Design of a two-way remote control door switch system based on voice alarm

introduction

Remote keyless entry (RKE) solutions have become a standard configuration for automobiles and an indispensable part of automobiles. The remote keyless entry system (RKE) is of great significance for improving the anti-theft and controllability of automobiles. Most RKE systems have car anti-theft and alarm functions as well as access control for cars and trunks. Some systems also include remote car start and car search functions. The one-way RKE system designed in the past consists of a control end and an execution end. The biggest inconvenience of this system is that only the user sends information to the car door, and the car door cannot feedback its own information to the user, which makes it impossible for the user to know the status of the car, which poses a hidden danger to the safety of the car. In addition, since the system is powered by a battery, how to minimize power consumption is also a relatively large problem. In order to solve these two problems, this paper designs a two-way RKE system based on Nordic's nRF401 transceiver, and uses a voice chip and its corresponding filter amplifier circuit to realize the system's automatic voice alarm through the built-in speaker.

The system mainly implements two functions: when the user sends the corresponding command, the car automatically executes the locking and unlocking functions, and when the user exceeds the transceiver range, the main transceiver can automatically send a warning message to inform the user that he has exceeded the safety range through voice. The wireless data transceiver function is realized by nRF401 chip; the voice alarm function of the system adopts ISD4004 series chip.

nRF401 Function Introduction

nRF401 is the latest single-chip RF transceiver launched by Nordic VLSI of Norway, designed for operation in the 433MHz ISM (industrial, scientific research and medical) frequency band. It is currently the most integrated wireless data transmission product. The chip integrates high-frequency transmission, high-frequency reception, PLL synthesis, FSK modulation, FSK demodulation, dual-channel switching and other functions, with the characteristics of excellent performance, low power consumption and easy use. nRF401 has very few peripheral components, only about 10. It only includes a 4MHz reference crystal oscillator (which can be shared with the MCU), a PLL loop filter and a VCO inductor, and the transmitting and receiving antennas are integrated. There are no debugging components, which brings great convenience to research and development and production. Its internal structure is shown in Figure 1.

nRF401 adopts 20-pin SSOIC package, and the internal circuit can be divided into transmitting circuit, receiving circuit, mode and low-power control logic circuit. The data to be transmitted is input through the DIN terminal (pin 9). The RF power amplifier has an output power of up to +10 dBm. The receiving circuit includes: low noise amplifier, mixer, FSK demodulator, filter and other circuits. In the receiving circuit, the low noise amplifier amplifies the input RF signal, and the receiving sensitivity is -105 dBm. The mixer adopts a 1-stage mixing structure. The output signal of the mixer is filtered by the intermediate frequency filter and sent to the FSK demodulator for demodulation. The demodulated digital signal is output at the DOUT terminal (pin 10).

ISD4004 Series Voice Chip Function Introduction

The ISD4004 chip uses CMOS technology and contains an oscillator, smoothing filter, automatic squelch, audio amplifier and multi-level flash memory array. The chip is a non-volatile device that can store data for up to 100 years without power supply. It has a long recording time and has a variety of chips with 8, 10, 12 and 16 minutes. It can record repeatedly for 100,000 times. The chip design is based on the fact that all operations must be controlled by a microcontroller, connected to the processor through the SPI bus, and the peripheral circuit is simple.

The ISD4004 voice chip can not only complete the recording process but also the playback process. The voice alarm discussed in this article mainly refers to the playback circuit installed in the host computer carried by the user, and does not include the recording circuit. In actual applications, the recording part stores the recorded content in the voice chip in advance and assigns a specific broadcast address. When an alarm is needed, it only needs to read the broadcast address, and then the alarm information will be automatically played by the voice chip through the speaker, realizing instant alarm.

System hardware overall design

The two-way RKE system consists of two parts: the upper computer and the lower computer. The upper computer is carried by the user and can be understood as a remote control key with an alarm function, while the lower computer is installed in the car door and is used to open the door. The entire two-way RKE system is shown in Figure 2.

The system is mainly divided into two parts: wireless transceiver part and playback part. The core controller uses MCU AT89C52 produced by ATMEL, which establishes a connection with the RF module through an asynchronous serial port. Pin 12 of the RF transceiver chip nRF401 is the channel selection: CS = "0" for channel #1 (433.9 MHz), CS = "1" for channel #2 (434.33MHz); Pin 18 is the power switch:
PWR UP = "1" for working mode, PWR UP = "0" for standby mode; Pin 19 is the transmission enable: TXEN = "1" for transmission mode; TXEN = "0" for receiving mode. They can be connected to the MCU's I/O ports P1.2, P1.3 and P1.4 respectively, for the selection of the working status of these control bits, while the data input bit DIN and the data output bit can be connected to the asynchronous serial ports TXD and RXD of the microcontroller respectively, so that the MCU can control the transceiver timing of the RF chip.

