Project Specification — USB-C WiFi BLE Environmental Node
Project Overview
Status: Draft schematic in progress.
A consumer environmental sensor node powered from USB-C 5 V. It measures ambient temperature and relative humidity with a digital sensor and reports data over Wi-Fi and Bluetooth Low Energy using a low-power ESP32-class radio module.
Intended Use
Indoor consumer environmental monitoring.
Powered from USB-C chargers, hubs, or ports providing default 5 V sink power.
Prototype-to-production-intent design with consumer safety and RF compliance considerations.
What the Device Should Do
Accept 5 V power from a USB-C receptacle in sink/device mode.
Tolerate common 0.5 A to 3 A USB-C source advertisements without requiring USB Power Delivery.
Protect downstream electronics from reverse current, overvoltage, undervoltage, and overcurrent events.
Generate a regulated 3.3 V rail for the MCU/radio and sensor.
Measure temperature and relative humidity digitally.
Communicate over 2.4 GHz Wi-Fi 802.11 b/g/n and BLE 5.x.
Support firmware flashing/debug over USB or a programming/debug header.
Main Features
USB-C 5 V sink input with independent CC1/CC2 Rd resistors.
Protected 5 V input path: ESD/TVS, overvoltage protection, reverse blocking, UVLO, and current limiting.
3.3 V low-noise supply for Wi-Fi/BLE radio bursts.
ESP32-C3/S3 class pre-certified module with integrated antenna.
I2C digital temperature/RH sensor.
Reset/boot controls, status LED, and test points.
System Architecture
Diagram
Hardware Subsystems
Power input: USB-C sink-only default 5 V input, no USB PD negotiation.
Protection: current-limited eFuse/power switch with OVP/UVLO/OCP/reverse-blocking behavior plus ESD protection.
Regulation: 5 V to 3.3 V regulator sized for ESP32 Wi-Fi transmit peaks.
MCU/radio: ESP32-class module supporting 2.4 GHz 802.11 b/g/n and BLE 5.x.
Sensor: I2C temperature and humidity IC with local decoupling.
Firmware/debug: USB serial/JTAG or boot/reset controls, plus optional test points.
Interfaces and Connections
USB-C receptacle: VBUS, GND, CC1, CC2, D+, D-.
3.3 V rail: MCU module, T/RH sensor, pull-ups, status LED.
I2C bus: SDA/SCL with one pull-up pair.
User/service: BOOT and RESET buttons, status LED, exposed test points.
Power and Runtime Expectations
Always-powered from USB-C; no battery runtime target.
Low-power firmware should use modem sleep/light sleep when not actively transmitting.
Design must survive source capabilities from 0.5 A to 3 A but firmware/hardware should not assume 3 A without CC/current-limit policy.
Power Tree and Power Budget
Table
Rail
Load
Sleep
Typical Active
Peak
3.3 V
ESP32 Wi-Fi/BLE module
~10-100 uA configured sleep
~80-240 mA
~500 mA radio burst
3.3 V
Digital T/RH sensor
<1 uA standby
<1 mA during measurement
<2 mA
3.3 V
Status LED
0 mA off
2-5 mA
5 mA
3.3 V
Pull-ups/debug leakage
negligible
<1 mA
<1 mA
Design target: size the 3.3 V rail for at least 600 mA peak. Input path target: at least 1 A continuous/current-limit capability with components rated for 5 V USB-C operation and tolerance of 3 A-capable sources.
Manufacturing and Assembly Expectations
SMD assembly, consumer-product-oriented.
Use a pre-certified radio module to reduce RF certification risk.
Add test points for VBUS_PROT, 3V3, GND, I2C, EN/RESET, and boot/debug signals.
Firmware-Relevant Hardware Requirements
Arduino/ESP-IDF compatible Wi-Fi + BLE firmware.
I2C T/RH sensor driver.
Bootloader entry and reset controls.
Wi-Fi/BLE status indication.
Optional low-power mode strategy for idle intervals.
Physical Design Expectations
Place the radio module antenna at a board edge with full antenna keepout on all layers.
Keep USB-C connector accessible at board edge.
Keep sensor away from regulator heat, USB connector heat, and the ESP32 module if possible.
Provide ventilation/opening guidance for final enclosure.
Important Design Decisions
Use an ESP32-C3/S3 module rather than a bare RF chip to reduce RF/layout/certification risk.
Use native USB-capable ESP32 variant if available to avoid an external USB-UART bridge.
Use a 3.3 V regulator sized for ESP32 radio bursts; consider buck if LDO thermal dissipation is too high.
Use an eFuse/power switch for combined OVP/UVLO/OCP/reverse-blocking input protection.