This is a heart rate monitor. The theory of plethysmography is all over the net. In brief the blood vessels pulsate with heart beating and we can see that with infrared light.
The circuit consist DC biased transimpedance converter and amplifier. The biasing remove the ambient light noises and keeps the photodiode in photovoltaic mode all the time. Notice that the polarity of the diode is not important. The integrator will compensate under or over the reference. Of course, the signal will be inverted or not.
The board is through hole with socked for the operational amplifier. I checked with TL062 and LM324. It works in either cases. The DC biasing of the TIA can have another function - the absorbtion of the RED or INFRARED light. This can be used for oxidation measurement because the oxidized and not oxidized blood absorb the red and infrared with some relation. I left a place in the board for red LED. The circuit can be expanded with microcontroller. I do not intend to do that, but if someone wants, please fell free to contact me. I could be useful if you want the board plane or future signal processing.
I used sound card scope from:
Thanks, Christian, you have build nice scope. If you intend to use sound card oscilloscope take in mind that the microphone input is band limited from 300Hz to 8kHz. It is not enough for this kind of signal, so you have to use line input.
Wow, 8 beats for 6 seconds after 3 cups of coffee at my age is not too bad :)