Infant sleep monitoring system based on MSP430 microcontroller

　　In today's society where the pace of life and work is increasingly tense, women often have to balance work and family, especially new mothers who have to take care of babies and do housework. New mothers are often in a rush. Babies spend most of the day sleeping. How to monitor their sleep, comfort them in time when they wake up, and how to solve the problem of wetness have always troubled busy new mothers. The vast majority of families choose to use convenient diapers, but in addition to the high cost of diapers, their absorbent layer is prone to breeding bacteria. If they are not changed in time, they will cause rashes, which is also a hidden danger to the baby's health. Pure cotton diapers are healthy and comfortable, but they need to be changed in time. Most of the existing wetness detection devices on the market are single-point measurements, which may be missed.

　　The remote baby urination monitoring system designed in this paper can effectively solve this problem for new mothers. In the abnormal alarm part, two situations are mainly detected: 1) the baby urinates; 2) the baby cries because of discomfort or needing care. The system uses multi-point urination detection and sound detection alarm, which can not only detect the baby's urination or crying, but also prevent missed alarms and false alarms caused by the baby's normal turning over, moving, talking, and other environmental sounds. This system embeds the multi-point urination monitoring module and the audio detection module under the waterproof layer of the urinal pad, which has low power consumption, is easy to separate and clean, and has practical value.

　　1 Overall system design

　　Figure 1 shows the system structure block diagram. This system consists of a multi-point urine monitoring module, a sound detection module, a wireless transmission module, and an audio-visual alarm module, all of which are controlled by a low-power single-chip microcomputer MSP4 30. The multi-point urine monitoring module monitors the humidity in real time. When the system detects that the humidity exceeds the threshold or detects that the baby is crying, the wireless transmission module sends a signal to make the receiving terminal sound and light alarm, so that mothers can work or do housework in other rooms without worrying about affecting the baby's sleep or not being able to comfort the baby in time when he wakes up.

　　2 System Hardware Design

　　Considering the low power consumption of the system, the microprocessor adopts the 16-bit single-chip microcomputer launched by Texas Instruments, which is characterized by ultra-low power consumption, a mixed signal processor with a reduced instruction set (RISC), and can operate at a voltage of 1.8 to 3.6 V. It has working mode and 5 low-power modes. In low-power mode, the CPU can be awakened by interrupts, and the response time is less than 6μs, which is particularly suitable for occasions where batteries are used for a long time. At the same time, it is small in size, has a complete interface, and has strong computing power, making it very suitable as the main control chip of the sleep monitoring system.

　　2.1 Multi-point temperature and urine detection module

　　Temperature measurement can make the system shut down automatically. When the temperature of multiple points is equal to the ambient temperature, it means that there is no baby sleeping on it. At this time, the system automatically powers off, greatly reducing battery consumption and extending the use time. The next time you use it, you only need to restart it, further realizing low power consumption design.

　　It is easy to miss a measurement using a single-point humidity sensor because:

　　1) The wetness sites of male and female babies are different, and a single sensor is prone to errors.

　　2) Babies move when they sleep. The humidity sensitive surface of the multi-point urine detection module is placed on the urine pad penetration layer, and the rest of the circuit is placed under the waterproof layer of the urine pad. When the humidity signal detected by one of the sensors exceeds the threshold, the main control microcontroller will trigger the RF chip and send a signal to the receiving end to sound and light alarm. Compared with the single sensor element of other similar products, multi-point measurement and control is more effective and reliable.

　　The multi-point temperature and urine detection module uses the DHT11 digital temperature and humidity sensor with calibrated digital signal output. The sensor includes a resistive humidity sensing element and an NTC temperature measuring element, and is connected to a high-performance 8-bit microcontroller. The product has the advantages of ultra-fast response, strong anti-interference ability, and extremely high cost performance. DHT11 uses 3.3-5.5 V DC power supply, humidity measurement range 20-90%RH, humidity measurement accuracy ±5%RH, humidity resolution 1%RH, fully interchangeable, long-term stability <±1%RH/year. The single-wire serial interface outputs digital signals, making system integration simple and fast. The ultra-small size and extremely low power consumption make it suitable for multi-point urine detection.

　　The connection method of the multi-point urine temperature sensor is shown in Figure 2. The positive and ground of the power supply are connected to the positive and ground of the microcontroller power supply, and the 4-way data interface is connected to the 5.1 k pull-up resistor and connected to the P1 port of the microcontroller.

　　2.2 Sound Detection Module

　　The sound detection alarm should only be triggered by the sound of a baby crying, and ignore other environmental sounds such as adult voices (30-3,000 Hz) and footsteps to avoid false triggering. Therefore, the system only detects sounds above 1,500 Hz. When the sound amplitude within this frequency band exceeds the threshold, a signal is sent through the wireless transmission module to cause the receiving terminal to sound and light an alarm. The sound threshold is adjusted by the user according to the actual environment.

