Soil moisture sensor

KiCad v9 Soil Moisture Sensor Project

Project Overview

Complete KiCad v9 project for a capacitive soil moisture sensor with interdigitated electrodes, temperature compensation, and LoRa transmission.

Files

• soil_moisture_sensor.kicad_pro - Main project file
• soil_moisture_sensor.kicad_sch - Schematic file
• soil_moisture_sensor.kicad_pcb - PCB layout file

Key Features

• Interdigitated Electrodes: Advanced capacitive sensing with soldermask protection
• Temperature Compensation: Air reference electrode for accurate measurements
• Guard Ring Design: Bootstrap guard ring reduces parasitic capacitance
• 74HC14 Oscillator: Schmitt trigger RC oscillator for frequency measurement
• ATTINY85 Microcontroller: Dual sensor reading and 1-wire communication
• CubeCell LoRa: Wireless data transmission module

PCB Specifications

• Size: 50mm x 35mm
• Layers: 2 (Top and Bottom)
• Thickness: 1.6mm
• Copper Weight: 1oz (35μm)
• Soldermask: Green
• Silkscreen: White
• Surface Finish: HASL or ENIG

Electrode Design

• Primary Sensor: 25mm x 15mm interdigitated electrodes
• Air Reference: 10mm x 10mm temperature compensation
• Finger Width: 0.5mm
• Finger Spacing: 0.5mm
• Guard Ring: 2mm wide, connected to oscillator output

Usage Instructions

1. Open in KiCad v9

1. Launch KiCad v9
2. Open soil_moisture_sensor.kicad_pro
3. The project will load with schematic and PCB files

2. View Schematic

1. Click "Open Schematic" or double-click .kicad_sch file
2. View complete circuit design
3. Check net connections and component values

3. View PCB Layout

1. Click "Open PCB" or double-click .kicad_pcb file
2. View 2D/3D PCB layout
3. Check component placement and routing

4. Generate Manufacturing Files

1. In PCB Editor: File → Fabrication Outputs → Gerbers
2. Select layers: F.Cu, B.Cu, F.Mask, B.Mask, F.SilkS, B.SilkS, Edge.Cuts
3. Generate drill files and pick-and-place files

5. Order PCB

1. Use generated Gerber files
2. Specify: 2-layer, 1.6mm thickness, 1oz copper
3. Include soldermask and silkscreen

Component List

• U1: 74HC14 (DIP-14) - Schmitt trigger oscillator
• U2: ATTINY85-20PU (DIP-8) - Microcontroller
• U3: AMS1117-3.3V (SOT-223) - Voltage regulator
• U4: CubeCell Module (SMD) - LoRa transmitter
• R1: 10kΩ (0603) - Timing resistor
• C1: 100nF (0603) - Decoupling capacitor
• J1: Power Connector (2-pin) - Power input
• J2: Programming Header (6-pin) - ISP programming

Net Assignments

• VCC: 3.3V power rail
• GND: Ground plane
• SOIL_FREQ: Soil sensor frequency input
• AIR_FREQ: Air reference frequency input
• ONE_WIRE: 1-wire communication line
• SENSOR_SEL: Sensor selection control

Manufacturing Notes

• Assembly: CubeCell requires reflow soldering
• Protection: Apply conformal coating over sensor area
• Programming: Use 6-pin ISP header for ATTINY85
• Testing: Verify oscillator frequency and 1-wire communication

Calibration

1. Dry Soil: Measure frequency in completely dry soil
2. Wet Soil: Measure frequency in saturated soil
3. Air Reference: Measure baseline frequency
4. Update Firmware: Adjust calibration constants

Support

For technical support and questions:

• Check the main project documentation
• Review the assembly instructions
• Contact the maintainers

License

This project is open source. See LICENSE file for details.