An IoT Card Rack: cardRack

This is a project description showing how to build a 3D-printed card rack that signals how many slots are used over LTE-M.

Hardware

The IoT rack uses the IR sensors APDS-9930 for presence sensing and the AVR-IoT Cell Mini board EV70N78A by Microchip as the controller and LTE-M board. Since the sensors use all the same I2C address, they have to be on separate I2C buses. However, all in all, the hardware is very minimal, as one can see in the Fritzing sketch below. The Feather board is actually the mentioned EV70N78A board and not the Feather 328P. Since they have identical pin assignments and there is no Fritzing part for the former one, I used the latter one.

While the initial version was built on a Featherwing doubler, which looks quite ugly, I have now created a PCB. The KiCAD design data can be found in the pcb directory.

Software

The software is quite simple. An interrupt is triggered if a card is removed or inserted. Then, the sensors are polled to check the number of used slots. Finally, a short message is sent via an HTTPS POST command to a webserver, where a CGI script stores the number of occupied slots. One can use the stored number in a Javascript script to display it on a web page.

To monitor that everything is working, the AVR chip is woken up every 2 hours, and a message with the number of occupied slots is sent. If the last message received is over 3 hours old, the information will no longer be considered valid.

3D Print Data

The 3D print data for the physical card rack can be found on printables.com.

Folders

The repository contains the following folders:

• IoTRack: The Arduino sketch
• pics: Some photos
• schematic: A Fritzing schematic of the electronics
• web: CGI script, an example bash script to trigger the CGI script, and an example web page
• pcb: KiCAD design files