This was my experiment in creating a low power Arduino that can communicate wirelessly to a Raspberry Pi using an HC-12. By low power I mean run for a couple of years on a 18650 battery. Encryption should be unhackable by today's standards.
I did a presentation for the Raspberry Pint group, the video is here.
The code is on my Github repo. I had to do some mods to the Arduino board to make it low power (removing the regulator and power LED resistor):
and programing the board is done with a serial borad (search "ftdi usb to serial" in AliExpress):
I worked out that if I send a message every minute the 18650 battery should last well over two years. With a message every four seconds the battery should last two months or so - I have been testing this, and it has managed three weeks, so I think my calculations are correct.