Wearable Environmental Monitoring PCB Design
Make a circuit that blinks a red LED once per second.
Power it with a 9V block battery through an on/off switch.
Use components that are easy to hand-solder.
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AeriSense Project Specification
Project Overview
Status: Draft schematic design.
AeriSense is a compact wearable environmental monitoring PCB for a science fair engineering prototype. It monitors asthma-trigger environmental conditions and transmits data wirelessly using an ESP32-WROOM-32D module.
Intended Use
Wearable battery-powered IoT prototype for real-time environmental sensing, demonstration, and data upload to a mobile/cloud workflow. Design priority is reliability, manufacturability, and fast bring-up by beginner-to-intermediate designers.
What the Device Should Do
- Measure temperature, humidity, pressure, and VOC/air-quality indicators.
- Estimate LiPo battery voltage and state of charge.
- Communicate wirelessly over Wi-Fi/BLE from an onboard ESP32-WROOM-32D module antenna.
- Charge a single-cell LiPo from USB-C.
- Provide status LEDs, boot/reset controls, and an external UART programming/debug interface.
Main Features
- ESP32-WROOM-32D module with onboard antenna.
- Bosch BME688 environmental sensor on I2C.
- MAX17043 LiPo fuel gauge on shared I2C bus.
- MCP73831 single-cell LiPo charger, 500mA charge target.
- AP2112K-3.3 LDO 3.3V regulator.
- USB-C power-only input with independent 5.1k CC pull-downs.
- 0603 passives for manufacturable SMT assembly.
- Programming pads exposing TX, RX, GND, 3V3, EN, and IO0.
System Architecture
USB-C 5V powers the charger and can power the system through a simple prototype power path. The LiPo feeds the 3.3V LDO. The ESP32-WROOM-32D communicates with the BME688 and MAX17043 over I2C, controls status LEDs, and exposes UART/debug pads.
Hardware Subsystems
Compute/Wireless
ESP32-WROOM-32D module, antenna at PCB top edge, no copper under antenna region during layout.
Environmental Sensing
BME688 placed at right edge near enclosure venting, physically separated from heat-producing power and charging circuitry.
Battery and Charging
Single-cell 3.7V LiPo, 1200-2000mAh target. USB-C 5V feeds MCP73831 charger set to 500mA using a 2k PROG resistor. Charge status LED included.
Regulation
AP2112K-3.3 low-noise LDO supplies ESP32 and sensors from battery/system input. Use required input/output capacitors and 3.3V bulk capacitance for Wi-Fi current bursts.
Debug/User Interface
Reset and boot tactile buttons, power/charge/status LEDs, and external UART programming pads.
Interfaces and Connections
- USB-C: VBUS, GND, CC1/CC2 pull-downs only; D+/D- intentionally unused.
- LiPo connector: BAT+ and GND.
- I2C: ESP32 SDA/SCL shared by BME688 and MAX17043, one pair of 4.7k pull-ups.
- UART/programming pads: TX, RX, GND, 3V3, EN, IO0.
Power and Runtime Expectations
Runtime depends on Wi-Fi duty cycle. For a 1200-2000mAh LiPo, always-on Wi-Fi will be hours; duty-cycled sensing/transmit can be much longer. Firmware should use light/deep sleep where practical.
Power Tree and Power Budget
Preliminary load assumptions:
Table
| Rail | Load | Typical | Peak |
|---|---|---|---|
| 3.3V | ESP32-WROOM-32D Wi-Fi active | 80-240mA | 500mA burst |
| 3.3V | BME688 | <5mA | <15mA heater-dependent |
| 3.3V | MAX17043 | <1mA | <1mA |
| 3.3V | LEDs/debug | 2-10mA each when on | 20mA budget |
Design target: 3.3V regulator and battery path sized for at least 600mA transient load. AP2112K-3.3 is acceptable for low-duty Wi-Fi prototypes but thermal margin must be reviewed if sustained current exceeds roughly 200mA from USB/fully-charged battery input.
Manufacturing and Assembly Expectations
- Approx. 50mm x 35mm board.
- JLCPCB-compatible 4-layer PCB, 1.6mm, ENIG, black solder mask, 1oz copper, rounded corners.
- 0603 resistors/capacitors; avoid 0402/0201.
- Prefer SMT assembly for small parts; hand solder larger connectors/buttons if needed.
Firmware-Relevant Hardware Requirements
- Define I2C pins, UART TX/RX pins, boot IO0, EN reset, and status LED GPIOs.
- Support sensor polling, fuel-gauge reads, Wi-Fi/BLE upload, battery-aware sleep behavior, and serial debugging.
Physical Design Expectations
- Top edge: ESP32 antenna and keepout.
- Left: USB-C, charger, regulator, power circuitry.
- Center: ESP32 module body.
- Right edge: BME688 near vented airflow area.
- Bottom: battery connector, buttons, programming pads.
Important Design Decisions
- Use an ESP32 module rather than bare RF design.
- Use AP2112K LDO for low-noise/simple power instead of a switching regulator.
- Use external USB-UART adapter to avoid onboard bridge complexity.
- Keep Rev A focused on core reliability; reserve future expansion only conceptually.
Assumptions
- External LiPo cell includes protection PCB or the product uses a protected LiPo pack.
- USB-C is power-only; ESP32 native USB data is not used in Rev A.
- ESP32-WROOM-32D programming uses manual BOOT/RESET with external USB-UART adapter on UART0.
- Component availability and exact footprints will be validated during library selection.
Change Notes
- Initial specification created from user requirements.
- Updated MCU selection to ESP32-WROOM-32D per user-provided available module.
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