In order to measure the heart rate signal, there are many techniques that can be applied, such as: blood measurement, heart sound measurement, ECG measurement and so on. This article discusses the use of the high opacity of blood and the great difference between the light transmittance of tissues and blood. The pulse signal is obtained by a photoelectric pulse sensor. After analog-to-digital conversion (A / D), the sampled data is digitally analyzed and processed to achieve Measurement of human heart rate . 1 Composition and working principle of heart rate detector The main components of the heart rate detector are shown in Figure 1. The pulse signal is collected by the photoelectric sensor, and the pulse signal data is obtained by pre-amplification, filtering, and sampling by the A / D conversion module of the single chip uPSD3234A and stored in the memory; the single chip performs digital signal processing on the obtained data and calculates the heart rate value , The result is sent to the display module and memory. Figure 1 Block diagram of the digital heart rate detector 1.1 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. The photoelectric pulse sensor can be divided into two types of transmission type and reflection type according to the light receiving mode. Not only can the reflective type accurately measure the change in the volume of the blood vessel, but in practical applications, the reflective type only needs to contact the sensor to any part of the body. The arterial pulse light pulse signal that periodically contracts and relaxes, thereby collecting the heart beat signal. This design uses a reflective infrared sensor. As shown in Figure 2, the photoelectric pulse sensor uses infrared pair tubes KP-2012F3C and KP-2012P3C, arranged in a reflective manner. KP-2012F3C has good skin illumination, the current is generally set at 20mA, the brightness is controlled by the software through PWM current, so that the infrared LED can work in the saturated area and emit stable light intensity. KP-2012P3C transistor adopts AC coupling structure to enhance the amplification of weak signals. The signal detected by the transistor is sampled in two ways. All the way is the DC signal line. It is the A / D conversion channel port 0 of the output of the transistor through the input and output of the microcontroller, which can be used to detect whether the transistor is in an effective working state; the other is the 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 single-chip microcomputer. The filter circuit is a two-stage band-pass filter circuit. Because the frequency spectrum of the pulse wave contains rich pathological information, special The spectrum in the range of 5 ~ 40Hz carries a lot of information related to coronary heart disease, so considering the future expansion of the function, the upper and lower limit frequencies of the preprocessing circuit are designed to be 48Hz and 0.86Hz. Figure 2 Pulse signal acquisition preprocessing circuit 1.2 uPSD3234 microcontroller This article uses ST (STMicroelectronics) company's new single-chip uPSD3234 as the core component of the system, which is based on the enhanced MCS-51 core 8032 single-chip microcomputer, has a wealth of peripheral equipment, integrated PSD (ProgrammableSystemDevice, programmable peripheral device) module , And contains large-capacity FLASH and RAM memory, integrated I2C and USB interface circuits, digital display (DDC) channels, 5 pulse width modulation (PWM) controllers, 4 8-bit AD converters, programmable logic devices (PLD ) Is a typical high-speed microcontroller with SOC characteristics. Therefore, the design requirements can be fully satisfied without adding complicated peripheral circuits. The USB module in uPSD3234 supports low-speed USB1.1 communication protocol. The heart rate detector sampling data and the data obtained during signal processing can be transmitted to the PC for storage and further analysis and processing. 2 Digital processing and algorithm of heart rate signal During the measurement process, the pulse signal detected by the pre-processing circuit is susceptible to external interference, and the interference noise needs to be processed. Generally, there are two ways to deal with noise: one is to increase the filter circuit; the other is to reduce noise through algorithms from the perspective of digital signal processing. If the filter circuit is added in the periphery, it will increase the cost and affect the portability of the instrument. In addition, due to the uncertainty of interference, the filtering effect will not be very good. Although software filtering will occupy a certain amount of system resources, it has the advantages of low cost, high reliability, and good stability. It has the advantages of flexibility, convenience, and strong functions under the conditions of processing speed. This article mainly adopts the method of digital filtering to process, and the most important algorithm is the matched filtering algorithm. Led Digital Tube Light use
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