We had another very interesting experiment as part of our BM605 course. This time we designed a simple pulse-oximeter (more accurately a pulse-volume meter) on a breadboard. It was done by students in just a single two-hour lab session. Another interesting thing about this experiment was that it worked with just ambient lighting.
We used an LDR (Light dependent resistor) as the primary sensing element for this. The LDR went into a voltage divider circuit. The resistor connected with the LDR for voltage division was carefully chosen so as to have a value equal to that of LDR resistance during finger placement in ambient lighting. The output of this voltage divider was fed to a band pass filter with cutoffs set at 0.1 Hz and 100 Hz.In the above figure, R1 is the thevenin equivalent resistance of the voltage divider.
This is the output that was achieved by two of our students Samrat and Aman. They placed another inverting amplifier stage after the filter. The signal is clear enough to locate the dichrotic notch.
Great going guys. Keep it up!