In summer, I have a lot of potted plants out on the balcony, and I need to know when to water them. This was an opportunity to learn about optimizing a WiFi based gadget for low power consumption.
The solution consists of an ESP32 based device, connected to 4 capacitive soil moisture sensors, which reports its measurements once an hour via MQTT.
It is powered by a pair of AAA batteries, which should last about 500 days.
A more detailed description can be found in my blog, and source code and hardware schematics are available in Github