It allows to manage all the fitting rooms of a clothing store, detecting attendance, climate or consumption and managing calls to staff.

Using the Arduino Cloud API to obtain IoT data for your MIT App Inventor apps.

How to Control IoT ONE Nixie Clock - Backlight & Nixie Tube & Display

How to Control IoT ONE Nixie Clock via Arduino IoT Cloud or Alexa voice Control

This device allows me to detect if the spool ran out of filament or if the print head is clogged !!

My first project running on a lithium ion battery is here: an IOT device that not only detects if there is water but sends data to the cloud

If you want use two Arduino, you can use a simple I/O port to communicate. The point of question is the Ground.

Arduino micro with RFID unlock your computer. Set one or many RFID tag, and unlock your computer with your personal password.