Soil Moisture Probe Design

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Best manual routing order for the 14 airwires
For this probe, the best order is to route by function and sensitivity, not randomly. In general, routing should prioritize net classes deliberately rather than one-by-one ad hoc; the project-modification guidance explicitly calls out routing by inferred net type and prioritization order . Since your board is a narrow sensor probe with a capacitive front end, I’d use this order:
1. Route the sensor-critical nets first
  • SENSE_A
  • SHIELD
These are the most sensitive nets and should get the shortest, cleanest paths with the most control over nearby copper. If you route power and digital first, you’ll box yourself in and force these onto noisier paths later.
Guidelines
  • Keep SENSE_A as short and direct as possible from U1 toward the sensing structure.
  • Route SHIELD to surround or guard SENSE_A only where intentional.
  • Avoid vias if possible.
  • Keep these nets away from SDA, SCL, 1WIRE, and any broad ground copper.
2. Route power next
  • 3V3
  • GND
Once the sensitive analog path is protected, lock in the supply distribution. This keeps decoupling loops compact and avoids awkward detours later.
Guidelines
  • Route J1 to U1 and U2 first.
  • Make the paths to C1 and C2 very short.
  • Keep return paths compact on the component half of the board.
3. Route the I2C bus after power
  • SDA
  • SCL
These are low-speed digital nets, but they are shared bus lines and should still be kept tidy and parallel only where necessary.
Guidelines
  • Route from J1 to U1, with short branches to R1 and R2.
  • Keep them away from SENSE_A and SHIELD.
  • Don’t snake them through the sensing-region boundary.
4. Route the 1-Wire net last
  • 1WIRE
This is usually the easiest remaining signal once analog, power, and bus routing are settled.
Guidelines
  • Route J1 to U2 with the pull-up at R3.
  • Keep it compact, but it does not need the same priority as the capacitive sensing net.
Practical airwire-clearing sequence
If you want a concrete hand-routing sequence, do it in this order:
  1. U1 SENSE_A
  2. U1 SHIELD
  3. J1 3V3 to local supply area
  4. 3V3 branches to U1, U2, C1, C2, R1, R2, R3
  5. GND return connections for U1, U2, C1, C2
  6. J1 to U1 SDA
  7. J1 to U1 SCL
  8. Short pull-up branches to R1 and R2
  9. J1 to U2 1WIRE
  10. Short pull-up branch to R3
Placement-aware tip
Because placement quality strongly affects trace length and routing success, it’s good practice to keep supporting parts clustered near their ICs and connectors at the edge before routing . For your board, that means:
Rule of thumb
  • Analog sensor nets first
  • Power second
  • Digital bus third
  • Lowest-priority digital last
If you want, I can turn this into a pin-by-pin routing checklist for J1, U1, and U2 so you can clear the 14 airwires in order without guessing.
Make pin checklist
Suggest rerouting moves
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Project Specification
Overview
A long-form soil moisture probe PCB for buried agricultural use. The board uses an external 3.3 V supply and exposes a bottom sensing region with interdigitated electrodes on both copper layers for capacitive moisture measurement.
Architecture
Diagram



  
    
  
  
    
  










3.3V GND SDA SCL node_1WIRE

1WIRE

3.3V

3.3V

CAP sense channels

CAP sense channels

Decouple

Decouple







"JST Connector"
"FDC1004 Cap Sensor"
"DS18B20 Temp Sensor"
"100nF Bypass"
"100nF Bypass"
"Interdigitated Electrodes Top"
"Interdigitated Electrodes Bottom"
Electrical Requirements
  • External regulated 3.3 V supply enters through a JST connector.
  • FDC1004 provides capacitance measurement for the PCB electrode structure.
  • DS18B20 provides soil temperature measurement.
  • Use 100 nF bypass capacitors for each IC supply.
  • All passives must be 0402 SMD.
  • Interface signals at the connector are 3.3 V, GND, SDA, SCL, and 1WIRE.
Mechanical Requirements
  • Board outline: 150 mm x 15 mm.
  • Two copper layers only.
  • Fabrication target: JLCPCB 0.8 mm board thickness.
  • Components must remain on the top half of the board.
  • The bottom half is reserved for sensing electrodes.
Sensing Region Requirements
  • Interdigitated sensing electrodes on both copper layers.
  • Electrode geometry target: 1 mm trace / 1 mm space.
  • No soldermask in the sensing region.
  • Keep component copper and ground copper away from the sensing region.
  • Use a wide top-side ground pour only in the electronics section.
Manufacturing Requirements
  • Final outputs must include manufacturable Gerbers.
  • BOM must be compatible with LCSC/JLC assembly where applicable.
Assumptions
  • The connector is implemented as a 5-pin JST-style interface because the required external signals are 3.3 V, GND, SDA, SCL, and 1WIRE.
  • A standard I2C pull-up network will be added for the FDC1004 interface.
  • A pull-up resistor will be added on the DS18B20 1WIRE signal unless the chosen system architecture explicitly places it off-board.

    Overview

    Architecture

    Electrical Requirements

    Mechanical Requirements

    Sensing Region Requirements

    Manufacturing Requirements

    Assumptions

Documents

    Project Specification

    Design Notes

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Soil Moisture Probe

Soil Moisture Probe thumbnail
150 mm x 15 mm 2-layer soil moisture probe using an external 3.3 V supply, FDC1004 capacitance sensor, DS18B20 temperature sensor, top-half component placement, and a bottom-half exposed interdigitated sensing electrode region on both copper layers with 1 mm trace/space for JLCPCB 0.8 mm fabrication.

Properties

Properties describe core aspects of the project.

Pricing & Availability

Distributor

Qty 1

Arrow

$1.96

Digi-Key

$2.82–$5.31

LCSC

$3.08

Mouser

$3.19

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