# Hardware Re-spec + Watchdog PCB Brief **Date:** 2026-06-19 · **Owner:** Leo Rodriguez (Helgen) **Strategy (Leo's call):** **phased** — ship a ruggedized interim to the fleet now (Phase 1), design the production PCB in parallel (Phase 2). **Drivers:** the RCA (`06-Testing-Validation/RCA-2026-06-field-failures.md`) and the ICAVE client review (prototype-grade, vibration, connectors, thermal, 4G failures). > This is an engineering brief, not a finished schematic. Specific part numbers > are **starting points** to price/validate — confirm against the measured power > budget (see §2) and your supplier availability. --- ## 1. Design requirements (the bar the fix must clear) > **Context update (2026-06-19):** the HBOX sits **inside an industrial cabinet** > that already provides surge protection, a breaker, isolated ignition sensing, and > a regulated **24 Vdc** supply (Mean Well DDR-60G-24) — see > `03-Architecture-Hardware/cabinet-electrical-bom.md`. So the HBOX can assume a **clean 24 V feed** > and **need not re-implement input surge/reverse-polarity**. The redesign focus > narrows to: **24 V→5 V**, the watchdog, the LiPo decision, and — now the top > issue — **thermal management** (see §thermal). | Requirement | Target | Why | |---|---|---| | Operating temp | **−10…+75 °C** internal, survive +85 °C | sealed **ASA** box **inside** a sealed ABS IP67 cabinet that **also holds the heat-dissipating Mean Well supply**, in full sun | | **Enclosure material** | **ASA (current) — keep**; UV-stable, ~95–100 °C | confirmed ASA (not PLA): margin over the cabinet interior; the case is not the liability | | Input voltage | **clean ~24 V** from the cabinet (design the buck 9–36 V for margin) | cabinet's Mean Well already regulates; isolated ignition provided | | Surge/transient | **handled at cabinet level** (Phoenix SPD + breaker); HBOX keeps a modest TVS | don't duplicate the cabinet's protection | | Vibration | survive continuous machinery vibration | no Dupont, locking connectors, retained boards | | Ingress | **IP67 maintained**, no internal condensation | coastal humidity; sealing constrains cooling (§thermal) | | 5 V rail | **≥3 A continuous**, handle LTE TX bursts (~2 A) | modem peaks brown out a 1 A hobby boost | | Maintainability | **SIM swap without disassembly**; labeled, keyed | client point #5 | | Serviceability | one PCB, documented, test points, repeatable | "apt for production" | | Observability | log temp + battery + link health | prove failures in the field | --- ## 2. First, measure the power budget (blocks everything) Before choosing a DC-DC or fuse, **bench-measure** the stack on the actual hardware: - Idle, 1 Hz, and **LTE-TX-burst** current at 5 V (modem bursts are the spike). - Inrush at power-on (caps + modem). - Repeat at 12 V and 24 V input. Expected ballpark (validate): avg ~0.3–0.6 A @5 V, **bursts 1.5–2 A** during LTE transmit. That burst is almost certainly **beyond the PowerBoost 1000C's ~1 A continuous rating** — a strong candidate for both the instability *and* the "under-rated fuse" the client flagged. Everything below assumes ~2.5 A peak @5 V; correct it once measured. --- ## 2b. Thermal & enclosure — now the #1 physical fix **The problem is a nested thermal trap.** A sealed **ASA** HBOX sits inside a sealed **ABS IP67** cabinet that **also contains the Mean Well supply** (60 W class, dissipating several watts continuously), in full Veracruz sun. Two failure drivers stack: - **The ASA shell is not the problem.** ASA is UV-stable and softens at ~95–100 °C, with margin over the cabinet interior, so the HBOX shell will **not sag/warp** at these temperatures. (Corrects an earlier PLA assumption.) - **The LiPo is in that same hot space** (charge limit 45 °C) → degradation, swelling, voltage sag → brownout. ### About adding a fan — read this first **A fan *inside* a sealed IP67 box does almost nothing.