Design and implementation of wireless blood oxygen finger clip based on MSP430F149 and nRF905

　　Blood oxygen saturation can reflect the patient's respiratory function and, to a certain extent, the changes in arterial blood oxygen. Therefore, it is of great significance in both clinical monitoring and home monitoring. When using a conventional multi-parameter monitor to monitor blood oxygen saturation, a blood oxygen finger clip is usually clamped on the finger or toe to collect the photoelectric pulse wave signal, and the signal is transmitted to the monitoring device through a cable for processing and calculation. Due to the influence of the cable, it is often inconvenient for the patient to turn over, and the cable is easy to fall off, resulting in incorrect measurement results, which seriously endangers the timely rescue of the patient. Although the single-module blood oxygen saturation measurement device is easy to carry, due to its high power consumption, the battery power supply limits the duration of monitoring: generally, such devices can only store the monitoring information inside the device, but cannot send the monitoring information in time, delaying the patient's rescue time. To this end, this paper proposes a design method for a blood oxygen saturation finger clip based on the RF chip nRF905 and the ultra-low power single-chip microcomputer MSP430F149, aiming to achieve monitoring functions such as no cables, ultra-long-term monitoring and timely sending of monitoring information.

　　1 Principle of non-invasive blood oxygen saturation measurement

　　Blood oxygen saturation (SpO2) is the percentage of oxygenated hemoglobin (HbO2) in the blood that is bound to oxygen to the total hemoglobin (Hb) capacity, that is, the concentration of blood oxygen in the blood. It is an important physiological parameter of respiratory circulation. Functional oxygen saturation (SaO2) is the ratio of HbO2 concentration to HbO2+Hb concentration. Therefore, SaO2 is often used to estimate the SpO2 level in monitoring. The theoretical calculation formula of SaO2 is as follows:

　　The measurement method is generally based on the Lambert-Beer theorem, using the different absorbances of different components in the blood, using red light and infrared light to illuminate the tissues respectively, and calculating the blood oxygen saturation value by measuring the intensity of the transmitted light. The formula is as follows:

　　In the formula, △I\'max is the maximum value of the AC component of infrared light, I\'max is the maximum value of the DC component of infrared light, △Imax is the maximum value of the AC component of red light, and Imax is the maximum value of the DC component of red light. This system uses 660 nm red light and 940 nm infrared light.

　　2 Overall system design

　　Figure 1 shows the overall structure of the system. The wireless blood oxygen finger clip uses the MSP430F149 microcontroller as the main control chip and uses the I/O interface of the single-chip microcomputer to drive the light-emitting diode. The system uses the finger-end blood oxygen finger clip produced by Mindray. The output of the finger clip is a current signal, which can be used to reflect the intensity of the transmitted light. After the current-to-voltage conversion, amplification, filtering and other signal conditioning, the current signal can be converted into a pulse wave signal, and finally the built-in 12-bit ADC of the MSP430F149 samples it and enters the single-chip microcomputer for processing, and the blood oxygen saturation value is obtained by calculation. The value is packaged and sent to the nRF905 module by the single-chip microcomputer, and then sent out through the antenna.

　　3 Hardware circuit of non-invasive blood oxygen finger clip

　　3.1 Signal acquisition and conditioning circuit

　　The signal acquisition of this system uses the ND78108494 fingertip blood oxygen finger clip produced by Mindray. The finger clip has one red light emitting diode and one infrared light emitting diode inside, which are connected in a reverse docking manner; in addition, there is a photosensitive diode that can be used to convert light intensity into current intensity.

　　The signal conditioning circuit includes four parts: current-to-voltage conversion circuit, amplifier circuit, filter circuit and voltage range adjustment circuit. The output is a relatively smooth pulse wave signal. The current-to-voltage conversion and amplifier circuit is shown in Figure 2, and its filter and voltage adjustment circuit is shown in Figure 3. [page]

　　3.2 RF Circuit

　　The nRF905 RF chip is a product developed by Nordic using VLSI SoctBurst technology. It can provide high-speed data transmission without the need for expensive high-speed MCUs for data processing/clock coverage. By placing high-speed signal processing related to the RF protocol into the chip, the nRF905 can provide a SPI interface to the microcontroller, and its rate is determined by the interface speed set by the microcontroller itself. Programming and configuring through the SPI interface can achieve very low current consumption. When the transmit power is -10 dBm, the transmit current is 11 mA, the receive current is 12.5 mA, and after entering the POWERDOWN mode, it will save more power.

　　3.3 Control Circuit

　　The control chip is the MSP430F149 from the MSP430 series of ultra-low power microcontrollers from Ti. This type of microcontroller has the advantages of ultra-low power consumption and can support C language development. It also has very strong processing capabilities, with a maximum rate of 2MIPS and an internal 12-bit ADC. It is a microcontroller with rich functions and powerful computing capabilities. Figure 4 shows the RF control circuit composed of MSP430F149.

　　4 System software process and test results

　　Figure 5 shows the software flow chart of this system. The author used the wireless blood oxygen finger clip of this solution to test an adult. The subject was a healthy adult male. The experiment collected pulse waves from the fingertips. In fact, the pulse waves processed by the signal conditioning circuit have less interference and less loss of details, and can be fully used for measuring blood oxygen saturation.

　　The collected pulse wave is sampled by AD and sent to the microcontroller for a series of calculations. Finally, the blood oxygen saturation value returned by the system is 99%. This result is completely consistent with the result measured by the PM-8000 portable monitor produced by Mindray, indicating that this system has basically met the expected requirements.

　　5 Conclusion

　　This article presents a hardware circuit and design method of a wireless blood oxygen finger clip based on MSP430F149 and nRF905. The finger clip can realize wireless monitoring function. On the one hand, it can overcome many problems caused by connecting cables. On the other hand, it also makes home monitoring and portable monitoring more possible. Due to the advantages of low power consumption, this system can realize ultra-long-term monitoring under battery-powered conditions, so it is easier to detect occasional diseases.

References:

[1]. MSP430F149 datasheet http://www.dzsc.com/datasheet/MSP430F149_.
[2]. MSP430 datasheet http://www.dzsc.com/datasheet/MSP430_490166.html.