F-UVC-WaterBottle (Prototype)

F-UVC-WaterBottle is a prototype for a portable, bottle-scale water treatment concept that stages:

1. Filtration to handle turbidity and many dissolved contaminants, then
2. UVC LED exposure to target microorganisms in the treated volume.

The current repo centers on STM32 firmware and a lid PCB that controls a gated UVC branch, backed by early MCAD for a two-chamber bottle (Outer dirty shell + inner filtered/UVC chamber) and a refillable filter cartridge.

⚠️ Safety / Disclaimer

• This is not a certified disinfection device and not a minimum viable / usable product.
• UVC is hazardous to eyes and skin. Misuse can cause injury.
• Current printed parts are not food-safe and not reliably watertight.
• Battery-powered systems can fail dangerously if miswired (overcurrent, shorts, thermal runaway).
• This project exists for engineering documentation and learning only.
  Do not rely on it for drinking water.

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What this project is trying to explore

User story (Target scenario)

“As someone who travels, camps, or works where tap water isn’t always viable,
I need a compact bottle that can handle turbidity, taste, and biological contamination,
so I can safely drink without hauling single-use filters, chemicals, or a stove.”

Rather than choosing filtration or UVC, this prototype explores a staged approach inspired by:

• Press-to-filter systems (e.g. Grayl GeoPress) for multi-barrier filtration, and
• Cap-integrated UVC (e.g. Larq PureVis 2) for in-bottle optical disinfection.

The long-term research question is:

Can a bottle-scale system combine filter-first architecture with geometry-aware UVC dosing and still be usable (Cycle times, charging, cleaning, and cost)?

For the full background, math, and validation plan, see the accompanying project write-up.

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Repository layout

• /Software/ – STM32L031 firmware project (STM32CubeIDE)
• /Hardware/ – hardware artifacts
  • /Hardware/ECAD/ – KiCad PCB files for the lid/control board (power path, MCU, interlocks, UVC gating)
  • /Hardware/MCAD/ – SolidWorks models & STEP files for the bottle's inner chamber, outer chamber, lid, and filter cartridge
• /Docs/ (planned) – validation notes, test logs, and design references

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System overview

Functional architecture

Treatment path

1. Outer chamber (Dirty water)

   • Filled from the source (Tap, stream, questionable faucet).
   • Houses a threaded, refillable filter cartridge.

2. Filter cartridge (FairCap-inspired)

   • Printable shell intended to be packed with cotton + activated carbon + media.
   • Goal: Reduce particulates, some metals/organics, and improve UV transmittance (UVT).

3. Inner chamber (Filtered water)

   • Receives water through the filter cartridge.
   • Interior will be designed to be reflective and geometrically favorable for UVC dose delivery.

4. Lid UVC subsystem

   • Contains UVC LED + driver, charger/power-path board, MCU, load switch, reed switch, and status LED(s).
   • UVC LED shines through a small UVC-transmissive window into the inner chamber.

Electronics & control

• Li-ion battery + BQ24074-class charger for USB/DC/solar input and load sharing.
• 3.3 V rail for logic (STM32L031K6).
• High-side load switch for the UVC branch (Gated by MCU and interlocks).
• Single push button for cycle control.
• RGB LED for state indication.
• Reed switch + magnet for lid-closed detection (firmware + hardware gating).

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Hardware stack

MCAD (Bottle chambers, lid, and filter)

The MCAD branch (SolidWorks) currently includes:

• Outer chamber: Holds untreated water.
• Inner chamber: Receives filtered water and forms the UVC treatment volume.
• Filter cartridge: A threaded, refillable block inspired by FairCap, intended to be packed with media (cotton + activated carbon + salt, etc.).
• Lid: Houses the electronics, UVC lens/window, RGB light pipes, and reed-switch magnet pocket.

Status / caveats

• Current prints are PETG and intended as “looks-like” prototypes only.
• Magnet pocket and wall thickness are still being refined (Early prints exposed the magnet cavity into the threads).
• Future revisions will move away from epoxy and toward M2 fasteners and gaskets for serviceability and sealing.
• Long-term, a metal inner chamber (Steel) would be preferable for UVC compatibility and cleanliness.

ECAD (Lid PCB, rev A)

The ECAD branch uses KiCad and currently implements:

• Power path & charging

  • BQ24074-class charger (Adafruit reference design)
  • Input: USB-C or solar (5-10 V)
  • Load sharing (Run from input while charging battery)
  • Input dynamic power management to avoid solar brown-out
  • Status: PGOOD, CHG exposed for future MCU use

• Logic rail

  • MCP1700 3.3 V LDO (or similar) from charger SYS/OUT rail
  • Local decoupling and layout tuned for stability

• UVC branch gating

  • TPS22918 high-side load switch
  • VIN from SYS rail, VOUT to UVC driver module
  • MCU-controlled EN, default-OFF on reset/boot
  • Optional CT pin for soft-start (In-rush shaping)

• MCU & I/O

  • STM32L031K6: Initially hosted on a NUCLEO-L031K6 dev board, with the custom PCB providing power and I/O connectivity.
  • Reed switch: Input for lid-closed detection.
  • Single user button: User input.
  • RGB LED (tri-color, PWM-driven): For state indication.