** It only stirs the same trapped air; with no path to outside air, **no heat actually leaves the box** (it even adds a little). Cooling requires either **moving heat out by conduction** or **exchanging air with the outside** — and air exchange **breaks IP67**, which in a salty, humid, dusty port invites corrosion. So: | Option | Removes heat? | Keeps IP67? | Verdict | |---|---|---|---| | Internal fan only (sealed) | ✗ (just stirs air) | ✓ | **Don't bother** | | Filtered vents + extractor fan | ✓ | ✗ (→ ~IP54) | Only with filters + maintenance; risky in this environment | | **Conduction: mount hot parts to a metal plate / metal enclosure** | ✓✓ | ✓ | **Best** — heatsink to the outside, stay sealed | | **Reduce solar gain** (white/reflective, **sunshade**, relocate out of sun) | ✓ | ✓ | **Cheap, do it now** | | **Reduce internal generation** (efficient DC-DC, **remove LiPo**) | ✓ | ✓ | **Do it** | | Gore pressure-vent | ✗ (condensation only) | ✓ (membrane) | Keep for humidity, not for cooling | | Active (Peltier / vortex / heat-exchanger) | ✓ | ✓ | Overkill/expensive — last resort | **Recommended thermal strategy (sealed, no IP compromise):** 1. **Keep the ASA enclosure** (UV-stable, ~95–100 °C — adequate). Optionally move to **metal** for extra heatsinking, or **eliminate the inner box** and mount the PCB directly to the cabinet plate. 2. **Conduct heat out:** bolt the DC-DC (and the new HBOX board's hot spots) to the cabinet's **metal mounting plate / gland plate**, or move to a **metal (aluminium) cabinet** that acts as a heatsink. Use thermal pads/gap-filler. 3. **Cut the solar load:** white/reflective cabinet, a **sunshade**, and mount out of direct sun where the machine allows. 4. **Cut internal heat:** a higher-efficiency 24 V→5 V buck and **removing the LiPo** both lower the steady-state interior temperature. 5. **Quantify it:** put a thermocouple/logger inside a representative cabinet for a full sunny shift — design to the measured number, not a guess. --- ## 3. Phase 1 — Interim ruggedization (do now, on existing modules) Goal: stop the bleeding on the deployed fleet without waiting for a PCB. All of this is achievable with off-the-shelf parts and rework. 1. **Kill the Dupont jumpers.** Replace every jumper with **locking, keyed connectors** — JST-XH (signal) / Molex Micro-Fit 3.0 (power). Crimped, not hand-twisted. This alone removes the #1 vibration intermittent. 2. **Replace the PowerBoost 1000C** with an **industrial DC-DC** stepping the cabinet's **24 V → 5 V, ≥3 A** (e.g., a sealed **Mornsun/Recom** module or a **TI LMR33630-class** buck for the LTE bursts). Design the input wide (9–36 V) for margin, but the normal feed is the cabinet's clean 24 V. Sized from §2. 3. **Resolve the battery (decision — see §6).** At minimum **relocate the LiPo** to the coolest wall, away from the modem/DC-DC, and add a temperature cutoff; better, **replace it** with LiFePO4 or a supercap hold-up, or **drop battery mode** if the client doesn't need parked tracking. 4. **Verify the cabinet fuse.** Read the **blade fuse fitted in the Phoenix PT 6-FSI/C** holder and compare to the measured worst-case + inrush — this is the "under-rated fuse" the client flagged. Surge/breaker are already handled at the cabinet (Phoenix SPD + breaker); the HBOX keeps only a **modest local TVS**. 5. **Mechanical lockdown.** Mount each board on **standoffs with vibration isolators/grommets**; retain the simpleRTK2B + 4G master in their sockets with a bracket or retention clip; **thread-lock + torque** the SMA bulkheads. 6. **Conformal-coat** the watchdog/discrete wiring (acrylic or urethane) against humidity and to stabilize solder joints under vibration. 