PulseFit-Pulse Sensor with AutoAdjusted Threshold

Description

All the DIY pulse sensors I have used previously have some sort of threshold that has to be adjusted to each new user. The threshold is tuned for each person or is generalized so that it fits any user. I haven't played with one (at least in the DIY space) that has an auto-adjusting threshold built in the hardware. I don't know how well it will work, or even if it will, I was just curious.

The circuit incorporates the basic pieces of a typical pulse sensor. It contains an LED to shine light through someone's finger, a photodiode to detect the light that passes through the finger, a transimpedance amplifier to amplifier the current from the photodiode, and an active bandpass filter to amplify the signal and filter out the noise. My main addition is a peak detector circuit at the output of the band-pass filter and a comparator.

*****NOTE: This is NOT a medical device

Details

*****NOTE: This is NOT a medical device*****

Demo!

Using a diode and capacitor pair, the peak detector captures the peak AC amplitude of the pulse waveform and holds the voltage (because of the capacitor). When we place a resistor in parallel with the capacitor, the resistor allows the capacitor to discharge at a rate equal to the time constant of the RC circuit. If we tune the time constant correctly, the peak detector circuit will hold the peak long enough to distinguish the peak pulse amplitude from secondary peaks due to the dicrotic notch. Also, because of the discharge of the capacitor through the resistor, the peak detector does not hold the voltage indefinitely, but it adjusts to the changing peak AC amplitudes of the pulse waveform due to stronger or weaker pulses.

Project Logs

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PulseFit GFX Board
Orlando Hoilett • 05/29/2019 at 13:46 • 0 comments

I've been working on this PCB off-line for a while now. Since originally posting this project, one of my students has been working on developing a PulseFit GFX board that allows the user to view the pulse waves in real-time. You can check out the schematic here: I still have a few things to tidy-up on the board layout, but it's almost there.
Flexible PCBs Here We Come!
Orlando Hoilett • 05/29/2019 at 13:36 • 0 comments

Submitted to Flexible PCB Concept Contest. I would like to experiment with moving the sensor to different parts of the body similar to ideas presented in my ring-based heart rate monitor, my wrist-worn heart rate monitor, or my finger clip heart rate monitor projects also shared on Hackaday. Such prototypes would require flexible PCBs to properly match the body's contours to main the signal. I've even published a research article on one of my wrist-worn heart and respiratory rate wrist-worn heart and respiratory rate monitors (please feel free to message me for a PDF of the full article). The Flexible PCB Contest would help me take my project further and advance my study into body-worn sensors.
Version 4 sent off to OSH Park
Orlando Hoilett • 09/10/2016 at 20:13 • 1 comment

Made my second @oshpark order for PulseFit. This version uses bottom entry SMD LEDs instead of the through hole types. I hoping to start selling these soon so having all surface mount components will make assembly a lot easier.
Finished PCB Design!
Orlando Hoilett • 07/09/2016 at 01:46 • 1 comment

Sent PCB to @oshpark Here's the link to order the board yourself.
Link to the .brd and .sch files on GitHub.
Almost done with the PCB Design
Orlando Hoilett • 07/05/2016 at 00:27 • 2 comments

I just need to organize the silkscreen a bit and I may re-adjust how I did the ground plane. The photodiodes are in the center of the heart. I will be using reflectance photoplethysmography as opposed to transmission photoplethysmography so I don't have to build a finger cuff. Can't wait to send this off to @oshpark!
Pulse Sensor and Me on Adafruit Show and Tell!
Orlando Hoilett • 07/01/2016 at 14:57 • 0 comments
Live Demo with Beating Heart
Orlando Hoilett • 07/01/2016 at 14:53 • 0 comments
Featured Instructable
Orlando Hoilett • 07/01/2016 at 14:50 • 0 comments

My project was Featured on Instructables! Check out that gold ribbon.
Live Demo!
Orlando Hoilett • 06/28/2016 at 01:08 • 0 comments
Test Circuit
Orlando Hoilett • 06/27/2016 at 22:22 • 0 comments