This is just an early beginning of project experimentation. I took two WiFi LoRa devkit by Heltec, soldered some pins on one of them and connected it to a GY-30 sensor (which had pins soldered on the wrong side long time ago! haha).

I then wrote some quick-and-dirty code that continuously reads luminosity from the sensor and sends it over low-level LoRa as a packet.

The other LoRa devkit, in absence of the sensor, just reads whatever it receives from the first devkit and displays it as well.

This is what you can see in a (somewhat blurry) video below. The secondary LoRa is on the left. Please excuse the cabling, it's a quick proof of concept! 

After making this work, I decided to read a bit more about LoRaWAN, and, damn, that was a mistake to start doing this @ 10pm! While I kind of started understanding the highest-possible-level properties of it, I ended up trying to shop around for a gateway, joining TTN and figuring out if there are any gateways within a reasonable proximity (and it looks like there aren't; I am in Maple Ridge, BC, Canada and it looked like I am at least 18kms away from one in Port Coquitlam). So this got me thinking. Should a mailbox notifier rely on a presence of a LoRaWAN gateway? Those things aren't particularly cheap.

Would it make sense for the device to support both? If it can't join a network, simply assume there is none and broadcast the signal in the hope there's a receiving device nearby. What do you think?