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Project Specification
Project Overview
Status: Draft schematic in progress.
Battery-powered environmental sensor board using an ESP32-C6-WROOM-1 module and Sensirion SCD41 CO2/temperature sensor. The board uses USB-C for charging and USB data/programming, a single-cell 3.7 V nominal 2000 mAh LiPo pouch battery, and a buck-boost regulator for a stable 3.3 V rail.
Intended Use
Portable or wall-mounted indoor air-quality sensor prototype with Thread-based intermittent communication and deep-sleep operation between measurements.
What the Device Should Do
- Measure CO2 and temperature using SCD41.
- Communicate wirelessly using ESP32-C6 Thread capability.
- Sleep intermittently to extend battery life.
- Charge from USB-C.
- Support USB-C data/programming/debug via ESP32-C6 native USB Serial/JTAG.
Main Features
- ESP32-C6-WROOM-1 MCU/radio module.
- SCD41 I2C CO2/temperature sensor.
- USB-C 5 V sink and USB 2.0 data interface.
- LiPo charging with power-path management.
- 3.3 V buck-boost regulator sized for ESP32 radio current bursts.
- Battery voltage sense path for firmware fuel indication.
- Boot/reset controls, status LED, and test points.
System Architecture
Diagram
Hardware Subsystems
- Power: USB-C VBUS input, ESD/overcurrent protection, LiPo charger/power-path, buck-boost 3.3 V regulator.
- Compute/wireless: ESP32-C6-WROOM-1, native USB Serial/JTAG, boot/reset support, antenna keepout required in layout.
- Sensor: SCD41 on I2C at 3.3 V with shared pull-ups and local decoupling.
- User/debug: USB-C data, boot/reset buttons, status LED, test points.
Interfaces and Connections
- USB-C: VBUS, GND, CC1/CC2 sink resistors, D+/D- to ESP32-C6 USB pins through ESD protection.
- I2C: ESP32-C6 SDA/SCL to SCD41 with one pair of pull-ups to 3.3 V.
- Battery: 2-pin JST-style single-cell LiPo connector.
- Debug: USB Serial/JTAG over native USB, BOOT and RESET controls.
Power and Runtime Expectations
The design targets intermittent Thread communication and deep sleep. Exact runtime depends on SCD41 measurement interval, ESP32-C6 radio transmit frequency, and firmware sleep strategy. The power architecture will be sized for ESP32 radio bursts rather than average current only.
Power Tree and Power Budget
Initial sizing target:
- 3.3 V rail peak target: at least 600 mA for ESP32-C6 radio bursts plus sensor/support margin.
- Buck-boost input current worst case at low battery is expected to be roughly 0.8 A peak before margin.
- USB-C non-PD operation: 5 V sink using CC pull-downs, no PD controller.
Final measured/calculated budget will be updated after datasheet values are extracted.
Manufacturing and Assembly Expectations
Default to SMD components for a compact professional PCB. USB 2.0 D+/D- should be routed as a controlled 90 ohm differential pair, preferably on a 4-layer stackup with a solid ground reference plane.
Firmware-Relevant Hardware Requirements
- ESP32-C6 GPIO assignment must reserve strapping pins appropriately.
- Native USB Serial/JTAG available for programming.
- I2C bus for SCD41.
- ADC-capable pin for battery voltage divider, divider should be switchable or high-value to limit sleep drain.
- Thread communication with intermittent wake/sleep cycles.
Physical Design Expectations
ESP32-C6 module antenna must sit at a board edge with the antenna keepout free of copper, traces, components, and ground pour on all layers.
Important Design Decisions
- Use buck-boost rather than LDO to preserve 3.3 V regulation across LiPo discharge.
- Use USB-C native USB data into ESP32-C6 instead of external USB-UART bridge.
- Use charger/power-path management so USB can power the board while charging.
Assumptions
- Single-cell LiPo pouch battery with 2-pin connector and no on-board fuel gauge unless added later.
- USB-C is 5 V sink only, no USB Power Delivery negotiation.
- SMD assembly is acceptable.
- Thread is the primary wireless protocol; Wi-Fi is not intended to remain continuously connected.
Change Notes
- Initial specification created from user requirements and selected buck-boost architecture.
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