7. **Thermal quick wins (see §2b — do NOT add an internal fan to a sealed box):** keep the **ASA** inner box (adequate) or remount the boards on the metal plate; cut solar gain (white/reflective cabinet + **sunshade**, relocate out of sun); **conduct** the DC-DC/modem heat to the cabinet's metal plate; keep a **Gore-style pressure-vent** for condensation only. 8. **SIM access:** bring the SIM to a **front-panel holder** so swaps don't need teardown. 9. **Flash firmware v2** (thermal logging + proper 4G recovery) on every unit. 10. **Instrument** at least one unit with an interior **thermocouple log** for a full sunny shift to quantify the thermal problem. Phase 1 is the "permanent-enough" stabilization while Phase 2 is fabricated. --- ## 4. Phase 2 — Integrated Watchdog Carrier PCB (the real fix) A single **4-layer, industrial-temp** carrier board that replaces all the hand-wiring and the hobby power boards, and onto which the simpleRTK2B and 4G NTRIP Master mount via **locking board-to-board** (or retained XBee socket). ### Block diagram ``` Cabinet 24V (already SPD/breaker-protected) ─► [keyed plug] └► modest TVS + reverse-polarity FET ─► bulk cap └► DC-DC buck 24V→5V (design 9–36V), ≥3A ─► 5V rail ├► STM32 watchdog (3.3V LDO) ── UART ──► simpleRTK2B RX1 (UBX) │ ├ sense: Vin, 5V, battery dividers (filtered) │ ├ status LED + SWD header + debug UART (dashboard) │ └ optional: 4G modem health/RESET line, RTC ├► high-side LOAD SWITCH / eFuse ─► switched 5V ─► simpleRTK2B + 4G stack └► hold-up: supercap bank OR LiFePO4 charger (see §6) Antennas: bulkhead SMA pigtails (GNSS L1/L2 + LTE) · SIM: front-panel holder/MFF2 ``` ### Key design choices vs the current discrete design | Current (discrete) | PCB replacement | Benefit | |---|---|---| | 2N3904 + IRF9540N + 10 k pull-up | integrated **high-side load switch / eFuse** (e.g., TPS25200 / TPS2596 / load-switch IC) | current-limited, fewer parts, defined turn-on, no gate-pull-up gotchas | | PowerBoost 1000C | automotive **buck ≥3 A** + protection front-end | survives LTE bursts, wide Vin, transient-tolerant | | LiPo in hot box | supercap hold-up **or** LiFePO4 (or none) | removes the thermal/safety liability | | **ASA inner box (UV-stable, ~95–100 °C)** | keep (adequate); optional metal for extra heatsinking, or mount PCB to the cabinet plate | survives the cabinet's internal temperature | | Dupont + screw terminals | **locking JST/Molex** + pluggable terminal block | vibration-proof | | Bare boards loose in box | retained board-to-board + isolated standoffs | no movement, no solder flex | | SIM buried in stack | **front-panel SIM** or **MFF2 eSIM** | client point #5 solved | | No local protection | **modest TVS + RP FET** (cabinet has the SPD/breaker) | survives without duplicating the cabinet | | No temp/observability | MCU temp + sense dividers + test points | provable diagnostics | ### Board-level details to specify - **Protection front-end (light — the cabinet does the heavy lifting):** the cabinet already provides surge (Phoenix SPD) + breaker + isolated ignition. The HBOX board needs only a **reverse-polarity P-FET** and a **modest TVS** on its 24 V input — don't duplicate the cabinet's SPD/eFuse. - **DC-DC:** qualify input 9–36 V; ≥3 A @5 V; good thermal pad to a copper pour / enclosure; spread-spectrum or proper filtering for GNSS RF cleanliness (switcher noise can hurt the L1/L2 front-end — keep it away from the antenna feed and filter it). - **MCU:** keep STM32F411 to reuse v2 firmware, or step to an **automotive/industrial-temp STM32 (G0/L4)**. Bring out **SWD** + a **debug UART** for the dashboard. Add the internal-temp logging already in v2; consider an external temp sensor near the LiPo/modem. - **GNSS RF integrity:** keep the switcher and digital noise away from the GNSS antenna trace; solid ground plane; the ZED-F9P front-end is sensitive — this protects fix quality (and therefore the watchdog's