Abstract : This paper introduces a home appliance remote control system composed of an AT89C51 microcontroller and an MT8870 DTMF decoding chip. This system can receive DTMF signals from telephone lines to control appliance switches, allowing users to remotely control their appliances by dialing a landline or mobile phone.
Keywords : telephone network; remote control; microcontroller; MT8870
Abstract : This paper proposes a remote control system of household appliances, which contains AT89C51 MCU and the DTMF decoder MT8870. The system receives DTMF signal through a phone line and remote-controls the electric switches, f the fixed telephone or mobile telephone. Thus it remote controls the household appliance or other equipments.
Keywords : Telephone Network; Remote Control; Single Chip Microcontroller; MT8870
With economic and technological development and the continuous improvement of people's living standards, the issue of intelligent living environments is receiving increasing attention. Remote control of home appliances via telephone networks will bring great convenience and enjoyment to people's daily lives, and is also one of the important indicators of intelligent communities and residences.
This article introduces a remote control system for home appliances based on telephone networks and microcontrollers. Using this system, users can conveniently control their home appliances via telephone. For example, before leaving work, users can turn on the rice cooker, air conditioner, and water heater in advance. When they arrive home, the rice is cooked, the room temperature is just right, and the water in the water heater is heated, providing both comfort and convenience. Furthermore, users can change the operating status of home appliances at any time, achieving energy savings and improving the security of unoccupied homes. The composition and control principle of this system are described in detail below.
1 System Composition and Working Principle
The system block diagram is shown in Figure 1. The ISD2560 is a voice storage chip.
Figure 1 System Composition Block Diagram
When a user needs to control home appliances, they can dial their home phone number using a mobile phone or landline. The ringing signal is transmitted to the home phone via the telephone network. At this time, the ringing detection circuit starts detecting the ringing current signal. Each ringing current signal generates a pulse, which is sent to the AT89C51 microcontroller for counting. If someone answers or the number of rings is less than 6 (configurable by software), the control system considers it a telephone call signal or a misoperation and does not take any action to avoid interfering with normal telephone services. When the number of rings exceeds 6, the system considers it a control request for sockets and appliances. The microcontroller controls the handset to be picked up and activates the voice prompt circuit to emit a prompt tone, requiring the user to enter a password. The DTMF decoding chip MT8870 converts the user's key input signals into corresponding digital signals and sends them to the microcontroller for judgment. If the password is incorrect, the voice prompt will prompt for re-entry. If the number of attempts exceeds 3 (configurable by software), the system will automatically hang up. If the system confirms the correct password, the voice prompt will prompt the user for the corresponding number keys for each operation. After confirming the operation, the system will automatically hang up. When a user presses a key on the phone, the time interval between key presses cannot exceed a certain period (usually set to 5-10 seconds). If the set time is exceeded, the system will automatically hang up.
2 System Hardware Design
2.1 Central Control Circuit
The central processing unit of this system is the AT89C51, which issues all control commands. After the system powers on and resets, the CPU continuously monitors the ringing signal status. Upon detecting a valid ringing signal, a counting program is started. After counting to the set number of rings, the CPU sends an off-hook signal, the control circuit automatically off-hook and sends an off-hook prompt tone, prompting the user to enter a password. Simultaneously, the microcontroller detects the STD signal from the MT8870. When the STD signal is valid, it reads the instruction code from the MT8870 and compares it with the preset password. If they do not match, the phone automatically hangs up. If they match, the user can perform further operations based on the system's voice prompts, such as checking the working status of appliances plugged into the socket or setting their status.
2.2 Ringing Detection Circuit
The ring detection circuit is shown in Figure 2. When the ringing signal is input through C21 and C22, the 75±15V, 25Hz AC voltage on the telephone line, after being rectified by bridge rectifier D4, filtered by C8, and regulated by D5, becomes a DC voltage that turns on the optocoupler. The +5V power supply voltage charges capacitor C13 through the optocoupler, R22, and D3. When the voltage on the capacitor reaches the opening voltage of 74L08, 74LS08 turns on, converting the analog signal into a digital pulse signal, which is then input to the T0 pin of the AT89C51 microcontroller for counting. Each ringing signal generates one pulse, and the microcontroller counts once. When the count reaches 6, the software sends a low level to the P2.1 pin of the AT89C51, completing the off-hook action.
