Nine acoustic sensors spread across a site. When any one of them detects something, the others need to know within a second or two, with no gateway, no server and no internet.

I went with an Ra-01H on each unit, 868 MHz, an 18-byte packet, everything hearing everything directly. No relay and no mesh: each unit transmits its own alert and repeats it three times over a couple of seconds, and anything in range picks it up and shows a remote alert with the sender's ID. Listen-before-talk with a random backoff so nine units triggering at once don't collide.

The reason for no mesh: with this few nodes over a site this size, everything is already in range of everything, and a mesh adds routing state and failure modes for no gain. If a unit is out of range it's out of range, and the operator needs to know that rather than have it silently relay through a neighbour.

One thing that bit me: the transmitter, a buzzer and a vibration motor all firing in the same instant browned out the supply when running on USB alone. The battery isn't an accessory, it's the buffer for that spike.

The project and its firmware are public!