The design of a portable blood glucose meter is introduced. From the perspective of low power consumption and accuracy, this design takes MSP430 series single chip microcomputer as the core and glucose oxidase electrode as the test sensor to quickly test the blood glucose concentration. In addition, the designed blood glucose meter also has a storage function to help users view the historical value and change trend of blood glucose concentration.

  Blood glucose test circuit:Free electrons are generated when blood is dropped at both ends of the enzyme electrode. Due to the excitation voltage at both ends of the electrode, a directional current will flow through the electrode. The excitation voltage is generated by the 1.5V voltage stabilizing provided by the ADC module through the resistance partial voltage, which is about 300mV, and it can generate μ Directional current of class A. Since the A / D conversion module measures the voltage, it is necessary to convert the directional current into voltage and amplify it. The system uses the circuit shown in Figure 2 to realize the conversion and amplification from current to voltage. The inverting end of the operational amplifier lm358 is connected to the enzyme electrode on the blood glucose test paper. When blood drips in, there is an equivalent resistance RX between the electrode and the ground, and the current flowing through the resistance is directly proportional to the blood glucose concentration in the blood.

The A / D module of MSP430 outputs 1.5V regulated voltage, and generates 300mV excitation voltage through R2 and R3 partial voltage, which is added to both ends of the electrode through the positive end of the operational amplifier. R4 plays the role of feedback amplification, which limits the output range of the operational amplifier to the conversion range of the A / D module. During PCB wiring, due to the long wiring between the output of the operational amplifier and the input I / O port of the ADC module of MSP430, in order to ensure the accuracy of the measured value, the test voltage needs to be filtered. C21 is used to filter, so as to reduce the impact of external interference caused by long wiring on blood glucose test. The operational amplifier directly connected to the capacitive load is easy to cause output oscillation. The function of R14 is to isolate the operational amplifier and the capacitor. Since there will be current flowing on the resistance R14, there will be a voltage drop at both ends of the resistance, and the voltage signal will change due to this influence. In order not to affect the accuracy of blood glucose test, the value of R14 cannot be too large. According to the empirical value, take 50 Ω.

 Temperature detection circuit:Because blood glucose measurement uses bioelectrochemical reaction, the important factor affecting the reaction is temperature. The activity of glucose oxidase was different at different temperatures. Even if the blood with the same blood glucose concentration adopts the same excitation voltage, the current generated by glucose oxidase oxidation is different at different temperatures. Therefore, it is necessary to compensate according to the temperature to obtain the correct blood glucose concentration. When the temperature is too high or too low, glucose oxidase will completely lose its activity. At this time, the blood glucose meter needs to give an alarm to remind the user that the instrument cannot operate at this temperature to avoid obtaining wrong detection values. The temperature test circuit is shown in Figure 3.

In the figure, R9 is thermistor et833, which has negative temperature characteristics. R10 is a high-precision resistor with a resistance value of 83k Ω. R9 is terminated with a 1.5V regulated voltage output by the A / D conversion module of MSP430. Since the 1.5V regulated voltage is also the reference voltage of the A / D conversion module, this connection method can eliminate the conversion error caused by the jitter of the A / D reference voltage. When the blood glucose meter works normally, calculate the size of thermistor R9 by measuring the voltage of p6.1 port, and then calculate the temperature value according to the special curve of et833 for temperature compensation.

  Data storage circuit:In order to facilitate users to view the change of blood glucose at any time, the blood glucose meter has the function of storing blood glucose value. Users can not only query the historical value of each measurement, but also query the change trend of blood glucose value in the last 28 days, and formulate the correct medication method according to the change trend of blood glucose, so as to achieve the purpose of controlling blood glucose concentration.

The system can store up to 1000 historical data. Each historical data needs 8b to save. The data includes two information: blood glucose value, concentration and test date, so 8000b storage space is required. 24lc64 is an E2 Prom chip produced by microchip company, which can store 8KB, so it is enough to select a 24lc64 chip.

The specific wiring mode between E2 PROM and MCU is shown in the figure. P4.0 ~ p4.3 are digital I / O ports of MSP430. P4.1 is a write protection pin, which is used to avoid the wrong write operation of EPROM due to external interference or program error. P4.2 and p4.3 are the connection ports for communication between 24lc64 and MSP430. P4.0 is used to supply power to 24lc64. The purpose of using I / O port to supply power to the chip is to reduce the overall power consumption during system operation. In addition, it also saves electronic switches, reduces costs and is conducive to wiring.

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