Arduino PM sensor system 2/3: sensor data on an SD card

In a series of three posts, I share my road towards a PM sensor system with help of Arduino and the SPS30 Sensirion PM sensor. This second post: storing the data on an SD card.

In the previous post I described how I got the sensor data on my computer. Preferably, measurements are done without continuously having a computer around. I wanted to add local data storage to the sensor system with a memory (micro SD) card.

1 Materials

Next to the setup part 1 materials, I bought the following materials:

DS3231 Real Time Clock
SD card module
Micro SD card (in bulk from Aliexpress much cheaper)
10 jumper cables

I found out that for a memory card, it was better to have a relatively small card (< 1GB). Furthermore, after running into battery failure of the real-time clock multiple times, I learned that the DS3231 charging system could destroy non-rechargeable batteries. To resolve this, the charging circuit had to be disabled (for example by removing resistor 201 from the DS3231 shield).

2 Software

Both the real-time clock and the SD card work with libraries that by default are included in the Arduino IDE installation. Additional software was not needed.

3 Connections

I made the following connections between the Aduino Mega and the SD and real-time clock modules:

SD module...Mega
GND.........GND
VCC.........5V
MISO........50
MOSI........51
SCK.........52
CS..........53

DS3231......Mega
GND.........GND
VCC.........5V
SDA.........SDA
SCL.........SCL

I also put the micro SD card in the SD module.

4 Sketch

I used the following sketches for the components:

Example1_sps30_BasicReadings of the sps30 library
ds3231 of the RTClib library
SoftwareSPI of the SDfat library

In the below sketch, these are combined.

See sketch

#include <SPI.h>
#include "SdFat.h"
#include <Wire.h>
#include "RTClib.h"
#include "sps30.h"

SPS30 sps30;
SdFat SD;
RTC_DS3231 rtc;

#define SD_CS_PIN SS
#define SP30_COMMS I2C_COMMS
#define DEBUG 0

File dataFile;
String data_filename= "measurement_data";
String month_cov = "";
String dataheader = "counter;datetime;PM1;PM2_5;PM4;PM10;NumPM1;NumPM2_5;NumPM4;NumPM10;PartSize";
String data ="";
String serialdata = "";
struct sps_values val;
String pmString = "";

long counter = 0; // For every restart, a counter is included

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  Serial.print("Initializing RTC module...");
  if (! rtc.begin()) {
    Serial.println("Couldn't find RTC");
    while (1);
  }

  Serial.print("Initializing SD card...");

  if (!SD.begin(SD_CS_PIN)) {
    Serial.println("initialization failed!");
    return;
  }
  Serial.println("initialization done.");
// set driver debug level
  sps30.EnableDebugging(DEBUG);
  // Begin communication channel TO sp30;
  if (!sps30.begin(SP30_COMMS)) 
    {
   while(1);
    }
  
  // check for SPS30 connection
 if (!sps30.probe()) 
   {
    while(1);
   }
  // reset SPS30 connection
  if (!sps30.reset()){
    
    while(1);
  }

  // start measurement
  sps30.start();    
  delay(10);
}

void loop() 
{
  DateTime now = rtc.now();
  sps30.GetValues(&val);
  long pm1 = static_cast<long>(floor(val.MassPM1+0.5));
  long pm2 = static_cast<long>(floor(val.MassPM2+0.5));
  long pm4 = static_cast<long>(floor(val.MassPM4+0.5));
  long pm10 = static_cast<long>(floor(val.MassPM10+0.5));
  long num1 = static_cast<long>(floor(val.NumPM1+0.5));
  long num2 = static_cast<long>(floor(val.NumPM2+0.5));
  long num4 = static_cast<long>(floor(val.NumPM4+0.5));
  long num10 = static_cast<long>(floor(val.NumPM10+0.5));

  pmString = String(pm1)+";"+String(pm2)+";"+String(pm4)+";"+String(pm10)+';'+String(num1)+';'+String(num2)+';'+String(num4)+';'+String(num10)+';'+String(val.PartSize);
  data = String(String(counter)+';'+now.timestamp(DateTime::TIMESTAMP_FULL))+";"+pmString;
  serialdata = String(String(counter)+'\t'+now.timestamp(DateTime::TIMESTAMP_FULL))+'\t'+pmString;
  Serial.println(serialdata);

  // Decide whether headers need to be included
  bool datafile_exists = SD.exists(data_filename);

  dataFile = SD.open(data_filename, FILE_WRITE);
  // if the file opened okay, write to it:
  if (dataFile) {
    if(!datafile_exists) {
      dataFile.println(dataheader);
    }

    dataFile.println(data);
    // close the file:
    dataFile.close();
          
  } else {
    // if the file didn't open, print an errors:
    Serial.println("error opening datafile");      
  }
  ++counter;
  delay(8500);
}

5 Operation and next

After uploading the sketch to the Arduino board, it started to collect data and store it on the SD card. I could get the data from the SD card. This is already sufficient for many measurement projects, but online real-time data would be even nicer. Post three: adding LoRa functionality to the sensor system.