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Project attempt: Pulseoximeter


Source :
http://www.oximeter.org/PulseOx/principles.htm
http://www.edaboard.com/attachments/24135d1178378436-cc2004-12_2018-pdf
http://www.edaboard.com/attachments/24304d1179237986-ti_app__notes_for_pulse_oximetry_1486-pdf
http://www.circuitcellar.com/microchip2007/winners/DE/abstracts/MT2278_Abstract.pdf
http://www.cypress.com/?rID=2696

Did some basic research…

Description of operation :
The device uses 2 LEDs, one bright red (about 640nm) and one IR (about 910nm). These LEDs transflects the finger (or other suitable tissue), and the light gets into sensor.
Sensor can be a photodiode (such as BPW34), or Light-to-Frequency converter (like TSL230…)
Because I cannot obtain TSLxxx (the easier solution), I’m forced to use photodiode BPW34.
Using TSLxxx leads to “almost” pure digital solution, simply introducing TSL’s output into InputCapture module of MCU, counting pulses, some digital filtering, AC part extraction & showing up results.
Photodiode is a bit trickier… Signal is very small, I have to use instrumentation amplifier with very low drift as an input opamp (transimpedance approach), filter DC part of the signal, extract the “pulse” wave, amplify it and then sample via ADC (then filtering, calculations & ->LCD).
There are many analog part solutions to this problem, but I didn’t find the one I could repeat.
(Cypress’ version is very sophisticated, but I cannot find any “cheap” PSoC chip near my localization, combining it with need for new programmers/debuggers rendering it very far from acceptable…)

Do you have any ideas, how to design analog part of this project ?

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  1. grabbo
    March 4, 2011 at 8:42 pm

    Hello, im working on oximeter either.

    There are 2 important books – How to design Pulse oximeter and another one Photodiode Amplifiers. These two are must to have 🙂

    Questions you should start with – how to drive intensity of leds (intensity). Analog part with filtering is not hard as digital filtration.

    Check TI pulse oximeter with Msp430. Easy to read and understand.

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