DTMF-based hospital nursing call intercom system

O Introduction

The hospital nursing call system is an important part of modern hospitals to realize informatization and modern management, which can effectively enhance the image of the hospital, strengthen institutional management, and ensure service quality. How to use advanced information technology to serve hospitals and improve the service quality of hospitals to a greater extent is an important focus of hospital information construction. The call system designed in this paper greatly reduces the labor intensity of medical staff, improves the hospital's work efficiency and service level, enhances the hospital's competitive strength, saves management costs, and makes the organization management smoother.

1 Working principle of DTMF call Intercom System

The schematic diagram of the entire system is shown in Figure 1. The host and extension of the system are controlled by single chip microcomputer, and the host is connected to each hospital bed through a single bus mode, and has a call intercom function. When the extension calls the host, the receiver encodes the DTMF and signal sent from the extension into a 4-bit binary code and sends it into the MCU. After the host receives the extension through the decoder and decodes it, it controls the ringing circuit, LED display, and LED indication Light prompt, the phone can be talked after picking up the phone; when the host calls the extension, after the extension receives the number dialed by the host, the number is compared with their respective numbers, if the number is the same, the phone is connected to talk. The corridor display screen of this system can display the call bed number, the number of current calls, the call care level, and the real-time time when there is no call.

2 Introduction of DTMF signal

Dual tone multi-frequency transmission (Dual Tone MulTI-Frequency, DTMF) is a new type of dialing information transmission method, which uses a combination of two tones to encode a dialing key, where the two tones are from different frequency groups : Low frequency group (697 ～ 941 Hz) and high frequency group (1 209 ～ 1 633 Hz), each group frequency includes 4 different frequencies, they constitute 16 kinds of combinations, representing O ～ 9, *, #, A, The B, C and D keys are shown in Table 1.

3 System hardware design

The call intercom system can be divided into three areas in the organization, namely the medical staff's duty room, corridor and ward. The host, telephone and display and instruction screen are installed in the duty room of medical staff, LED digital tubes are installed in the corridor, and the extension is installed in the ward. The system block diagram is shown in Figure 2.

3.1 System working principle

The host is the core of the entire system. On the one hand, it is responsible for the reception and transmission of extension and telephone numbers, on the other hand, it controls the connection and disconnection of extensions and telephones. According to the patient's condition, special care, level 1, level 2 are set Nursing level, control bed and time display, level and call indication, off-hook detection circuit and display screen display, etc.

In the system, when the extension calls the host, the receiver encodes the DTMF signal sent from the extension, translates it into a 4-bit binary code and enters into the MCU, and the transmitter decodes the 4-bit binary code on the data bus into a DTMF signal output The host computer stores the sent data; after the data of the CPU is latched, the corresponding number and bit code are translated through the decoder, and the extension number and time are displayed by the nixie tube; after the phone picks up the answer, the CPU detects the picking signal , The ringing signal is cleared, the host and the extension talk, and the host hangs up after the call. When the main unit calls the extension, after the phone is off-hook, the CPU receives the dialed extension number after decoding through DTMF, and then sends the extension number to the extension; if several extensions dial the main unit at the same time, the main unit answers the extension with a high nursing level For calls, the general nursing level is only for display instructions.

3.2 Main hardware design of the system

3.2.1 DTMF signal transmission circuit

The DTMF sending circuit is mainly undertaken by the HT9200A / B. It encodes the 8421 code sent from the host into a DTMF signal and transmits it to the extension or telephone after being amplified. The numbering circuit is shown in Figure 3.

3.2.2 DTMF signal receiving circuit

The number-receiving circuit decodes the DTMF signal sent from the telephone into a 4-bit binary code for the host to read. It is essentially a decoding process, and the decoding of DTMF signals is mainly undertaken by MT8870. The interface circuit of MT8870 and AT89C52 microcontroller is shown in Figure 4.

Its working principle is as follows: if MT8870 receives a valid DTMF signal, the EST terminal first becomes a high level, and the control input terminal ST rises through the integration circuit. If the ST terminal level is lower than the threshold level, the 4-bit binary code inside the MT8870 remains unchanged, and the STD terminal remains low; if the ST terminal is higher than the threshold level, the 4-bit binary code inside the MT8870 is updated, The STD output changes from low level to high level, and an interrupt request is sent to AT89C52 to notify the MCU main console that control information is now issued. After AT89C52 receives the interrupt request, it is sent to STD of MT8870, and the STD terminal responds accordingly , And read the decoded data of MT8870 through P0.O ~ PO.3, and control the display and instructions of the display, light-emitting diode, etc. according to the content of the decoded data. The invalid DTMF signal (phone line noise, people's voice signal, etc.) or DTMF signal continuous distortion will not cause the MT8870 STD terminal changes.

3.2.3 Pick-up and hang-up detection circuit

In the standby state of the telephone, the 40V DC power supply is the working voltage for the telephone circuit board. In fact, after picking up the phone, the voltage at both ends of the phone is only about 6 to 12V, and the remaining voltage falls on the phone's long-distance communication line. Because the circuit is very close and the resistance value is very small, a constant current source circuit is provided to replace the high resistance value of the remote communication circuit of the telephone. This is the off-hook circuit shown in Figure 5.

The constant current source circuit is composed of R1, R2, Q1, and D1, and its current is (VD1-VBE1) / R2. This current is forward-stabilized by D2, D3, and D4 to generate a voltage of 2.1V to provide GDl photocoupler with luminous energy. The HUAJI1 and HUAJI2 nodes of the phone are open in the standby state (on-hook), the constant current source circuit and the optocoupler circuit are not working, the optocoupler is cut off, and the high level is output through U1A and UlB; The HUAJI1 and HUAJl2 nodes form a path, the constant current source circuit and the optocoupler circuit start to work, and the current on the left side of the optocoupler is (VD2 + VD3 + VD4-VGD1) / R3 = (O.7 + O.7 + O.7-1.8) /120=2.5 mA, meet the working conditions of optocoupler (1 ~ 50 mA), its internal light-emitting diode emits light, the phototransistor is turned on, and the low level is output through U1A and U1B. It can be seen that off-hook and on-hook can make U1B output different levels, so that the single-chip computer can distinguish the off-hook state of the phone. The constant current source circuit keeps the working current of the telephone set off-hook at (VD1-VQBE1) / R2 = (12-O.7) /1.5=7.5 mA, which meets the normal working conditions of the telephone set. The one-chip computer judges the off-hook by checking the high and low level of INTl.

3.2.4 Voice call circuit

Fig. 6 is the amplifying circuit that the host computer sends the conversation signal to the extension, adopting LM386 low-voltage audio power amplifier.

4 Conclusion

The research in this article focuses on the design of a practical and low-cost ward care intercom system. It should be said that the design goals have been achieved through a series of efforts. However, the research on the hospital nursing call system is an ever-evolving subject, so further research is needed on this basis to strive to make the system more complete, adding functions such as broadcast function, voice reporting function, and infusion completion warning.