Design 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 reflect the changes in arterial blood oxygen, so it is of great significance in clinical monitoring and family monitoring. When monitoring blood oxygen saturation with a conventional multi-parameter monitor, a blood oxygen finger clip is usually clamped on the end of 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, the patient is often inconvenient to turn over, and the cable is easy to fall off, causing error in the measurement result, and seriously endangering the timely rescue of the patient.

0 Preface

Blood oxygen saturation can reflect the patient’s respiratory function, and to a certain extent reflect the changes in arterial blood oxygen, so it is of great significance in clinical monitoring and family monitoring. When monitoring blood oxygen saturation with a conventional multi-parameter monitor, a blood oxygen finger clip is usually clamped on the end of 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, the patient is often inconvenient to turn over, and the cable is easy to fall off, causing error in the measurement result, and seriously endangering the timely rescue of the patient. Although the single-module blood oxygen saturation measurement equipment is easy to carry, due to its high power consumption, the use of battery power limits the duration of monitoring: generally this type of equipment can only store the monitoring information inside the device, but cannot store the monitoring information Send out in time, delay the patient’s rescue time. For this reason, this paper proposes a design method of blood oxygen saturation finger clip based on radio frequency chip nRF905 and ultra-low power single-chip MSP430F149, which aims to realize monitoring functions such as no cable, ultra-long time monitoring and timely sending of monitoring information.

1 Principle of non-invasive blood oxygen saturation measurement

Blood oxygen saturation (SpO2) is the percentage of the volume of oxygen-bound oxyhemoglobin (HbO2) in the blood that accounts for the total volume of hemoglobin (Hb) that can be combined, that is, the concentration of blood oxygen in the blood, which is an important physiology of the respiratory cycle parameter. The functional oxygen saturation (SaO2) is the ratio of HbO2 concentration to HbO2+Hb concentration. Therefore, SaO2 is often used in monitoring to estimate the level of SpO2. The theoretical calculation formula of SaO2 is as follows:

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

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

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

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 red light AC component, Imax is the maximum value of the red light DC component . This system uses red light at 660 nm and infrared light at 940 nm.

2 System overall design

Figure 1 shows a block diagram of the overall structure of the system. This wireless blood oxygen finger clip uses MSP430F149 microcontroller as the main control chip, and uses the I/O interface of the microcontroller to drive the light-emitting diodes. The system adopts 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. The current signal can be converted into a pulse wave signal after signal conditioning such as current-voltage conversion, amplification, filtering, etc., and finally sampled by the built-in 12-bit ADC of MSP430F149 into the single-chip microcomputer for processing, and the blood oxygen saturation value is obtained through calculation. After the value is packaged, it is sent to the nRF905 module by the microcontroller, and then sent out through the antenna.

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

3 Hardware circuit of non-invasive blood oxygen finger clip

3.1 Signal acquisition and conditioning circuit

The signal collection of this system uses ND78108494 finger end blood oxygen finger clip produced by Mindray Company. The finger clip has one red light and one infrared light emitting diode inside, which are connected by reverse docking; in addition, there is a photodiode, which can be used for Convert light intensity to current intensity.

The signal conditioning circuit includes a current-to-voltage conversion circuit, an amplifying circuit, a filter circuit, and a voltage range adjustment circuit. The output is a smoother pulse wave signal. Among them, the current-to-voltage conversion and amplification circuit is shown in Figure 2, and Figure 3 is the filtering and voltage adjustment circuit.

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

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

3.2 Radio frequency circuit

The nRF905 radio frequency chip is a product developed by Nordic using VLSIShoctBurst technology, which can provide high-speed data transmission without the need for expensive high-speed MCUs for data processing/clock coverage. By putting the high-speed signal processing related to the RF protocol into the chip, nRF905 can provide an SPI interface to the microcontroller, the speed of which is determined by the interface speed set by the microcontroller itself. Programming and configuration through the SPI interface can achieve very low current consumption. When the transmitting power is -10 dBm, the transmitting current is 11 mA and the receiving current is 12. 5 mA, and after entering the PowerDOWN mode, it will save more power.

3.3 Control circuit

The control chip is MSP430F149 in the MSP430 series of ultra-low power single-chip microcomputers of TI Company. This type of single-chip has the advantages of ultra-low power consumption and can support C language development. At the same time, it has a very strong processing capability, its rate can reach up to 2MIPS, and the internal 12-bit ADC. It is a single-chip microcomputer with rich functions and powerful computing power. Figure 4 shows the radio frequency control circuit composed of MSP430F149.

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

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 program to test adults. The subject is a healthy adult male. The experiment collects the pulse wave from the finger tip. In fact, the pulse wave processed by the signal conditioning circuit has less interference and less detail loss, so it can be used for the measurement of blood oxygen saturation.

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

The collected pulse wave is sampled by AD and sent to the single-chip microcomputer to go through a series of calculations. Finally, the blood oxygen saturation value returned by the system is 99%. This result is the same as the result measured by the PM-8000 portable monitor produced by Mindray. They are completely consistent, indicating that the system basically meets the expected requirements.

5 concluding remarks

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 the 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 likely to be realized. Because the system has the advantages of low power consumption and other advantages, it can realize super long-term monitoring under the condition of battery power supply, so it is easier to find occasional diseases.

The Links:   LM64183PR G150XTN062

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