Legacy article: this content was migrated from the old Hive76 wiki and may be outdated or inaccurate.
Multiple sensors are used in the system, the main sensor being the moisture sensor.
Moisture Sensor
Moisture sensors are simple and act on measuring capacitance between 2 points. As the soild dries, the capacitance lowers.
So simply put, you have 2 options for the sensor, DIY or commercial: