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Electronics => Projects, Designs, and Technical Stuff => Topic started by: lgbeno on April 30, 2013, 06:32:43 pm

Title: ViaDesigner Pulse Oximeter
Post by: lgbeno on April 30, 2013, 06:32:43 pm
So I'm all setup with a ViaDesigner license from the amp hour and I think that I'm going to try my hand at designing a pulse oximeter circuit.  For now ill just use a red led and photo diode to measure a waveform.

Basic architecture is to use a opamp and pnp to create a constant current led drive.  Then another opamp as a TIA.  The output of the TIA will be ac coupled to a gain stage followed by a low pass filter and the a buffer amp to drive a ADC.

Anyone have experience with such a circuit or any words if wisdom?

I'll use this thread as sort of a project log.
Title: Re: ViaDesigner Pulse Oximeter
Post by: babysitter on May 01, 2013, 08:18:08 am
make the led current controllable, not fixed. so you can make it brighter with big thumbs and dim it when a lot of light reaches the receiver. pulse the led to discern from ambient light. its not a pulseoxi, its a photoplethysmograph (if you plot optical attenuation over time) or pulse sensor.

written with baby on my knees
Title: ViaDesigner Pulse Oximeter
Post by: lgbeno on May 15, 2013, 06:37:16 pm
Thanks for the advice!  Sorry I haven't been on the forum for awhile but have been working in this project.

I did notice exactly what you are talking about wrt the variability of Rx light through the thumb vs finger.  If I use the same led current through a finger, it saturates the diode however works great through the thumb.

To address this, I made my current reg variable and placed an op amp to regulate the DC common mode of the Tia output (filtered through LPf of 2meg ohm w/ 2uf).  This still presents a problem because that regulator also cancels out my pulse signal because this as well is very low frequency!  I thought that time constant would crush it but I can still see my pulse leak through on the current output.

It might be time to setup a dac and some logic instead...
Title: ViaDesigner Pulse Oximeter
Post by: lgbeno on May 15, 2013, 06:39:03 pm
By the way, like the dedication of staying on the forum while caring for kids.  When my two year old sits on my lap I have the hardest time keeping her away from pressing keys in the keyboard.  "Super letters" as she calls them.
Title: Re: ViaDesigner Pulse Oximeter
Post by: babysitter on May 15, 2013, 08:46:24 pm
OK, you talked about a DAC taking care of the measured signal, so I guess there is a Microcontroller around, too. Use a PWM output and a low pass filter to make a little DAC to control the light intensity. In the ┬Ác, you can play with the time constant of the LED intensity regulator however you like it.. even measuring it once, keep it on a fixed value until the fixed value doesnt work anymore because the ADC is over/underfed.

Fixed intensity with a resting test person can tell you a bit about the tendency of the blood pressure rising or falling.

Yeah, kids are great - mine always wants too see what I am doing, but at nine months still quite easy to keep off of the devices i am playing with :)