SteadyBreath - Wearable Heart Rate Sensor

Project Description

We're building a wearable wrist watch that measures heart rate (PPG) and motion (IMU). When you’re sitting still but physiologically “amped,” it vibrates a guided breathing rhythm to help you calm down. 

Core Functions

Core functions:

• Heart-rate sensing (MAXM86161 PPG)

• Motion tracking (BMI270 IMU)

• Smart haptic feedback (DRV2605L + coin motor)

• (Potentially) bluetooth app for visualization, steps, sleep, and ML fall detection

Electrical / Mechanical / Functional Requirements:

• Compact, low-power wearable (battery-operated)

• Stable 3.3 V logic for MCU + sensors

• Rechargable Li-ion with UCB charging

• Reliable optical coupling to skin and vibration isolation

Circuit Categories Fulfilled:

• Compute - ESP32-S2-SOLO-N4 Microcontroller

• Power - MCP73831 charger + 3.3 V LDO

• Sensing - PPG + IMU 

• Actuation - DRV2605L haptic driver + coin motor

PCB Design Feedback & What we learned! 

• SDO (serial data out) and CSB (chip select bit) lines are for SPI protocol, whereas I2C uses SDA (serial data) and SCL (serial clock) for timing synchronization

• Interrupt pins useful to wake the microcontroller from sleep

• Voltage dividers are not suitable for power applications because a high current can alter the output voltage of the divider

• Ground plane to reduce power dissipation and noise. 

• Via stitching near decoupling capacitors to reduce inductance and control return currents. 

• Decoupling capacitors: Place them as close to IC power pins to provide local energy and avoid voltage drops by trace resistance and inductance. 

• Buck/boost converter uses inductors, capacitors, and diode to provide a stable DC output voltage that can be higher, lower, or inverted compared to the input voltage. In our project, we converted between 5 V and 3.3 V

Board Bring Up!

Challenges:

• Soldering small IC components

• We soldered components with plastic (the USB-C connector) before putting it in the oven

• We forgot to order a stencil (LOL)

• We accidentally sucked some of our small resistors with the solder gun, when trying to desolder them. :(((

Future Plans:

• Adding testing points and LED's for ease of debugging the design and code

• Either order a stencil or pay extra to get it assembled for us