Radio

This LED (5mm), looking like a water tower, is a detail of RadioLINK module. In the background is another one.
Our house has some smoke alarms. But A: not enough, B: too old, and C: not interconnected.
Every house here, by law, needs to have smoke alarms. Not in all rooms, kitchen and bad for instance are excluded.
Because of before mentioned A, B and C  we finally bought a whole series of new alarms, now also with a carbon monoxide detector.
The radiolink modules are to enable interconnection. If the alarm goes off in the top floor, far away from the stairwell, and far away from our bedroom two floors lower, with the old system we would not hear it.
Using the radiolink modules all alarms will go off. At least in theory.
The alarms/detectors can also "talk" with an app on a mobile. And with a handset, just in case.
The alarms have a battery that will last 10-11 years, enough, since the alarms need to be replaced after 10 years anyway.
Hopefully I will not have interference with alarms in neighbouring houses.
On the other hand, then they would also know about smoke/fire in our house if we are out :)