Peter Balch's Arduino Nano-Powered Health Monitor Can Now Track Blood Volume, Too

Maker Peter Balch has put together the perfect project for anyone interested in keeping an eye on their vital signs, using an Arduino Nano and SPI display to graph an electrocardiogram and photoplethysmogram simultaneously — tracking heartbeats and blood volume over time.

"A plethysmogram is a graph of blood volume vs. time," Balch explains of the metrics his health monitor is designed to track, "and a photoplethysmogram (PPG) uses light to measure blood volume, often with a finger probe. A probe clips on your finger and passes infra-red light through it. The amount of IR received is a measure of the blood volume in your finger."

Balch's latest project builds on one he published two years ago, as a response to the COVID-19 pandemic. Where the original project merely traced an electrocardiogram — albeit in three neat modes, including as a Poincaré plot — this upgraded variant adds a photoplethysmogram trace, monitoring the user's blood oxygen saturation at the same time.

The core technology in the PPG sensor is simple: an infrared LED, and optionally a red LED, is shone through the finger, and the light picked up by a phototransistor below. As the volume of blood in the finger changes as a result of the wearer's pulse, the amount of light varies too. "It's straightforward to build a PPG that measures red and [infreared] absorption," Balch explains of his reason for going infrared-only, "but calibrating it is a problem."

In addition to the sensor circuit, and an optional build-it-yourself amplifier design which can be used in place of an Analog Devices AD8232 amplifier, Balch has also designed a 3D-printable finger clip for the PPG. Updated source code for the machine's driving Arduino Nano is included, adding the PPG trace and also automatically calculating the "QT interval" between the start of a heartbeat's Q-wave to the end of the T-wave.

"This is not a medical device, it's something you're building for fun," Balch warns. "If you were building this as a medical device, you'd use different filtering. You'd worry about the phase-shifts introduced by the filters — Chebyshev and Butterworth filters have different phase-shift. And you'd use a better quality op-amp!"

The full guide is available on Instructables, along with the original ECG project.