Soil sensor

Soil Sensor

A BLE soil moisture + temperature sensor for houseplants: a custom fork-shaped PCB (CC2340R5MODA + resistive probe, CR2032 coin cell) plus an Android app that logs and charts the history.

soil-sensor/
├── firmware/   TI SimpleLink BLE5-Stack firmware (FreeRTOS, ticlang)
├── android/    Kotlin/Compose companion app
├── hardware/   KiCad project + JLCPCB fabrication/assembly output
└── docs/       screenshots, BLE protocol reference

Fork-shaped PCB (top / bottom). Render by KiCad.

How it works

The board samples its resistive fork probe every 10 minutes and keeps a rolling 7-day buffer. The Android app connects, pulls only the samples it hasn't seen yet, and keeps its own unbounded local history.

The app can also push a signed firmware image to the sensor over BLE (on-chip OAD via MCUboot).

Hardware

KiCad project + JLCPCB fabrication/assembly output in hardware/.

• MCU: CC2340R5MODA module (to simplify RF part)
• Sensor: two exposed copper tines on the PCB itself act as a resistive soil probe (based on SparkFun SEN-13322), gated by a P-MOS switch so current only flows through the soil during a reading. Read via the CC2340's own ADC
• Temperature: CC2340's on-die sensor
• Battery: CR2032 coin cell; level estimated from VDDS via the CC2340's internal Battery Channel ADC (no fuel-gauge IC)
• Debug: standard 10-pin Tag-Connect

Firmware

SDK basic_ble example, stripped to a peripheral-only role plus on-chip OAD:

• app/app_soil_sensor.c - the glue: a 10-min Clock takes a sample (soil + battery), stores it, notifies subscribers, and reports its own version over Device Information at boot
• app/soil_sensor.c - GPIO-gated ADC for the fork probe + die temperature
• app/soil_history.c - RAM ring buffer (1008 samples ≈ 7 days) with a never-reset sequence number per sample so a client can resume a sync
• app/battery_gauge.c - VDDS battery estimate via the internal Battery Channel ADC
• app/Profiles/* - GATT services: custom soil sensor, Battery (0x180F + a custom mV characteristic), on-chip OAD glue

Building

Requires the SimpleLink Lowpower F3 SDK, SysConfig, and the ticlang compiler.

The makefile pulls SYSCONFIG_TOOL and TICLANG_ARMCOMPILER from the SDK's imports.mak ($SDK/imports.mak), which ships with placeholder /home/username/... paths. Edit those two ?= lines once to point at your real installs, e.g.:

SYSCONFIG_TOOL      ?= /home/you/ti/ccs2100/ccs/utils/sysconfig_1.28.0/sysconfig_cli.sh
TICLANG_ARMCOMPILER ?= /home/you/ti/ccs2100/ccs/tools/compiler/ti-cgt-armllvm_5.1.1.LTS

Then just:

cd firmware/freertos/ticlang
gmake

Outputs:

• soil_sensor.hex / soil_sensor.out - full-flash image for a debugger.
• soil_sensor_ota.bin - the OAD image. Version is MAJOR.MINOR.<git commit count> (e.g. 1.0.6), stamped by tools/oad_version.sh. Build from a repo checkout; a CCS build copies the sources out of git and the commit count falls back to 1.0.0.
• soil_sensor_v1.bin / _v2.bin - SDK fixed 1.0.0 / 2.0.0 placeholders, handy for OAD upgrade testing.

First flash (one-time, needs a debugger)

OAD updates a running device; the first flash needs three images at fixed addresses. Flash all three with UniFlash:

Use the prebuilt MCUboot hex. A self-built one defaults to a generic flash map that never boots the app slot.

Firmware updates over BLE (OAD)

Tap the update icon next to "Disconnect" in the app, pick soil_sensor_ota.bin, and confirm the version/size shown.

Tuning

• Sample interval: SOIL_SAMPLE_PERIOD_MS in app/app_soil_sensor.c (currently 10 minutes).
• Moisture thresholds: the raw ADC code from the resistive divider is ratiometric against VDDS (higher code = lower soil resistance = wetter) and depends on your probe geometry, resistor values, and soil/pot. Use the CSV export to work out real thresholds for your plant.

Android app

Location: android/. Kotlin + Jetpack Compose, Material 3 (dynamic color), Room, no third-party BLE or charting libraries.

Building

cd android
./gradlew assembleDebug
adb install -r app/build/outputs/apk/debug/app-debug.apk

Requires local.properties pointing at an Android SDK (sdk.dir=...). It is not checked in, so create your own. minSdk 31 (Android 12), targetSdk/compileSdk 37.

BLE protocol

Services, characteristics, and record layout: docs/PROTOCOL.md.