view of "link health"). - **Connectors:** all locking/keyed; bulkhead SMA with lock washers; a single keyed vehicle-power plug. - **Mechanical:** mounting holes with vibration isolators; design for conformal coat or selective potting of the power section. - **DFM/serviceability:** silkscreen labels, test points on every rail + UART, a fab/assembly package, and a documented BOM (this is literally the "apt for production" the client asked for). --- ## 5. Indicative BOM (validate prices/availability/temp grade) | Function | Interim (Phase 1) | PCB (Phase 2) | |---|---|---| | DC-DC **24→5 V** ≥3 A | Recom/Mornsun/TI buck module | TI LMR33630-class on-board buck | | Load switch | keep NPN+P-FET (retained) | TPS25200 / TPS2596 / load-switch IC | | Input protection | modest TVS + RP diode (cabinet has SPD/breaker) | RP FET + modest TVS on-board | | Hold-up / battery | relocate LiPo + temp cutoff | supercap bank **or** LiFePO4 charger | | **Inner enclosure** | **ASA — keep** (adequate); or mount to plate | metal box / mount PCB to cabinet plate | | MCU | existing Blackpill | STM32F411 (or G0/L4) on-board | | Interconnect | JST-XH / Molex Micro-Fit | board-to-board + locking I/O | | Antennas | re-torque bulkhead SMA | bulkhead SMA pigtails | | SIM | front-panel holder | front-panel holder or MFF2 eSIM | | Coating | acrylic conformal coat | conformal coat / pot power section | | Thermal | sunshade + **conduct heat to metal plate** (no internal fan) | metal cabinet as heatsink; sealed (keep IP67) | --- ## 6. Open decisions for Leo (these change the design) 1. **Battery mode — keep, change, or drop?** - *Keep tracking while the vehicle is off* (anti-theft / parked location)? Then use **LiFePO4** (wider temp, safer) or a relocated/temperature-managed cell — **not** a LiPo baking next to the modem. - *Only need ride-through* across brief power dips? A **supercap** hold-up is simpler, safer, and temperature-tolerant — but won't give long parked tracking. - *Vehicle-only* acceptable? **Drop the battery** entirely — removes the single worst thermal/safety liability. **(Recommended if the client doesn't require parked tracking — confirm with ICAVE.)** 2. **SIM strategy:** swappable **front-panel SIM** vs **MFF2 eSIM** (eliminates the swap problem but commits the carrier). Depends on how often SIMs change. 3. **MCU:** reuse F411 (fast, keeps v2 firmware) vs move to an industrial-temp STM32 (more margin, small port effort). 4. **Inner enclosure material:** **confirmed ASA** (UV-stable, ~95–100 °C) — adequate for the nested-cabinet temperatures; no material change required. Optionally eliminate the inner box and mount the PCB to the cabinet plate for better conduction. 5. **Cabinet thermal path:** keep ABS IP67 + sunshade + **conduct heat to a metal plate**, or move to a **metal (aluminium) cabinet** as a heatsink (best, costs more). **An internal fan in a sealed box is not a fix (see §2b)** — only conduction or (IP-breaking) ventilation removes heat. 6. **Make vs buy the modem:** keep the ArduSimple 4G NTRIP Master, or move to a qualified industrial LTE module with a known wide-temp rating. --- ## 7. Suggested next actions (feeds `PLAN.md`) - Bench power-budget + inrush measurement (§2) → picks the 24→5 V buck. - **Quantify cabinet temperature** (thermocouple in a representative cabinet, full sunny shift) → design target for §2b. - Decide §6.1 (battery), §6.4 (inner-box material), §6.5 (cabinet thermal path) — the biggest levers. **The ASA enclosure is adequate — no material swap needed; focus on the conduction path + active thermal.** - Phase-1 ruggedization kit BOM + rework one unit as the reference build. - Phase-2 schematic → layout → fab/assembly; pilot 2–3 boards before fleet. - Cabinet BOM is industrial-grade and stays — see `03-Architecture-Hardware/cabinet-electrical-bom.md`.