Detailed explanation of circuit design of portable pulse test and measurement system

Pulse measurement is a measurement that detects the presence or absence of a pulse. When there is a pulse, the light is blocked, and when there is no pulse, the light is transmitted. The sensors used are infrared receiving diodes and infrared emitting diodes. There are generally two methods of pulse measurement used for sports measurement: finger pulse and ear pulse. Both measurement methods have their own advantages and disadvantages. Finger pulse measurement is more convenient and simple, but because there are many sweat glands on the fingers and the finger clips are used all year round, pollution may reduce the measurement sensitivity. Ear pulse measurement is relatively clean, and the environment where the sensor is used is polluted. Few and easy to maintain. However, because the ear pulse is weak, especially when the seasons change, the measured signal is significantly affected by the ambient temperature, resulting in inaccurate measurement results.

Pulse signal pickup

The pulse signal pickup circuit is shown in Figure 1. IClA is connected as a unity gain buffer to generate a 2.5V reference voltage.

Infrared receiving diodes can generate electrical energy when illuminated by infrared light. A single diode can generate a voltage of 0.4 V and a current of 0.5 mA. The operating wavelength of BPW83 type infrared receiving diode and IR333 type infrared emitting diode is 940 nm. In the finger clip, the infrared receiving diode and infrared emitting diode are placed opposite to obtain the best directional characteristics. The greater the current in the infrared emitting diode, the smaller the emission angle, and the greater the emission intensity produced. In Figure 1, the RO selection of 100 Ω is based on the sensitivity of the infrared receiving diode to sensing infrared light. If R0 is too large, the current through the infrared emitting diode is too small, and the BPW83 infrared receiving diode cannot distinguish between signals with pulse and without pulse. On the contrary, if R0 is too small, the passing current will be too large, and the infrared receiving diode cannot accurately distinguish the signals with and without pulse. When the infrared light emitted by the infrared emitting diode directly illuminates the infrared receiving diode, the potential of the inverting input terminal of IC1B is greater than the potential of the non-inverting input terminal, and Vi is "O". When the finger is in the measuring position, two situations will occur: one is the pulseless period. Although the finger blocks the infrared light emitted by the infrared emitting diode, due to the dark current in the infrared receiving diode, there is still a dark current of 1 μA, which will cause the Vi potential to be slightly lower than 2.5 V. The second is the pulse stage. When there is a beating pulse, the blood vessels make the light transmittance of the finger worse, the dark current in the infrared receiving diode decreases, and the Vi potential increases.

From this point of view, the so-called pulse signal pickup is actually obtained through the infrared receiving diode, the weak changes in the dark current when there is a pulse and when there is no pulse, and then amplified by IClB. The signal picked up is a voltage signal of about 2μV.

　signal amplification

The low-pass amplifier is designed based on the calculation that the maximum number of human pulse beats after exercise is 240 times/min. It consists of IC2A and C04, as shown in Figure 2. The turning frequency is determined by R07, C04, R08 and C05, and the amplification factor is determined by the ratio of R08 and R06.

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