Mad_PlantBug

Low-power capacitive soil moisture sensor with BLE connectivity.

Features

• powered via CR2032 coin battery
• transmits telemetry via BLE
• measures ambient light and temperature
• visual indication via LED

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How it works

This sensor determines the amount of soil moisture by measuring changes in capacitance of the probe submerged in soil.
Insertable part forms a parasitic coplanar capacitor which uses soil as a dielectric medium.
As the environment around the capacitor changes depending on whether the soil becomes wetter or drier, so does it capacitance.
Together with a resistance the probe forms an RC low-pass filter.
Changes in capacitance shape the output waveform observed by the sensor.
Measurement pipeline logic is as follows:
environment changes -> capacitance changes -> cutoff frequency changes -> peak amplitude changes

Capacitance measurement

• Clean PCB measurement in air: ~59pF
• Fully submerged in water: 2nF

Assembly & measurements

Current issues

• CR2032 footprint and power pins connection is incorrect.