Designing Within Reach: A Wearable Heart Rate Monitor Constrained by Available Fabrication

Abstract

This project set out to build a heart rate monitoring headband with Bluetooth connectivity, as an alternative to smartwatches for users whose preferences are not served by wrist-worn devices. The designer began with no prior experience in soldering, sewing, wiring, or laser fabrication, and completed a working prototype in eleven days.

The device was built around the materials and tools available. Every component was selected on the condition that it could be hand-soldered, laser-cut, or sewn. This constraint drove the selection of the GY-MAX30102 optical sensor over more capable but non-hand-solderable alternatives. The MAX30102 and an ADXL345 accelerometer are mounted at the temple in a laser-cut 3 mm black cast acrylic carrier, wired through the interior of the headband to a Seeed Studio XIAO ESP32C3 microcontroller, a 500 mAh LiPo battery, and a tactile button housed in a second acrylic module at the back of the head. The device recharges over USB-C, streams heart rate over standard Bluetooth Low Energy so that generic heart rate applications can connect to it, and pairs with a custom application that records timed activities and exports heart rate and motion data.

The prototype was tested against a Garmin Forerunner 255 as a commercial reference. At rest, it recorded a mean absolute error of 1.33 bpm. Recording while the user was running drove the mean absolute error to 14.65 bpm, approximately eleven times greater than at rest. The device operated for more than five hours on a single charge and maintained a Bluetooth connection to approximately 128 feet. Total device material cost was $52.07.

The device is accurate at rest and is not accurate in motion. The accelerometer transmits motion data, but that data is not applied to the heart rate calculation. Motion-artifact rejection was the accelerometer's stated purpose in the design and was not implemented; doing so is the most likely route to reducing the error observed under motion.

The primary outcome of this project is not the device but the fabrication capability behind it: design, cut, solder, sew, wire, and verify.