LoRa Soil Monitor Design Overview

flux

0:27

Short answer: this design conditions a sensor signal with the MAX40005 and reads it on the MCU.
From the schematic:
  • Sensor input path:
    • The soil probe connects at the IN net portal, then through R5 to the nano-power amplifier/comparator IC1 input (Net 22 → IC1:IN).
    • The amplifier’s output (IC1:OUT) is on Net 17, which goes to the MCU’s PA8 pin on U2. C3 is tied to that path (to OUT and GND) to filter noise.
    • C4 provides local rail decoupling for the analog stage.
What this implies functionally:
  • The soil probe changes its electrical characteristics with moisture (typically resistance or conductivity). That varying signal is presented through R5 into IC1.
  • IC1 conditions it and produces a clean logic/analog level the MCU can read on PA8. With suitable firmware, PA8 can:
    • Use a digital threshold (treat IC1 as a comparator) for wet/dry or multi-level thresholding.
    • Or sample as an analog input (if PA8 is configured as ADC on L0 variant) for quasi-analog moisture level, depending on MCU pin capabilities chosen in firmware.
Important caveats:
  • I do not see an explicit AC excitation or true resistive bridge that is common for corrosion-free soil sensing. As-is, this likely uses a simple DC-based measurement, which can work but may cause probe electrolysis over time.
  • There is no obvious sensor connector net labeled specifically for a two-electrode probe except the IN line plus GND. That suggests a single-ended measurement versus ground.
If you share how your probe is connected (two-wire resistive vs. capacitive pad), I can recommend:
  • For resistive probes: low-duty-cycle AC excitation via GPIO and measure with an RC/ADC method to reduce electrolysis, plus input protection.
  • For capacitive probes: drive an excitation signal and measure amplitude/phase or frequency using a timer/capture pin, or add a simple relaxation oscillator front end.
Would you like me to propose a concrete sensing front end and firmware readout approach for your specific probe type?
Ask about probe type
Request improved analog front end
Ask for AC excitation approach
Clarify MCU measurement method
Add connector and protection
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    LoRa soil monitor Reference Design.png

    LoRa soil monitor Reference Design.png

    LoRa soil monitor Reference DesignThumbnail

    LoRa soil monitor Reference Design

    LoRa soil monitor Reference Design thumbnail
    This is a LoRa soil monitor Reference Design. It uses a STM32L031G6U6S microcontroller and a RFM95W-915S2 LoRa transceiver, integrated with sensor interfacing and LED indicators. Communication occurs via USART and SPI. The system is powered using a battery. #referenceDesign #simple-embedded #stm #template #reference-design

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