Pulse sensor (verified) (photoplethysmography of pulse waves)

    This sensor converts light signals into electrical signals and amplifies them to a measurable level. Through actual detection, this signal can be directly detected by an oscilloscope or acquired by the ADC of an STM32F103 and displayed on a host computer. The
acquisition site is the capillaries in the finger.
The PPG signal is generated based on the Lambert-Beer law. When a beam of monochromatic light passes through a material of uniform density, three phenomena generally occur: reflection, absorption, and transmission. That is, some of the light energy is absorbed, and the rest is reflected and transmitted. The Lambert-Beer law is as follows:
 
Where A represents absorbance; T represents transmittance, which can be obtained by calculating the ratio of transmitted light intensity to incident light intensity; K represents the molar absorptivity; b represents the thickness of the absorbent layer; and c represents the concentration of the substance.
When light passes through the skin, part of the light is absorbed, part is transmitted, and some is reflected. The absorbed and transmitted light changes regularly with the pulsation of blood vessels, thus causing changes in the reflected light. Therefore, absorbance is directly proportional to the thickness of the absorbing layer and the concentration of the absorbing substance, and inversely proportional to the transmittance.
Consequently, we can conclude that when blood vessels dilate, the amplitude of the corresponding waveform is higher; when they constrict, the amplitude of the corresponding waveform is lower.
 
Welding Note: Pay attention to the orientation of the ambient light sensor!