Custom PCB Design Inquiry

cjam28

cjam28

I'm designing a battery-powered water level sensor for a residential saltwater swimming pool. I have a working hardware concept but want your recommendations on improvements, alternative parts, simplifications, or a custom PCB design that integrates the modules.
================================================================ GOAL
Detect pool water level continuously, report via Home Assistant, and trigger a Zigbee-controlled solenoid valve to top up evaporation losses. Hard cutoff via independent float switch if the primary sensor fails open.
================================================================ HARD CONSTRAINTS (non-negotiable)
  1. Mounts entirely UNDER the pool skimmer lid. Skimmer is set into a concrete pool deck — no through-wall access. Lid is the only serviceable surface.
  2. Sensor sees the water surface from 13-20 cm (5-8 inches) above waterline, hanging vertically downward.
  3. Skimmer cavity is ~20 cm wide. Sensor beam angle should not bounce off cavity walls.
  4. Saltwater chlorine generator pool (~3000 ppm chloride). All wetted parts must be plastic — no stainless steel, no brass in contact with water/spray.
  5. Pool surface is 88-95°F, hot tub use. Ambient under-lid air is hot and condensing.
  6. Battery powered. Replaceable cell. Target ≥6 month life on a single 18650-class cell, ideally 1+ year.
  7. Two sensors with orthogonal physics (ultrasonic primary + mechanical float backstop) — single-sensor failures must not flood the pool.
  8. Integrates with Home Assistant. Already running ESPHome and Zigbee2MQTT on a Cauldron-HA instance.
================================================================ CURRENT BOM (challenge any of this if there's a better option)
  • MCU: Seeed XIAO ESP32-C6 (WiFi 6 + Thread + Zigbee + Matter, 15 µA deep sleep, ESPHome openthread component support, 21×17.5mm)
  • Primary sensor: DFRobot A02YYUW (IP67 ultrasonic, 3-450cm range, 3cm blind zone, 60° beam, UART 9600, ESPHome a02yyuw component, 8mA active)
  • Backstop: Madison M8000 vertical float switch (1/8" NPT, all-polypropylene stem and float, hermetically sealed reed switch, UL/CSA/NSF certified)
  • Charger: TP5000 module set for LiFePO4 3.6V mode
  • Battery: 18650 LiFePO4 1500mAh with integrated BMS
  • Enclosure: 83×81×56mm IP67 ABS with M16 cable glands
  • Refill plumbing: GiEX QT06 Zigbee solenoid valve with flow sensor + Watts 9D-M2 atmospheric backflow preventer
  • Protection: silicone conformal coating on PCBs, indicating silica gel in enclosure, no resin potting (battery must remain replaceable)
================================================================ DESIGN CHOICES I MADE — challenge if you disagree
  • GPIO power-gating the ultrasonic sensor instead of a dedicated MOSFET (8 mA load << 40 mA GPIO limit on ESP32-C6)
  • Float switch on GPIO with internal pull-up + delayed_on/off filtering for debounce, also serves as deep-sleep wake source
  • 30-minute polling interval, only trust readings when pool pump has been off ≥10 minutes (water surface settling)
  • HA-side automation handles refill logic; ESP only reports raw distance
    • float state and acts on overfill cutoff
  • WiFi day-one via ESPHome, with planned migration to Thread/openthread or Zigbee end-device firmware later for ~3× battery life
================================================================ WHAT I WANT FROM YOU
A) Recommend alternative parts where you see a better fit. Specifically:
  • Is there a better ultrasonic, ToF, capacitive, or radar sensor for this geometry that I missed? (Note: ToF lasers ruled out due to condensation on optical windows. Through-wall mounting impossible.)
  • Is there a better float switch with better saltwater resistance or smaller form factor than the Madison M8000?
  • Better MCU for ultra-low-power Thread/Matter end devices? (I considered XIAO MG24 / Silicon Labs EFR32MG24 — 1.95 µA sleep — but ESPHome support is poor. If non-ESPHome firmware is acceptable for a 10× battery life win, recommend it.)
B) Design a custom PCB that integrates the XIAO carrier, TP5000 charge circuit, USB-C power input, battery management, GPIO breakouts for the ultrasonic and float switch with proper ESD protection, an LDO or boost if needed, and test/programming pads. Goal: replace the loose modules with one ~50×40mm board that mounts inside the IP67 enclosure.
C) Schematic-level recommendations for:
  • ESD/TVS protection on external sensor lines (cables exit the enclosure)
  • Reverse polarity protection on battery input
  • Optional power-good LED that does not parasitically drain the cell
  • Proper Thread/Matter antenna placement and ground plane considerations
  • Low-Iq LDO recommendation if 3.3 V regulation is needed beyond what the XIAO provides
D) BOM with Mouser/DigiKey part numbers (NOT Amazon — production parts), plus suggested PCB stackup and fab-house options (JLCPCB, OSHPark, PCBWay).
E) Identify any failure modes I haven't accounted for. Be specific — "consider EMI" is useless; "the TP5000 switching at 1.2 MHz can couple into the A02YYUW UART line if traces run parallel — recommend minimum 5mm separation and ground pour between" is what I want.
Format the response as:
  1. Quick verdict on each starting-point part (keep / swap / consider)
  2. Recommended alternative parts with rationale
  3. Block diagram / schematic notes
  4. Custom PCB BOM with Mouser part numbers
  5. Top 3 risks I should design around
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SDIO-DATA1_GPIO21_D3
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Seeed Studio XIAO ESP32C6

Seeed Studio XIAO ESP32C6 thumbnail
Carrier PCB for Seeed Studio XIAO ESP32C6 with USB-C, Li-ion Power, and Complete Breakout Headers

Properties

flowchart LR subgraph Module ESP[XIAO ESP32C6 Module] end subgraph Power Subsystem USB(USB-C Input) BAT(Li-ion Battery Input) PROT(Protection: TVS & Fuse) VREG(Voltage Regulator) TP3(3.3V Testpoint) TP5(5V Testpoint) end subgraph Interfaces PROG(Programming Header: BOOT, EN) IO(I/O Breakout: I2C, SPI, UART) end USB --> PROT --> VREG BAT --> PROT VREG --> TP3 VREG --> TP5 TP3 --> ESP ESP --> PROG ESP --> IO

U

113991254

files.seeedstudio.com/wiki/SeeedStudio-XIAO-ESP32C6/XIAO-ESP32-C6_v1.0_SCH_PDF_24028.pdf

seeedstudio.com/Seeed-Studio-XIAO-ESP32C6-p-5884.html

wiki.seeedstudio.com/xiao_esp32c6_getting_started/

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