Oxygen Levels in Blood

[krismoly]

Hi folks,

Can any help.

I am trying to design a circuit that will connect to a microcontroller to display pulse and Sp02 levels in blood.

I have limited budget. I have managed to contruct simple circuit to detect heart beat. this works well. This part of the circuit uses a high intensity red led.

From all the articals I have read, it is best to use infrared for measuring blood oxygen levels(SpO2).

Can anyone give me any pointers or ideas as to were to begin with this part of the circuit?

Any comments or suggestions welcome!

Regards

Kris

 

[Sceadwian]

**broken link removed**

Regardless of the area of the body you take it (forehead in the case of the above link) or in the finger. Oxygen content measurments of the human body taken with an IR transmitter and receiver are ALWAYS relative and there is no other method of obtaining an absolute blood oxygen content without the sensor actually being in contact with a blood sample.

 

[FMadi]

Hi, I used to work servicing pulse oximeters, and I can tell that although the principle of operation is simple, the actual practical application is not.
You can read about the principle of operation here, as you can see you need both the infrared and red light source, since the SPO2 is a function of the difference between the emitted and received light of both sources (you need the 2 different wavelengths).

What I can tell you is that, after digitalizing this light signal, there is a lot of signal processing to do if want to get a usable measurement, the algorithms are quite complicated and a play a big part in the quality of the waveform, the oximeters I have worked with were always constructed with a main board, and a separate oximetry board, (usually manufactured by a third party, I’ve seen mostly Respironics and Nonin and Nellcor boards), the oxymetry board would do all the measurements and signal processing and feed the main board (based around a 80C31) the measured values and waveform, so the main board would just be a display driver and an alarm and power manager.
I have only serviced low-cost oxymeters, and some of the boards (specially the cheaper ones with no brand) were barely unusable, the wave forms were always jittered and the SPO2 value would not be stable nor accurate (we used an index2 simulator by Fluke). There’s a lot of research involved in those algorithms.

Sorry about the long (and not very helpful) post, if I wore you I would try to get one of those boards or maybe their documentation, its not a trivial task what you are trying to do but I give you my best wishes that you succeed. Please keep us posted on your progress.

 

[krismoly]

Thanks for your replies folks.

I understand that you need both (visable) red light and infrared for this to work. (I have read articals that use different colours such as blue, yellow)

From the research I have done. Measurements are taken individually (ie a measurement of red light is taken then a measurement of infrared light this process is continuous)

From the red light I have a varying dc level, I have used an opamp as a comparator (to set threshold) which illuminates an LED. This LED illuminates as the heart beats.

The experiments I have conducted with the infrared are the same for the red. I get a varying dc level. I have used the same amplifier (3 stage transistor) and connected the output to oscillascope. I get same varying dc level. (As one level falls the other one rises. red compared to infra red)

Am I on the right track? Do I need to use different gains for the different wavelengths of light? If I do then can anyone point me in the direction of how to calculate this?

I will be using a pic with a/d convertors. so the problem of converting this signal to digital will be taken care of by the PIC. I can get the help I need with the programming that is not a problem. (as long as I know the equations).

I basically need to know if I am on the right track so far?

I am only a hobbyist and have never actually studied electronics formally since leaving secondary school.( over 15 yrs ago now ouch I'm getting old...he he)

Any comments or suggestions are welcomed

Thanks again for your help
Kris

 

[Sceadwian]

It appears to be a differential signal. Ie it's the ratio of RED to IR absorption, not either one individually that matters. I'd make sure your amplifiers are very linear. The main problem with a device like this is also calibrating it. You really need a medical quality device that is in known calibration and good working order to really compare the values you come up with on your homebrew setup.