ESP32-Based IoT Wearable Sports Sensor: Design & Development

Client: Sports Wearables Startup

Timeline: 4 weeks (from schematic to Gerber files)

What We Delivered: Schematic, PCB, Gerber files, Bill of Materials with links to sources, and Pick-and-Place file.

The customer required a compact PCB designed for a wearable sports sensor. The wearable sports sensor would be used by athletes and fitness enthusiasts.

The basic requirements were simple enough but challenging: design and manufacture an ESP32 based board which contains an IMU, can be powered by LiPo battery, and can be charged via USB-C, all in a size that allows for it to be worn while performing at high intensity levels.

Core specifications:

• ESP32 as main processor (Wi-Fi + BLE for data transmission)
• 6-axis or 9-axis IMU for motion tracking
• LiPo battery (single-cell, 3.7V nominal)
• USB-C for charging and programming
• Compact form factor for wrist or body attachment
• Low power consumption for extended wear

The client specifically requested:

• Schematic files
• PCB layout files
• Gerber files (ready for manufacturing)
• BOM with sourcing links (DigiKey, Mouser, etc.)
• Pick-and-place file for automated assembly

The constraints for this project included:

• No enclosure design was required. The client handled all the mechanical integration.
• The focus was on manufacturability and would be for small to medium volume quantities (100-1000 units).
• Component availability through national distributors was a critical aspect of the project.
• The layout of the board should not affect the RF performance of the ESP32 in any way.

Microcontroller Types: ESP32, ESP32-S3, and ESP32-C3

The client did not specify which version of the ESP32 they wanted, so I had to choose which variant of the ESP32 I would like to use. After researching all variants, I selected the ESP32-S3-WROOM-1 module as my module for the project.

Reason for ESP32-S3: USB support had eliminated the need for using a dedicated USB to serial converter chip (CP2102 or CH340). This saves board area and BOM costs. The extra GPIOs also give the user more possibilities for adding future sensors to the design.

IMU (Inertial Measurement Unit) Selection: Motion Sensing for Sports

The IMU must be capable of maintaining very rapid changes in speed by measuring the direction of moving as well as locations of physical objects (accelerometers) while performing a sport. I assess two options:

We chose the MPU-9250 for this design because it has 9 axes of measurement so it can monitor how something is moving and what position it is in compared to Earth's magnetic field.

The magnetometer will help with some sports because determining your absolute direction is important (running routes, swimming laps).

Alternatively, the LSM6DSO32 may have an acceptable sensing range of ±32g, which would be beneficial for contact sports (basketball, boxing), but the customer indicated that they focus primarily on general physical activity.

Charging and Battery Management

The following components must be included in the system:

• 5V USB-C power input
• Lithium Polymer (LiPo) charging circuit; single-cell, 3.7 to 4.2 volt
• 3.3V regulation for the ESP32 and the sensors
• Battery voltage sensing

Key Components:

• TP4056 LiPo charger (1A max, 450mA typical for this type of device)
• TPS63031 buck-boost DC-DC voltage converter to keep the output at 3.3V regardless of the battery voltage
• Battery connector: 2-pin "JST-PH" (which is the standard connector used with LiPo batteries)

Why Buck-Boost? A standard Low Dropout (LDO) regulator or buck converter cannot maintain a 3.3V output from the battery voltage when the battery drops below 3.4V.

The TPS63031 buck-boost converter will boost the voltage to 3.3V when the battery voltage is low and step down the voltage when the battery voltage is high, thus providing the maximum amount of discharge time (or usable battery energy) from the LiPo battery.

Power Distribution Network

The TP4056 charge current was programmed to 450mA (R_PROG=2.4kΩ) which approximates the 1C charge rate (one hour) for a lithium polymer battery (approximate range of 400-500mA/h). An LED indicates charging status by being on when charging and off when charge is complete.

USB-C Implementation

ESP32-S3's native USB support simplified the USB-C implementation significantly. Only required:

• 5.1kΩ pull-down resistors on CC1 and CC2 pins (USB-C configuration)
• ESD protection diodes on D+ and D- lines
• VBUS directly to TP4056 charger input
• No external USB-to-serial chip needed — this saves ~$2 in BOM cost and valuable board real estate. Programming occurs via ESP32-S3's built-in USB-serial-JTAG controller.

IMU Interface

The MPU-9250 connects via I²C (default address 0x68). Required:

• 4.7kΩ pull-up resistors on SDA and SCL lines
• Separate 3.3V supply with 0.1µF decoupling capacitor near the IC
• INT pin routed to ESP32 for data-ready interrupts

The I²C bus is shared with any additional sensors (planned for future revision), supporting up to 3.4MHz high-speed mode.

ESP32-S3 Minimum Support Circuitry

Layer Stack-up

I chose a 4-layer PCB for this wearable design:

The dimensions for the board are 35 mm x 35 mm which is small enough to fit in the wrist but has enough area for placing the components.

Why are 4 layers better than 2 layers?

• Improved RF performance with controlled impedance for the ESP32's antenna
• Better power integrity due to the presence of a dedicated power plane
• Reduced EMI because of the ground plane separating the signals on the top and bottom of the board
• Small form factor can be obtained through the tighter routing of the components

Critical Design Decisions

Keep-out area for the ESP32 Antenna: The PCB Antenna used in the ESP32-S3 module will require an area on all 4 layers to not be used. I have used the area of 15 mm x 10 mm of board space in my design for the following reasons:

• No copper pour underneath the antenna
• No components should be within 10 mm of the edge of the antenna
• The ground plane should be cut-out on the inner layers below the area of the antenna

IMU Placement: The MPU-9250 was placed near the board center to minimize lever arm effects during rotation. No high-current traces run beneath the IMU to prevent magnetic interference with the magnetometer.

USB-C Connector Placement: Edge-mounted on board perimeter for easy access when worn. ESD protection diodes placed within 10mm of the connector pins.

Design Rule Check (DRC) Settings

These rules are compatible with JLCPCB and PCBWay standard manufacturing processes.

Sourcing Strategy

The client required sourcing links to DigiKey and Mouser — major distributors with consistent stock. I structured the BOM to prioritize parts available from these distributors with reasonable lead times.

Critical Component Sourcing

Alternative Parts (Supply Chain Risk Mitigation)

Given component shortages affecting the industry, I provided alternatives:

Gerber Files

Generated standard RS-274X Gerber files including:

Output format settings:

• Format: 4:4 (inch)
• Zero suppression: Leading
• Coordinates: Absolute

Pick-and-Place File

Created centroid file (CSV format) for automated assembly.

Assembly Drawing

Included assembly drawing layer in PCB files showing component outlines and reference designators. This aids manual inspection even when automated pick-and-place is used.

Electrical Rule Check (ERC)

• All ESP32 inputs float are pulled down using a resistor of 10kΩ on GPIOs that are not in use.
• The sequence at which the power supply is connected was confirmed. The 3.3 V rail has power provided right after the battery is connected.
• Pull-ups for I²C (4.7kΩ) on both the SDA/SCL lines were verified.

Thermal Analysis

The TP4056 charge IC produces heat while charging, causing the charge IC to use thermal vias located under the "Exposed Pad" of the IC to transfer heat to the ground reference plane. The TPS63031 has a similar design.

RF Analysis

Proper placement of the ESP32 module ensures the antenna is located on the edge of the PCB with sufficient clearance from any copper near the antenna. The 4-layer stack-up with one layer being a ground reference plane will provide a good return path for RF signals to reduce any detuning effects from adjacent components.