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:
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
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
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