I can track the amount of light reflected in green, red and IR and compare with ECG identified R waves and chest diameter through the strain gauge. The top plot below has vertical lines at times when an R wave is detected, as well as the three reflected light amplitudes. The reflected light amplitudes here are all high pass filtered with a 1Hz critical frequency. The heart beats are clear, but it’s also obvious that identifying heart beats from this signal will be worse than using the ECG signal. The bottom plot has the same data with a filter frequency of 20 seconds, and in black is the chest strap tension filtered with the same filter. It appears from this that breath and heart rate can both be extracted with decent accuracy (at least while the test subject sitting at rest) from PPG.
I was surprised that the signals are clearest with the green LED, however as usual, I shouldn’t be because PPG sensors meant to detect heart rate without SPO2 use only green leds because the scattering distance is shorter with green wavelengths than with red or IR ones.