A separate constant-current UVC LED board (i.e. IO Rodeo 275nm module) currently handles LED drive. In future revisions its topology could be folded into a dedicated UVC daughterboard or an integrated lid PCB.

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Firmware: STM32L031K6

Firmware lives under /Software/ and targets STM32L031K6Tx, typically on a NUCLEO-L031K6 for bring-up.

High-level behavior

• Gated UVC branch

  • Controls the UVC load-switch EN line.
  • OFF by default at boot/reset.
  • Forced OFF on:
    • lid-open (reed switch)
    • cycle cancel
    • (future) fault conditions

• Cycle selection (single button)

  • Single tap → short cycle (~1 minute), LED amber (R+G).
  • Double tap (within ~400 ms) → long cycle (~3 minutes), LED blue.
  • Tap during an active cycle → cancel cycle, force UVC OFF, return to idle.

• Reed-switch safety

  • Input is configured with pull-up:
    • LOW → lid closed / magnet present (safe)
    • HIGH → lid open (unsafe → force UVC OFF)
  • If lid opens during a cycle:
    • UVC branch is shut off immediately.
    • State machine resets (no lingering taps or modes).
    • LED returns to idle (off).

• Auto-stop

  • Each cycle tracks a target end-time.
  • At expiry, UVC branch = OFF, mode cleared, LED back to idle.

I/O mapping (expected)

Inputs

• REED_SW_Pin – reed switch (input with pull-up).
• B2_Pin – user button (EXTI, falling-edge, active-low).

Outputs

• Control_Pin – UVC load-switch enable:
  • LOW → UVC branch OFF
  • HIGH → UVC branch ON
• RGB LED via TIM2 PWM channels (common-anode assumed):
  • PA3 → TIM2 CH4 → RED
  • PA1 → TIM2 CH2 → GREEN
  • PA0 → TIM2 CH1 → BLUE

Some SPI/OLED-related pins may remain configured from CubeMX templates. They’re reserved for future debug UI and don’t affect UVC logic.

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Build & flash (STM32CubeIDE)

• Toolchain: STM32CubeIDE
• Target MCU: STM32L031K6Tx

1. Clone the repository.
2. Open STM32CubeIDE → File → Import… → Existing Projects into Workspace….
3. Select the firmware project under /Software/.
4. Build the Debug or Release configuration.
5. Connect ST-LINK (NUCLEO onboard or external).
6. Click Run or Debug to flash.

After flashing, for bench testing:

• Power the board from a regulated 3.3 V rail (or the NUCLEO’s default power path).
• Connect the reed switch, button, RGB LED, and load-switch control line as per the I/O mapping.
• Confirm that behavior matches the “High-level behavior” section above using a safe stand-in load (e.g. an OLED display or resistor) instead of a UVC LED.

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Validation status (UVC effectiveness)

A detailed biodosimetry-based validation plan is drafted (MS2 coliphage, collimated-beam-derived k(λ), RED/VF framework, EPA 186 mJ/cm² target), but:

🔬 No biological tests have been executed yet.
No log-reduction claims are made for this prototype.

Until validation is complete:

• All cycle times, UX modes, and design equations should be treated as engineering placeholders, not safety guarantees.
• Any use of this design with real water is strictly at your own risk and strongly discouraged.

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Sponsorship (PCB fabrication)

PCB fabrication for this prototype was kindly sponsored by PCBWay, who covered the cost of the 4-layer boards and DHL shipping via store credit.

From a bring-up perspective the experience was exactly what I was hoping for:

• The 4-layer FR-4 S1000H TG150 stack-up (78 × 78 mm, 1.6 mm, ENIG, 1 oz Cu inner/outer) matched the KiCad design with no surprises.
• Drill hits, solder mask, and silkscreen registration were all on-point; pads were fully exposed and silkscreen stayed off fine-pitch pads.
• ENIG pads wetted cleanly during hot-air/QFN reflow, and the USB-C, BQ24074, and header footprints all fit mechanically without any bodge-wires or footprint hacks.

This repository and write-up reflect my own design decisions, testing, and documentation. Sponsorship support did not include editorial control or performance claims; any results here are based on my own prototyping and are shared for transparency and learning.

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Roadmap / next steps (WIP)

Planned expansions to this repo:

• ECAD

  • Integrate or document a dedicated UVC LED + heatsink board.
  • Tighten power-integrity checks (inrush, brownout behavior, EMC, thermal).

• MCAD

  • Add outer bottle, finalized filter cartridge, and realistic sealing strategy (O-rings, glands/grooves, fasteners).
  • Explore food-safe materials (Metal inner chamber, UVC-stable windows, and gaskets).

• Firmware

  • Add explicit fault/error states (Blink codes, fault latch, charging/solar status UI).
  • Integrate charger status (PGOOD / CHG) into UX.

• Testing & validation

  • Execute UVC biodosimetry tests and publish RED / DVal results.
  • Map measured performance back into Design Equations 1 & 2 to tune cycle times and power.

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License

MIT License — see LICENSE.