If your data is more sensitive and you don't want to share it across the Wi-Fi network or you don't have Wi-Fi connectivity, a solution is to store your data on an SD card.

Let's see how data can be stored on an SD card.

Necessary hardware that will be used:

Since the microSD card is a shield for the Wemos D1 mini, it is easy to stack them; you just need to solder the pins that are coming into the package:

Let's determine the size of the SD card with the following sketch.

Include the SPI.h and the SD library:

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

Set up variables using the SD utility library functions:

Sd2Card card; 
SdVolume volume; 
SdFile root; 
const int chipSelect = D8; 

In the setup function, we will determine if the card is inserted or not and the card details will be read over SPI:

void setup() 
{ 
  Serial.begin(115200); 
  Serial.print("\nInitializing SD card..."); 

Use the initialization code from the utility libraries...