Wearable Heartbeat Monitor with LED Visualizer
A wearable heartbeat monitor that flashes an LED in real-time synchronization with the user's pulse.
Intermediate Project
— Integrating electronics into clothing requires managing flexible wiring and motion-related signal noise, which is more complex than a standard stationary prototype.
Assumptions:
- The device will use a finger or earlobe-based optical pulse sensor for the prototype.
- Battery power is required for wearable integration.
- The LED needs enough current to be visible through or on clothing.
Bill of Materials
Compatibility Notes
- The SEN-11574 sensor requires 3.3V-5V; the recommended XIAO-RP2040 provides a 3.3V output which is compatible.
- The WP7113SRD LED requires a current-limiting resistor (approx 150-220 ohms) when driven from the MCU's 3.3V GPIO.
- The MAX1555 charger requires a 5V input (from USB) to charge the LiPo battery.
You'll Also Need
- Conductive thread or flexible silicone-coated wire for clothing integration
- Velcro or snap buttons for mounting the sensor to the skin
- 150-ohm resistor (0805 or through-hole) for the LED
- Breadboard or small protoboard for assembly
Estimated BOM Cost: $75-85 (based on live distributor pricing)
Design Considerations
Signal Filtering
Heartbeat signals from optical sensors are prone to motion artifacts. You must implement a software band-pass filter (0.5Hz to 4Hz) or a moving average window to prevent the LED from flashing due to body movement rather than a pulse.
Battery Management
Since the device is wearable, use a LiPo battery with an integrated protection circuit (PCM) to prevent over-discharge. The MAX1555 should be paired with a physical slide switch to completely disconnect the battery when not in use, as LDO quiescent current can drain a small cell over weeks.
Motion Sensitivity
Optical Pulse Sensors (PPG) are highly sensitive to the pressure applied against the skin. For clothing integration, use an elastic band to maintain constant, light pressure; too much pressure restricts blood flow (killing the signal), while too little introduces massive noise.
Skin Contact and Sweat
Sweat can short-circuit exposed pads on the sensor. The sensor back should be coated with a thin layer of clear epoxy or wrapped in heat-shrink tubing (leaving the optical window clear) to provide moisture resistance against skin contact.
Interrupt-Driven LED
To ensure the LED flashes in 'exact sync', use an ADC interrupt or a hardware timer to sample the sensor at a fixed rate (e.g., 100Hz). Detect the peak of the waveform in software and trigger the LED immediately to minimize perceived latency.
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