Milos Rasic has published a fully open-source Bluetooth chest-strap heart rate monitor on GitHub, giving makers a reproducible alternative to commercial units.
The hardware is direct: an AD8232 handles ECG signal acquisition, a Seeed Studio XIAO ESP32C3 manages Bluetooth transmission, and a single 3.7V cell powers the system through a boost converter stepped up to 5V. There is no charge controller, so battery management is the builder's problem. Firmware loads via Arduino IDE, which means porting to other ESP32 variants is a realistic modification, not a theoretical one.
Rasic is not a one-project builder. He gave a talk at Hackaday Europe 2026 dissecting the accuracy failures of consumer blood pressure monitors, and this chest strap sits inside the same larger effort to open up personal medical hardware. The full repository and that earlier talk are both worth your time.
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