Design of hospital wireless nursing call system based on nRF401 chip

1 Introduction

With the continuous deepening of medical system reform and the rapid development of medical industry, more and more people need to get various medical services from hospitals quickly and conveniently, which will inevitably make the competition among hospitals increasingly fierce.

This means that the comprehensive level of a hospital is no longer limited to the construction of software and hardware, but also to service. Clinical call assistance devices are an important means of transmitting clinical information, which is related to the safety of patients. Traditional wired call systems have always been widely valued by major hospitals. If wireless transmission is adopted, it will save money on wiring and line transformation, save costs for hospitals, and be timely, accurate, reliable, simple and feasible. It is more recognized by hospitals and patients than current similar products, has stronger competitiveness, and can be widely promoted.

Traditional ward call systems use wired transmission, which is difficult to be concealed and beautiful, and are inconvenient to install and maintain, and have weak electrical interference resistance. To overcome the above shortcomings, if a wireless ward call monitoring system is used, a call button is installed next to each bed in the hospital ward. When the patient needs help, press the call button, the indicator light of the corresponding room number on the call display board in the nurse's office lights up and a voice prompt is given. At the same time, an electronic display board is installed in the corridor so that the nurse on duty can know in time which room the patient needs help or rescue.

This article uses a dedicated RF module nRF401 and uses a single-chip microcomputer control. Its principle is simple, the chip used is highly integrated, the performance is stable, and the cost is relatively low. Each extension has a unique address code. The host stores the incoming number to ensure that the call information is not lost. The terminal digital tube cyclically displays the call address and sound alarm. The system is stable and reliable and has a good application prospect.

2 System Hardware Design

The system is divided into a calling extension and a receiving host. The extension is used to make calls, and the encoding is completed by a single-chip microcomputer. The core circuit of the extension is the connection circuit between the single-chip microcomputer and the radio frequency chip. The host is responsible for receiving the signal sent by the extension, decoding, displaying and alarming. The host is equipped with a keyboard for reviewing and deleting.

2.1 System principle block diagram
The system principle block diagram is shown in Figure 1 and Figure 2.

2.2 nRF401 Introduction

2.2.1 Main performance of nRF401
nRF401 is the latest single-chip RF transceiver launched by Nordic VLSI Company of Norway, designed for operation in the 433 MHz ISM (industrial, scientific research and medical) frequency band. nRF401 uses FSK frequency (Frequency Shift Keying) modulation with strong anti-interference ability, which improves the system performance in a noisy environment. It adopts DSS+PLL frequency synthesis technology, and the operating frequency is stable and reliable. Compared with ASK amplitude shift keying and OOK on-off keying, this method has a wider communication range, especially in situations where similar equipment is working nearby.

Its main features are as follows:
(1) The operating frequency is the internationally common digital transmission frequency band;
(2) FSK modulation, strong anti-interference ability. Especially suitable for industrial control occasions;
(3) PLL frequency synthesis technology is adopted, and the frequency stability is excellent;
(4) High sensitivity, reaching -105 dBm (nRF401);
(5) Low power consumption. 250 mA in receiving state and only 8 A in standby state;
(6) Maximum transmission power reaches +10 dBm;
(7) Low operating voltage (2.7 V) to meet the requirements of low-power devices;
(8) With multiple channels, the operating frequency can be easily switched;
(9) The maximum operating rate can reach 20 kb/s (RF401);
(10) Only one crystal and several resistors, capacitors and inductors are connected externally, and basically no debugging is required;
(11) Due to the design of low transmission power and high receiving sensitivity, no license is required for use, and the maximum distance of use in open areas can reach 1000 m (related to the specific use environment and the number of components).

2.2.2 nRF401 pin introduction
The pins of nRF401 are shown in Figure 3.

CS: Channel selection, CS=0 selects working channel 1, i.e. 433.92 MHz; CS=1 selects working channel 2 (i.e. 434.33 MHz). Connect to P2.5 pin of AT89C51;
Dout: Data output, connected to RXD of AT89C51 serial port;
Din: Data input, connected to TXD of AT89C51 serial port;
PWR-UP: Energy saving control, PWR-UP=1 normal working state,
PWR-UP=0 low power saving state. Connect to P2.6 pin of AT89C51;
TXEN: Transmit and receive control, when TXEN=1, nRF401 is in transmitting state. When TXEN=0, nRF401 is in receiving state, connected to P2.7 pin of AT89C51;
ANT1 and ANT2 are the input of the signal when receiving, and the output of the power amplifier when sending. The antenna connected to nRF401 is connected to nRF401 in differential mode, and the recommended load impedance at the antenna end is 400Ω.

