In the field of consumer electronics, portable electronic products have become more and more popular among consumers due to their small size and light weight, and have become an indispensable part of people's lives. Based on this idea, we designed a portable heart rate meter, which can replace the traditional method of measuring with pulse stethoscope and so on. It is very convenient to use. The product consists of three main parts: signal acquisition, data processing, and LED display and alarm circuits.
Overall system design
Figure 1 system block diagram
As shown in Figure 1, the pulse signal detected from the sensor is converted into a voltage signal and sent to the voltage follower, which acts as a buffer to isolate the front and rear stages from mutual interference. The output signal is preamplified and sent to a high-pass filter to filter out the thermoelectric interference of the sensor, and then filter the high-frequency interference in the environment through a low-pass filter. The processed signal is sent to the subsequent stage to continue to amplify to obtain a signal with small interference and clearness. The signal is rectified by the comparator and the diode and directly sent to the single chip processing to drive the display circuit and the alarm circuit.
System hardware circuit schematic
Figure 2 is a schematic diagram of the circuit. The following describes each module one by one.
Figure 2 system hardware circuit schematic
1 voltage follower and preamplifier circuit
The input signal of the voltage follower, that is, the pulse sensor signal is input from the V+ terminal, and the feedback resistor is set to zero to form an in-phase follower, which acts as a buffer to isolate the influence of the front and rear stages. The function of the heart sound pulse amplifier is to amplify the mV level heart sound signal to the V level for display and recording.
According to the characteristics of the heart sound pulse signal, the amplifier is required to have the following characteristics:
1. A sufficiently high gain, about 800 times.
2, there is a suitable frequency bandwidth (0.78 ~ 3.33Hz)
3. Because the heart sound pulse signal is relatively weak, the interference and noise are relatively large, the circuit is required to have high input impedance to reduce the signal loss, and the high common mode rejection ratio (greater than 80dB) is used to suppress interference and noise.
Due to the interference of external signals in practical applications, and considering the stability of the amplifier, the primary amplifier cannot achieve such a large gain, so the voltage amplifier generally consists of two stages. Among them, the front stage uses a negative feedback differential amplifier circuit to improve the common mode signal rejection ratio. The key to this part is how to suppress various noises and avoid letting noise get into the back-stage circuit. Therefore, in the system, the micro-powered instrumentation amplifier LM358 based on dual op amp circuit is used as the preamplifier of the heart pulse signal. In order to prevent the occurrence of nonlinear distortion so as to impair the common mode rejection ratio of the circuit, the amplification factor of the portion should not be too high, and the selection is about 1000 times.
Voltage follower and preamplifier circuit
2 high and low pass filter circuit
In this design, the signal frequency is low, between 0.78 and 3.33 Hz, so the design of the filter becomes the key to this circuit. First, a 0.5 Hz high-pass filter is used to filter out the thermoelectric interference of the sensor, and then pass through a low-pass filter to filter out most of the interference of the heart sound signal. In the implementation circuit, it is difficult to filter such a low signal by a common filter, so a Butterworth filter with a relatively flat gain variation is used in this design. Among them, Qualcomm is a second-order Butterworth filter, and low-pass is a Butterworth filter with a cutoff frequency of 5 Hz. Figure 3 shows the schematic of low-pass filtering.
Figure 3: Low-pass filtering schematic
Figure 3: Low-pass filtering schematic
3 post-stage amplification and comparison rectifier circuit design
After the heart sound signal is amplified by the preamplifier, the amplitude has not reached the ideal application value, and there is still some interference, so the post amplifier needs to continue to amplify to meet the use requirements. The entire circuit uses a general inverting amplifier module circuit. The function of the comparison rectifier circuit is to convert the processed signal into a square wave that does not contain a negative pulse, and send it to the microcontroller for processing. The circuit consists of a zero-crossing comparator and a rectifier circuit. Since the signal sent to the microcontroller requires a positive voltage, after the rectifier circuit, the signal will all be converted into a square wave of the positive edge.
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