USB Sensor Node PCB
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Current project state:
- Schematic has been built for the USB-C WiFi/BLE temperature/humidity sensor node.
- Key blocks are present: USB-C input, ESD protection, TPS25940 eFuse/protection, MP28301 3.3 V buck regulator, SiWG917 Wi‑Fi/BLE module, SHT41 sensor, debug/reset/ISP controls, and status LED.
- Supporting project files were created: specification, firmware starter, bring-up plan, and regulatory/TCF notes.
What remains:
- Inspect current PCB layout state to confirm whether placement, routing, or both are needed.
- Run component placement with constraints: SiWG917 antenna at board edge, SHT41 away from heat sources, USB-C at edge, buck loop compact.
- Route traces and resolve airwires.
- Run DRC/ERC after layout and fix remaining issues.
- Assign final MPNs to generic passives before manufacturing export.
Inspect layout state
Run ERC/DRC
Review BOM MPNs
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Project Specification — USB-C WiFi BLE TH Sensor
Project Overview
Status: Draft schematic in progress.
A consumer USB-C powered temperature and relative humidity sensor node with Wi-Fi and Bluetooth Low Energy connectivity. The board uses a protected USB-C 5 V sink input, local 3.3 V regulation, a low-power Wi-Fi/BLE MCU module, and a digital I2C T/RH sensor.
Intended Use
- Indoor consumer environmental monitoring node.
- Powered from USB-C 5 V chargers, hubs, or ports.
- Designed for prototype-to-production learning; not medical, industrial safety, or outdoor/weatherproof use unless later specified.
What the Device Should Do
- Measure ambient temperature and relative humidity.
- Connect over 2.4 GHz 802.11 b/g/n Wi-Fi.
- Support BLE 5.x provisioning or local telemetry.
- Power from USB-C default 5 V sources.
- Survive common consumer input faults with reverse-current, overvoltage, undervoltage, and overcurrent protection.
Main Features
- USB-C receptacle configured as sink-only with independent CC1/CC2 5.1 kΩ Rd resistors.
- Protected 5 V input sized for 0.5–3 A capable sources, while the node itself is expected to draw well below 500 mA average.
- 3.3 V logic rail for MCU and sensor.
- Native USB data/programming path where supported by the selected MCU.
- Boot/reset controls, debug/programming access, and status LED.
System Architecture
Diagram
Hardware Subsystems
- Power input: USB-C receptacle, CC pull-downs, VBUS ESD/TVS, protected power switch/eFuse or load switch with OVP/UVLO/OCP/reverse-current blocking.
- Regulation: 5 V to 3.3 V rail sized for Wi-Fi transmit current peaks.
- Compute/radio: certified ESP32-family module preferred to avoid custom RF matching and simplify regulatory work.
- Sensor: digital I2C temperature/humidity sensor with local decoupling and one shared I2C pull-up pair.
- User/debug: reset and boot controls, status LED, USB programming, optional exposed test/debug header.
Interfaces and Connections
- External: USB-C 5 V power and USB 2.0 D+/D- for programming/debug.
- Internal: 3.3 V rail, GND, I2C SDA/SCL, boot/reset, status LED GPIO.
- RF: integrated 2.4 GHz antenna module; PCB layout must provide the module antenna keepout.
Power and Runtime Expectations
- Source: USB-C 5 V default power only; no battery in this revision.
- Source capability planning: tolerate 0.5 A, 1.5 A, and 3 A capable USB-C sources. Do not assume the device may draw 3 A unless CC current advertisement is measured/implemented.
- Low-power behavior: firmware should sleep between samples and radio transmissions, but USB-powered operation makes absolute sleep current less critical than thermal and regulatory behavior.
Power Tree and Power Budget
Table
| Rail | Load | Sleep | Typical active | Peak |
|---|---|---|---|---|
| 3.3 V | Wi-Fi/BLE MCU module | ~10–100 µA deep sleep | ~80–240 mA radio active | ~500 mA short Wi-Fi TX burst |
| 3.3 V | T/RH sensor | <1 µA standby | ~0.5–1 mA measuring | ~1 mA |
| 3.3 V | Status LED | 0 mA off | ~2 mA on | ~5 mA configured max |
| 3.3 V | Pull-ups/leakage | ~1–2 mA worst when lines low | negligible average | ~1.4 mA for two 4.7 kΩ pull-ups low |
Estimated 3.3 V rail sizing target: 250 mA typical active, 510 mA transient peak. For an LDO from 5 V, input current is approximately output current; thermal at 250 mA is about (5.25 V - 3.3 V) × 0.25 A = 0.49 W, so a small SOT-23 LDO may be marginal during sustained Wi-Fi. Preferred choices are a high-current low-Iq regulator in a thermally capable package or a quiet buck regulator if sustained radio current/thermal limits demand it.
Manufacturing and Assembly Expectations
- Default assembly: SMD, production-intent consumer PCB.
- Use stocked, non-EOL parts where available.
- Add test points for 5 V protected rail, 3.3 V, GND, reset/boot, and I2C if space permits.
Firmware-Relevant Hardware Requirements
- Define actual GPIO pins for I2C SDA/SCL and status LED.
- Provide USB programming or serial/JTAG path.
- Support Wi-Fi + BLE provisioning, periodic sensor reads, and telemetry output.
- Include boot/reset access for field recovery.
Physical Design Expectations
- ESP32 module antenna must be placed at board edge with no copper/traces/components in the keepout on all layers.
- Sensor should be isolated from regulator/MCU heat sources and exposed to ambient airflow.
- USB-C connector should be board-edge mounted and mechanically reinforced.
Important Design Decisions
- Use an ESP32-family certified module rather than a bare RF SoC.
- Use a digital I2C humidity/temperature sensor.
- Use USB-C sink-only 5 V; no USB-PD negotiation in this revision.
- Include a protected VBUS front end instead of relying on a fuse alone.
Assumptions
- Indoor consumer use, ambient operating range roughly 0–50 °C unless later expanded.
- No enclosure dimensions or board size provided yet.
- No cloud protocol specified; firmware examples will print readings and include Wi-Fi/BLE placeholders.
Change Notes
- Initial requirements captured from user request on 2026-07-17.
Project Overview
Intended Use
What the Device Should Do
Main Features
System Architecture
Hardware Subsystems
Interfaces and Connections
Power and Runtime Expectations
Power Tree and Power Budget
Manufacturing and Assembly Expectations
Firmware-Relevant Hardware Requirements
Physical Design Expectations
Important Design Decisions
Assumptions
Change Notes
