Design of wearable heart rate signal acquisition preprocessing circuit

　　Heart rate signal acquisition preprocessing circuit: The pulse signal acquisition preprocessing circuit mainly converts the pulse wave into an electrical signal and performs preliminary high-frequency filtering preprocessing. The key part is the photoelectric pulse sensor. Photoelectric pulse sensors can be divided into two types according to the light receiving method: transmission type and reflection type. The reflection type can not only accurately measure the volume change of blood vessels, but also in practical applications, the reflection type only needs to touch the sensor to any part of the body. When the blood flow of the irradiated part changes with the heartbeat, the infrared receiving probe will receive the arterial pulsation light pulse signal that contracts and relaxes periodically with the heart, thereby collecting the heartbeat signal.

　　Analysis: This design uses a reflective infrared sensor. The photoelectric pulse sensor uses infrared tubes KP-2012F3C and KP-2012P3C, which are arranged in a reflective manner. KP-2012F3C has good epidermal illumination, and the current is generally set at 20mA. The brightness is controlled by software through PWM current, which enables the infrared LED to work in the saturation area and emit stable light intensity.

　　Pulse signal acquisition preprocessing circuit

　　The KP-2012P3C transistor uses an AC coupling structure to enhance the amplification of weak signals. The signal detected by the transistor is sampled in two ways. One is a DC signal line. It is the transistor output and input to the A/D conversion channel 0 of the microcontroller through an emitter follower, which can be used to detect whether the transistor is in an effective working state; the other is an AC signal line. It is first input to the two-stage filter shaping circuit through an emitter follower and then input to the A/D conversion channel 1 of the microcontroller. The filter circuit is a two-stage bandpass filter circuit. Since the spectrum of the pulse wave contains rich pathological information, especially the spectrum in the range of 5~40Hz carries a lot of information related to coronary heart disease, considering the future expansion of functions, the upper and lower limit frequencies of the preprocessing circuit are designed to be 48Hz and 0.86Hz.