;      // Chip select pin of SD card module

DHT dht(DHTPIN, DHTTYPE); // Declaring DHT object
File sd_file;             // Declaring File object

In the setup function, we initialized the serial communication at 9600 baudrate. Here serial communication is used only for displaying logs in the Arduino Serial Monitor. The dht.begin() function will initialize the Arduino for reading data from DHT sensor. Similarly sd.begin() function will initialize the Arduino for write data to SD card module. After that we will open the file “data.txt” from SD card using sd.open() function. Then we will write titles ‘Time’, ‘Humidity’, ‘Temperature_C’, ‘Temperature_F’ and ‘Heat_index’ in it. After that we will close the connection to save the file.

void setup()  {
  Serial.begin(9600);
  pinMode(CS_pin, OUTPUT);
  dht.begin(); // Initialize the Arduino for reading data from DHT sensor
  // SD Card Initialization
  if (SD.begin())  {
    Serial.println("SD card is initialized. Ready to go");
  } 
  else  {
    Serial.println("Failed");
    return;
  }

  sd_file = SD.open("data.txt", FILE_WRITE);

  if (sd_file)  {
    Serial.print("Time");
    Serial.print(",");
    Serial.print("Humidity");
    Serial.print(",");
    Serial.print("Temperature_C");
    Serial.print(",");
    Serial.print("Temperature_F");
    Serial.print(",");
    Serial.println("Heat_index");

    sd_file.print("Time");
    sd_file.print(",");
    sd_file.print("Humidity");
    sd_file.print(",");
    sd_file.print("Temperature_C");
    sd_file.print(",");
    sd_file.print("Temperature_F");
    sd_file.print(",");
    sd_file.println("Heat_index");
  }
  sd_file.close(); //closing the file
} 

In the loop function, we will open the SD card “data.txt” file again. If it is opened successfully we will call senddata() function, otherwise it will show ‘error opening file’ in the serial monitor.

void loop()  {
  sd_file = SD.open("data.txt", FILE_WRITE);
  if (sd_file)  {
    senddata();
  }
  // if the file didn't open, print an error:
  else  {
    Serial.println("error opening file");
  }
  delay(1000);
}

In the senddata() function, we have made a loop which will count the time. Then we have read the temperature, humidity from DHT22 sensor and heat index is calculated. After storing these values in the variables, we will write these values to the SD card file. After that we will call sd_file.flush() to ensure that all data is stored in the SD card. These steps are repeated with a delay of 2 seconds.

void senddata()  {
  for(long seconds = 00; seconds < 60; seconds=seconds+2)  {
    float temp = dht.readTemperature(); //Reading the temperature as Celsius and storing in temp
    float hum = dht.readHumidity(); //Reading the humidity and storing in hum
    float fah = dht.readTemperature(true);
    float heat_index = dht.computeHeatIndex(fah, hum);

    sd_file.print(hours);
    sd_file.print(":");
    sd_file.print(minutes);
    sd_file.print(":");
    sd_file.print(seconds);
    sd_file.print(",  ");
    sd_file.print(hum);
    sd_file.print(",    ");
    sd_file.print(temp);
    sd_file.print(",      ");
    sd_file.print(fah);
    sd_file.print(",      ");
    sd_file.println(heat_index);

    Serial.print(hours);
    Serial.print(":");
    Serial.print(minutes);
    Serial.print(":");
    Serial.print(seconds);
    Serial.print(",  ");
    Serial.print(hum);
    Serial.print(",    ");
    Serial.print(temp);
    Serial.print(",       ");
    Serial.print(fah);
    Serial.print(",      ");
    Serial.println(heat_index);

    if(seconds>=58)  {
      minutes= minutes + 1;
    }

    if (minutes>59)  {
      hours = hours + 1;
      minutes = 0;
    }

    sd_file.flush(); //saving the file

    delay(2000);
  }
  sd_file.close(); //closing the file
}

Importing Logs to Excel

For viewing or evaluating the logged data, you can take the SD card from the module and connect it to a PC via a SD card reader.

1. Open Microsoft Excel
2. Press Office button or File
3. Click Open
4. Select the file from the SD card and open it.

After opening the file, all data will be shown on the screen. Now we can plot graphs by using these values.

Generating a Graph

• Go to Insert tab
• Click on Line option
• Select 2D Line

The graph will look like this. You can also customize this graph by using other options.

Video