The playback part uses the chip ISD4004, whose power supply voltage is 3V, while the power supply voltage required by the single-chip microcomputer is 5V, so the interconnection between them needs to go through a certain level conversion circuit. The P1.6 of the single-chip microcomputer is connected to the chip select pin /SS of the ISD4004 through the level conversion circuit to control whether the ISD4004 is enabled; P1.7 is connected to the serial input pin MOSI of the ISD4004 through the level conversion circuit, and the playback address is read from this pin; P3.2 and P3.3 are respectively connected to the serial clock pin SCLK and the interrupt pin /INT of the ISD4004 through the level conversion circuit; AMCAP is the automatic mute terminal, which is grounded through a capacitor when used; the connection required by the ISD4004 chip also includes the audio signal output pin AUDOUT, which is connected to the speaker through a filter amplifier circuit. The principle of the playback circuit is shown in Figure 4. Since the four pins of the single-chip microcomputer need to be connected to the voice chip, four level conversion circuits are required as shown in the figure (only one is drawn here).

System software design

Communication Protocol

The two-way RKE is mainly composed of a host computer and a slave computer. To realize the locking/unlocking function and the security alarm function, the wireless data communication between the host computer and the slave computer must be completed. All parts of the entire system serve the purpose of wireless data transmission. Therefore, in the software design of the entire system, the transmission of wireless data is the most important part. Wireless transmission is carried out in the form of data frames. The format of the data frame is shown in Table 1.

The leading code is mainly used to prevent useful data from being interfered with; the vehicle ID is mainly used to mark the information of a specific vehicle for correct identification. If the ID does not match, the data frame will be ignored; the information code is used to indicate the user's operation, which includes locking, unlocking, displaying an alarm, etc. When the user sends the corresponding command, locking and unlocking are performed respectively, and when the user exceeds the transceiver range, the main transceiver can automatically send a warning message; the checksum is used to check whether the received data is correct. If it is incorrect, the data frame needs to be resent.

System software process design

Since the entire RKE system is a two-way system, the communication between the data terminal and the main transceiver is duplex, so when the actual system is running, the data transmission direction is constantly changing. Therefore, in order to achieve coordinated communication between the two parties, a more suitable receiving and sending order must be designed. The upper computer receiving and sending flow chart is shown in Figure 4. After the system is powered on, the upper computer is in standby mode. When it is detected that the upper computer sends a key command, it means that the user wants to perform a certain operation, and the operation information is sent to the speaker at the same time, and the user is informed of the operation to be performed by sound. At this time, the upper computer turns on the transmitter and sends the corresponding information code (such as unlocking, locking, etc.), while the upper computer is in the receiving state at other times. If the address code sent by the lower computer is not received, the upper computer can judge that the user is out of the sending and receiving range at this time, so the alarm information that will be pre-stored in the upper computer is read into the playback address through P1.7, and the speaker sends a voice signal to warn the user. If the upper computer does not receive the ACK (correct response signal) sent back by the lower computer, the upper computer will ask the user to press the button again until the ACK command is received, and then the lower computer will perform the corresponding operation.

After the system is powered on, the lower computer is first initialized, and then the receiver is turned on to determine whether there is a user pressing a key to issue a command. If no key command is detected, the lower computer turns on the transmitter and continuously sends the address code to the upper computer to determine whether both parties are within the communication range to prompt the user. When a key command is detected, the lower computer first verifies the operation information code sent by the upper computer. If the verification is incorrect, the user is required to press the key again; if the verification is correct, an ACK command is sent to confirm that the user's operation is valid. In this way, two-way communication between the upper computer and the lower computer can be effectively realized, so that the two-way RKE system can work stably. The lower computer's transceiver flow chart is shown in Figure 5.

Conclusion

The two-way RKE system composed of RF chip nRF401, AT89C52 MCU and voice chip ISD4004 can let users express the executed commands more intuitively through voice, making the system convenient and practical. At the same time, the chips selected in the system are all low-power, which is very important for the battery-powered host computer. In addition, the size of the selected chips is very small, and the interface between MCU and nRF401 and ISD4004 is very simple, without too many peripheral circuits, which is conducive to the integration of the system to some extent and convenient for users to carry with them. On the basis of this system, if a pressure sensor is added to the car door, it can greatly prevent the car from being damaged by human beings and further improve the safety of the car. In addition, the system can also be used in other occasions, such as digital voice communication, wireless remote control anti-theft door, intelligent wireless meter reading and so on, and can be greatly promoted.