[$] Using low-cost wireless sensors in the unlicensed bands

When it comes to home automation, people often end up with devices
supporting the Zigbee or Z-Wave protocols, but those devices are
relatively expensive. When I was looking for a way to keep an eye on the
temperature at home a few years ago, I bought a bunch of cheap
temperature and humidity sensors emitting radio signals in the unlicensed
ISM (Industrial, Scientific, and Medical) frequency bands instead. Thanks to rtl_433 and, more recently, rtl_433_ESP and OpenMQTTGateway,
I was able to integrate their measurements easily into my home-automation
system.