2.2.3 Typical connection of nRF401
The typical application connection diagram of nRF401 is shown in Figure 4, which can be directly used for asynchronous transmission of the RS 232 serial port of a microcontroller or computer.

As can be seen from Figure 4, there are very few peripheral components, including a reference crystal oscillator and several passive devices, no debugging components, and the antenna is directly designed on the circuit board using a microstrip antenna, which brings great convenience to research and production. The L1 inductor in the figure needs to use a high-Q value and high-precision patch winding high-frequency inductor (Q>45), the crystal oscillator X1 needs to use a high-stability crystal oscillator, and the capacitor component should use a high-stability patch component such as an NPO high-stability capacitor to ensure its performance.

2.3 Extension circuit design
The extension is portable and battery powered. When selecting components, power consumption and volume must be considered, as well as the minimum voltage of the chip. Therefore, the MCU is AT89C2051, which has only 20 pins, a compact structure, a small size, low power consumption, and can work stably at 3 V. It has the core of AT89C51 and the same instruction system. The extension requires very few I/O ports, and AT89C2051 can fully meet the requirements.

The extension uses an 8-bit dial switch to manually locate the extension address. If the extension needs to be moved to another bed, you only need to change the state of the dial switch to change the extension number. If you need to add beds, you only need to increase the number of extensions. Each extension is exactly the same in software and hardware. You only need to set the address code on the dial switch. There is no need to make any changes on the host, which is very convenient. nRF401 has three working states: standby, receive (RX) and transmit (TX). From the pin function of nRF401, it can be seen that the switching between these three states can be determined by the state of PWR-UP and TXEN. DIN and DOUT are serial communication ports, which are respectively connected to the serial communication port of the microcontroller, and the CS pin selects the working frequency. The connection circuit of nRF401 and AT89C2051 is shown in Figure 5.

In this design, nRF401 is used to communicate with the microcontroller via serial port. You only need to connect its data input port (DIN) and data output port (DOUT) to the TXD and RXD of the microcontroller respectively.

2.4 Host circuit design

2.4.1 Signal transceiver processing part
The host uses AT89C51 as the control chip. When working, it also needs to perform state switching, frequency selection and serial communication settings. The implementation method is the same as that of the extension. The connection circuit between nRF401 and AT89C51 is the same as that of the extension.

2.4.2 Design of display circuit
The display circuit mainly includes a large LED digital tube BSI20-1 (common anode, digit net height 12 cm) and a high voltage and high current driver ULN2003. Each segment of the large LED digital tube is composed of multiple LED light-emitting diodes connected in series and parallel, so the conduction current is large and the conduction voltage drop is high. ULN2003 is a high voltage and high current Darlington transistor array circuit. It has 7 independent inverting drivers. The output current of each driver can reach 500 mA. The output voltage is about 1 V when it is turned on and the output voltage can reach 50 V when it is turned off. Pins 1 to 7 of ULN2003 are signal input pins, and the corresponding output terminals are pins 16 to 10, and pin 8 is the ground terminal. When the driving power supply voltage is +12 V, if the conduction current of each segment of the digital tube is required to be 40 mA, the current limiting resistor of each segment is 50Ω. Then one ULN2003 just drives 7 segments of an LED digital tube. The large digital tube adopts a common anode connection method, and the low level is effective. The level of the latch output is inverted by NPN transistor 9014, amplified by ULN2003 and drives the large digital tube display.

2.4.3 Alarm circuit design
After receiving the call, the host will first alarm to inform the duty personnel. The alarm circuit can use the single-chip microcomputer P2.0 to output 1 kHz and 500 Hz audio signals, which are amplified and drive the speaker to make an alarm signal. The 1 kHz signal is required to ring for 100 ms, and then the 500 Hz signal is required to ring for 200 ms, alternating. The audio amplifier LM386 is used here. Its operating voltage is 4~12 V, the maximum output power can reach 1 W, and the input impedance is 50 kHz.

3 System Software Design

3.1 Extension system software flow chart
The single-chip computer scans the transmit key. If the transmit key is pressed, the system scans the state of the dial switch to determine the address code, and then puts the RF chip into the transmit state and starts to transmit the address code. After the address code is transmitted, the RF chip returns to the receive state to wait for confirmation information. After the confirmation information is received, the confirmation light is turned on for 1s, and then the sleep state waits, and the work cycle is repeated. The process is shown in Figure 6.

3.2 Host system software flow chart
When the host receives the call signal, it stores it, then calls the display subroutine for loop display, and then sends a response signal to the pager. After sending, the RF chip is placed in the receiving state again to wait for information. The main flow chart is shown in Figure 7.