# Pulse oximeter - Update 02/09/12

I am currently in the engineering phase to build a pulse oximeter. I mainly make use of the following references:

Normally a pulse oximeter analyzes the light absorption of two wavelengths from the pulsatile-added volume of oxygenated arterial blood (AC/DC) and calculates the absorption ratio:

SpO2 (O2 saturation value) is than simply computed by a look-up table stored in the memory of the MCU.

But there is fortunately another way to calculate SpO2. Only the AC level is used and SpO2 calculated by following formula:

where

(IAC)λ1=light intensity at 660 nm and

(IAC)λ2=light intensity at 940 nm.

We also know that  photodiodes produce a leakage current that is directly proportional to the intensity of the light. The current is converted into a voltage by an according current-to-voltage-converter. The current is now again directly proportional to the voltage. This means, we can use the voltage instead of the intensity of the light to calculate SpO2.

We now firstly need to call a function which determines the maximum and minimum peak of the two signals in a certain time interval. We call it Vmin λ1/2 and Vmax λ1/2.

To remove the DC component from the signals, we compute:

The remaining root mean square voltage is then:

Now we can directly compute SpO2:

After figured that out, I have started working on the circuit diagram. A first version can be found attached. The amplifying and filtering stages are re-drawn from the Freescale page. As the values of the capacitors and resistors are not very clear, I had to re-calculate all the high-/low-pass-/Notch-filters and gains though.

I'll build a prototype the next days/weeks. If I get it working, I'll compare the data with a commercial oximeter. I'll update this blog as I make progress.

Update September 2, 2012

Currently the oximeter sensor is under construction.

I am still waiting for the pin photodiode to arrive in the mail.

Technical data of the photodiode:

AttachmentSize
Oximeter_circuit_v.1.JPG238.62 KB

## Comment viewing options

Just a comment to a now deleted spam comment :-)

Tomm, it's not cool to register here and just drop links in a so called comment which I believe is not really to the topic but matching with your product. I already did report your comment twice, before with the other link to the wedding toppings (if i remember right). Now I also reported your profle as spam...so please do yourself a favor and stop this behavior. If you want to post a link to your shop and it's at lease a bit related to our stuff here then go to the shop section and tell the people what they can expect there, DO NOT SPAM in comments...that's so un-cool and we have a good spam control here since all the members (except the spammers) are taking care of our website.

A way to high math stuff for me...as I told you during lunch...but i will try to understand :-) Thumbs up for this Markus

The thinking to be the smartest guy is not unknown to me as well :-) but i already met much smarter guys (Markus is probably the most smartest i know in person) but some of them are just 1/3 of my age...this is what bothers me LOL

I was asking Markus WHY he is doing this project and he told me that he just want to do something else...his way to say "Because I can!" haha

I am only the smartest guy in a room, if I'am alone:)

Never stop to learn and discover. That's what I can tell ya. And I've learned a lot from you both too.

I love those times when I can look around a crowded room and think to myself "I'm the smartest guy here."  This is not one of those times.

And this is what it feels like to be on the other side of that.

,_,

Very cool, Markus!  I can't wait to see (and maybe even copy) the results!

:D i had the same feeling . mind you as a high school drop out this happens to me often on LMR , but that's ok i try my best with what i have :D i think it was your work with oddbot's board that impressed  me the most

Markus nice work , i wonder if this could be applied to other fluids and gases