　　The sound detection module circuit shown in Figure 3 uses AD7794 to convert the analog signal collected by the microphone into a digital signal, which is filtered and processed by the microcontroller. The RF chip is triggered only when the processing result is judged to be a baby crying sound. The AD7794 chip is suitable for high-precision measurement applications and has the advantages of low power consumption, low noise, and complete input terminals.

　　2.3 Wireless transmission and reception module

　　RF transmission and reception need to ensure the reliability of transmission, low bit error rate, strong anti-interference ability, and a transmission distance of more than 15 meters. In addition, combined with the requirement of low power consumption, nRF24 L01 wireless chip is selected as the RF integrated circuit. nRF24L01 is a single-chip wireless transceiver chip produced by NORDIC, which mainly works in the ISM band of 2.4-2.5 GHz, and the conversion time between frequency bands is less than 200 μs. The output power channel selection and protocol settings can be set through the SPI interface. When working in the transmission mode and the transmission power is 0dBm, the current consumption is 11.3 mA, and the current consumption in the receiving mode is 12.3 mA. The current consumption is lower in the power-down mode and standby mode. The wireless transmission module consists of a signal transmitting unit and a signal receiving unit. The two interact with each other through wireless transmission to complete the wireless transmission of data. Its indoor transmission distance can reach 30 m, and the wireless transmission speed can reach 1Mbit/s. In use, only one nRF24 L01 and a few peripheral components are needed to complete the RF transceiver function, which greatly reduces the size of the system.

　　When connecting nRF24L01 and MSP430 microcontroller, the serial communication module USART1 of MSP430 microcontroller is used as SPI to provide synchronous communication, mainly for command and data communication between the two. The pins P1.4, P3.0, P1.3, P3.3, P3.2, and P3.1 of MSP430 are configured as general I/O pins, which are respectively connected to CE, CSN, SCK, MOSI, MISO, and IRQ of nRF24L01 to control the working mode of nRF24L01. The connection circuit is shown in Figure 4.

　　This system is a portable system, powered by batteries, and requires low power design. The RF chip transmits the signal only when the microcontroller detects that the measured signal exceeds the threshold, otherwise the nRF 24L01 is in a power-off state to reduce power consumption. [page]

　　2.4 Sound and light alarm module

　　The purpose of this design is to transmit the baby's wetness and crying information to the device (such as a wristband) carried by the guardian through wireless transmission. This component receives the signal and makes the alarm on the device work. This system uses a two-level sound and light alarm at the receiving end. First, it will emit a light reminder. After 30 seconds, if the guardian does not turn off the reminder, the buzzer will sound a reminder. The volume of the buzzer can be adjusted.

　　3 System Software Design

　　3.1 System Main Process

　　The system software design includes the software design of the urine wetness and crying measurement module and the software design of the receiving alarm module. The main flow charts are shown in Figures 5 and 6. After the urine wetness and crying measurement module is powered on, the system initialization is completed first, and the multi-point temperature measurement module completes the temperature measurement. When the temperature of each point remains equal to the ambient temperature for more than 5 minutes, the system is powered off; otherwise, the humidity and sound are measured. When the measured humidity sound signal exceeds the threshold, the RF chip is started to send a wireless signal, and then the system enters a low power mode, waiting for the module to be turned off or the button to be turned on again. After receiving the alarm signal, the receiving alarm module first starts the photoelectric alarm. If it is not turned off after 30 seconds, the sound alarm is started until the button is stopped.

　　3.2 Sound Detection Process

　　The frequency of a baby's cry is generally in the mid-frequency band and greater than 1,500 Hz, so it is required to be able to filter out sounds such as adult voices. However, the sound after filtering is not necessarily a baby's cry, so the amplitude must also be detected. Only when both the frequency and amplitude meet the conditions will an alarm be triggered to achieve higher accuracy.

　　The filtering part is implemented by a custom bandpass filter function. The signal within the frequency band (LowFreq, HighFreq) is then detected by the amplitude detection function to see if the signal amplitude is greater than the threshold value Voice_T. If it is less than the threshold value, it is ignored. If it is greater than the threshold value, the RF chip is started and a signal is sent to make the sound and light alarm module alarm. The threshold value can be adjusted by the user to meet different needs.

　　4 System Testing

　　For several modules of this project, the false alarm rate and missed alarm rate of the crying detection and urine detection modules are measured. Generally, the distance between the baby room and the receiving point is less than 25 m. The urine and crying detection is carried out at a distance of 25 m to observe the success rate and accuracy of the transmission and reception. The specific experimental results are shown in Table 1.

　　From the experimental results, it can be seen that the system's anti-interference ability, stability and reliability can meet general usage requirements and has certain promotion value.

　　5 Conclusion

　　In view of the difficulties in taking care of babies in reality, a low-power infant sleep monitoring system with crying detection and urine detection functions based on MSP430 was designed, and the shortcomings of products on the market were improved to facilitate parents to take care of babies in time. The test proves that the infant sleep monitoring system has high reliability, strong practicality, low power consumption, and can be used for a long time. It is easy to maintain and expand more functions, and has product significance.