Designing a really low cost Pulse Oximeter

Losing LRC(Lunar Rover Challenge 2013 shaastra) sure was upsetting but it kind of reset my thoughts.

I have finally decided to move back to building healthcare devices,
The grand vision requires assembling several diagnostic equipment into one wearable system which could predict a disease before the symptoms manifest.
First of I’ve decided to make a ultra low lost pulse oximeter,
Well despite there being several ways to monitor a person’s heart beat (ECG, Echocardiogram) Pulse oximetry remains to be the simplest both in design and processing.
We all have at some point of time tried shining a flashlight through our finger only to find it illuminated in red.

Shining a flashlight through my finger

Shining a flashlight through my finger

Blood generally gets the red color due to the presence of hemoglobin, which absorbs the oxygen captured by our lungs. So technically the amount hemoglobin present can be correlated with the amount of oxygen absorbed by our body.  That is the basic principle behind the calculation of oxygen saturation.
Pulse oximeters work by shining red and infrared light down the veins calculating the amount of radiation absorbed or reflected.  The principle is that light at two wavelengths (650nm , 940nm ) are sent through the vein and the intensity of light absorbed or emitted is measured and when this is plotted against time, We get something that provides us an insight about our heart.


Circuit? –

I hooked up a red led and a photodiode I had lying around to a single stage amp opa381 and fed the output my trusty scope, I modified a simple cloth clip to hold the red led and photodiode with my finger clipped.


When I connected it to a scope-:

A scope trace

A scope trace

Things to do- :

What I’ve done right now is nearly no where close to what I intend to do, I merely plotted a photoplethysmogram which can only provide heart rate. What I really need to do is to measure the intensity of light transmitted for red and IR led’s and find oxygen saturation.
I got some TSL235 light intensity sensors lying around so will use it to interface it with a MSP430 launchpad next.

Until Next Time,
Peace homies!





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