A state of the art fitness monitor should combine three C’s: Convenience; Comfort and Continuous monitoring of cardiovascular parameters.
Convenience is a single unit without the need for external sensor attachments; comfort allows a full range of motion in physical activities – wrist worn instead of chest straps — and continuous monitoring is the opposite of snapshots in time.
Photoplethysmography has emerged as a most promising technique for achieving three C’s. It uses light to detect the dilation of blood vessels — caused by cardiac activity – in the subcutaneous tissue. The skin is illuminated by an LED and the light transmitted (or reflected) to a photodiode. Because blood flow to the skin is modulated by many physiological conditions, this optical approach can also monitor blood oxygen levels, breathing and blood pressure among other circulation functions — in theory.
In practice, optical techniques are first finding their way into wrist-worn devices for measuring heart rates only, but are easily upgradable to blood oxygen measurements once those are deemed reliable by the FDA.
To build your own wrist-worn device that uses optical techniques you would need: LED and drive circuitry; photodetector and associated signal conditioning; user-motion sensor; ultra-low-power microcontroller for running calculations; data storage; wireless module for communicating with other devices, and power supply – a lithium-polymer rechargeable battery with charger and fuel gauge. Certain commercially available analog front ends integrate LED drivers and photodiode signal conditioning into one package.
Sounds daunting? Check out the Optical Heart Rate Monitor Reference Design with BLE Connectivity. It includes components plus schematics, layout and Gerber files. It may be the easiest way to get started. Customize with your own features.