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Photoplethysmography

A project log for Aman :The low cost wireless stress monitoring sock

Quantify stress comfortably by measuring Electrodermal Activity(EDA) and Heart Rate along with contextual info like Humidity,Temp & Motion

Naveen SridharanNaveen Sridharan 09/28/2016 at 04:040 Comments

Photoplethysmography forms the back bone of today's wearables ranging from the Apple Watch to fitbits. It works by sensing the change in blood flow volume by measuring the transmittance or reflectance of the 535nm light(Green).

This wavelength is ideal as it as at this wavelength peak absorption of heamoglobin happens. I have hence chosen to use the simple yet effective Pulse Sensor Amped from WorldFamousElectronics cheaper ebay knockoffs work fine too.


Since I cant have a bunch of wires running through a socks and expect the user to be comfortable, I had to connect this to some fabric ribbon cables.

I ran into some issues initially as the wire would snap under the stress of my toe moving, This was an issue I would have to deal with later. But for now nothing a bit of E-6000 glue cant fix.



Now that the cable is all stable and rigid, I got to the conductive textile ribbon cable part.
I coincidentally had some FabricKit three channel cables lying around in my bench, These cables look and feel like textile but are actually solderable and conduct electricity.

As it comes with it's own snap like connectors this part became easy. I just soldered the pulse sensor cable to the connector snap.



Now I soldered the conductive ribbon cable to the male snaps.


After some struggle with some solder bridges, I managed to connect it properly.
Now for the moment of truth, Does it work?

Thankfully yes.

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