（Photoplethysmography （PPG））
When a beam of light of a certain wavelength shines on the fingertip skin surface, the contraction and dilation of blood vessels with each heartbeat affects either the transmission or the reflection of the light. As the light passes through the skin tissue and then reflects back to the photosensitive sensor, there is some attenuation of the light. While the absorption of light by muscles, bones, veins and other connected tissues is essentially constant (provided there is no significant movement at the measurement site), arteries are different, and because they are pulsating with blood, the absorption of light will naturally vary. When we convert the light into electrical signals, it is because the arterial absorption of light has changed while other tissues to the absorption of light is basically unchanged, the signal obtained can be divided into DC signal and AC signal. By extracting the AC signal from them, the characteristics of blood flow can be reflected.
Introduction to the working principle of heart rate sensor chip
The heart rate sensor chip includes two parts: transmit led and PD receive.
When the chip is working, the light from Led hits under the skin and reflects back to the receiving end. The reflected light intensity contains the pulse information. After processing, the light intensity is converted into a value by ADC, which will change periodically with the pulse beating.
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