|Investigation of soil humidity sensors|
A number of types of soil humidity sensors exist. Many have the problem that the electrodes corrode over time because of electrolytic effect. The one used in this project uses capacitance to measure soil humidity avoiding direct electrical contact between the electrode and the soil.
See this video by Andreas Spiess that compares a couple of types.
I got these capacitive ones from Aliexpress. The electrodes themselves are covered in solder mask. I put heatshrink wrapping around the top part of the probe, to protect the electronics from moisture.
It's connected to a Wemos D1 mini ESP8266 board, as follows:
- probe GND (black) to Wemos D1 mini pin D5
- probe VCC (red) to Wemos D1 mini pin D0
- probe AOUT (yellow) to Wemos D1 mini pin A0
This way, all connections on the Wemos D1 are right next to each other and the wire with the 3-pin connector can simply be plugged into the Wemos D1 mini without having to modify the cable.
The software puts a low level on pin D5 (to emulate GND) and a high level on pin D0 (to emulate VCC). The idea is that the operating current of the probe is low enough for this trick to work. I havent actually measured this current.
See my github repository.
The software takes a measurement at a regular interval and sends it over MQTT.
The measurement value is a 'humidity percentage', with 0 representing a completely dry environment and 100 a completely wet environment. I don't know the optimal value yet, but typically my plants are at about 60%.