Maybe some theoretical background could help the user who posted this message in the top of this page to get through the design of the project.
A pulse oximeter technically is a modified version of a simple absorption UV/VIS spectroscope. Like the preceding user told, there are two basic modes of operation for these devices. One is transmission and the other one is reflection.
(Okay let's explain this a little bit)
In transmission, the device works using absortion spectroscopy techniques, where a pair of LEDs face a photodiode through a translucent part of the patient's body (usually fingertips, earlobes). One led is Red with a wavelenght around 660 nm while the other one is on the IR range around 900s nm. The difference between the absorption of oxyhemoglobin and its deoxygenated form at those wavelenghts allows the calcuation of the ratio of absorption from both leading to the estimation of the concentration of blood oxygen.
Like all absorption techniques this requires some correction to get the real value shown in the screen or monitor, so isobestic point principle is configured in the microcontroller unit of the device to perform the calculation and display the result.
In reflection mode the device should work some like diffuse reflectance spectroscopy where detection unit collects and analyzes scattered IR energy. Thus pulse oximetry uses reflected light on a single-sided monitor, but the principles of operation of this mode are the same as for transmission oximetry.As far i understand (because i am not a medical technician- i'm more closer to analytical chemist), research in Finland made during the early 1990s indicated, reflection oximeters performed well during partial ECC (extracorporeal circulation) giving earlier readings at lower pressure whereas transmission oximeters failed to give readings in partial ECC. It also shown that during partial ECC the accuracy of heart rate data was closer to electrocardiograph readings in reflection oximeters.
(if you want to read more details you can check here http://www.springerlink.com/content/pj5714057jv82578/
But because these results were too far specific for a certain medical procedure (that is likely would occur during a surgery due to some sort of inherited disease), the study concluded that in standard and normal heart rates, both sensors gave good results.
In my opinion as (none specific details were given to the use of this device), you could work in a transmission oximeter as seems more easier to design without the need to buy a probe already made. Such project would be nice for a fair science or hobby electronics.
If you want more details about isobestic point principles you can check Wikipedia's entry regarding that matter (nicely explained) on http://en.wikipedia.org/wiki/Isosbestic_point
More theoretical background is shown here http://courses.cs.tamu.edu/rgutier/cpsc483_s04/pulse_oximetry_notes.pdf
A design of pulse oximeter used in Mice made by Pensylvania state university is available on http://www.bioe.psu.edu/seniordesign/SD2006/DFord/bioe450%20web%20page.htm
includes circuit and the picture of the sensor used.
Although not what you are looking for, on this page there is another circuit embedded to a phone that monitors pulse rate using a LM324 chip. http://www.nandadoes.com/stetho-phone/stetho-phone-v0-1-2009/hardware
I hope this information would be good to you,