Figure 2 Ringing detection circuit
2.3 DTMF Decoding Circuit
The decoding circuit is shown in Figure 3. The function of the DTMF decoding circuit is to receive the dual-tone multi-frequency (DTMF) signal input from the telephone and convert it into binary code, then input it to the microcontroller for data processing to achieve control functions. The MT8870 is a complete DTMF receiver, which can be used as an interface separation filter and digital decoder. The filtering section is used for high and low frequency filtering of the switching capacitor, and the decoder is used for digital querying, converting the detected 16-bit DTMF signal into a 4-bit code. The minimum value of the external counter is preset by different input amplifiers, clock oscillators, and three latched state values. The signal on the telephone line is coupled through capacitor C23, attenuated by resistors R13 and R14, and input to pins 2 and 3 of the MT8870 for decoding, converting the telephone dialing signal into the corresponding binary code. After the user operates the keys on the telephone keypad, this information is transmitted through the telephone line in dual-tone mode.
2.4 Voice Prompt and Monitoring Circuit
The ISD series voice chips avoid the quantization noise and distortion of typical solid-state voice circuits, and can reproduce the natural sound of voice better. The ISD2560 supports direct address manipulation of voice information, allowing designers to operate on voice segments and combine voice data without needing to know their addresses, only the relative sequence numbers of the segments. The ISD2560 operates in six different modes based on the different combinations of high and low levels on pins A1 to A8, achieving different functions. When activating the monitoring function, simply changing the level of the corresponding port converts the ambient sound into a signal and sends it to the telephone line; the circuit is simple and easy to use.
Figure 3 DTMF decoding circuit
2.5 On/Off Circuit
The off-hook/on-hook circuit is shown in Figure 4. Normally, when the phone is on-hook, both telephone lines are open, and the voltage on the lines is applied to the ringing circuit of the phone. When the phone is off-hook, the ringing circuit is disconnected, and the telephone lines are connected. When making a call, the high-voltage ringing signal from the telephone lines generates a pulse signal through the ringing current detection circuit, which is sent to the P3.5 port of the microcontroller for ringing pulse counting. When the ringing count reaches 6, the microcontroller program controls the P2.1 port to output a low level. Then, the high level of the P2.1 port is sent to pin 1 of the MC1413 integrated switch driver circuit for amplification, and a high level is output from pin 16 to the base of the 9013 transistor, causing the transistor to saturate and conduct. At this time, the +5V power supply is grounded through the relay coil and the transistor. The relay coil is energized, causing the normally open contact of the relay to close, and the call is connected. When the user enters an incorrect password or the operation ends, the system software controls the P2.1 port to output a high level. Through the MC1413 driver circuit, pin 16 outputs a low level, controlling the 9013 to cut off, the relay coil is de-energized, the normally open contact opens, and the telephone line is in an open circuit state, realizing the hang-up.
Figure 4. On/Off Circuit
2.6 Control Circuit
The design utilizes a power socket with relays to achieve final control of home appliances such as rice cookers, water heaters, and air conditioners. Appliances simply need to be plugged in, and the main control microcontroller can control the power supply to these appliances via the relays in the socket. This method is simple and easy to implement. The circuit diagram is shown in Figure 5.
Figure 5 Control Circuit
3 Software Design
After the control system completes the off-hook operation, it prompts for a password. If the password is incorrect, the system prompts for re-entry, with a maximum of three attempts. After three attempts, the system automatically hangs up. If the password is correct, the system will provide operation prompts according to the set menu. For ease of operation, the system includes a back button; pressing this button returns to the previous menu. After all settings are complete, press "#" to end the settings, and the system will automatically hang up. The system flowchart is shown in Figure 6.
Figure 6 Program Flowchart
4. Conclusion
This system uses a microcontroller as the main control unit, combined with software programming, to achieve remote control via a voice interface. The system adds password authentication, enhancing the security of personal property and meeting the requirements of smart homes. Furthermore, when users are away, they can use the system to turn indoor lights, stereos, televisions, and other appliances on and off at any time, creating the illusion that someone is home, thus improving home security.
References
[1] Mao Zhaorong, Xu Hua. Design of a remote control system based on telephone. Electronic Technology, 2000, (3): 30-32
[2] Yu, Gaoxiang. A simplified implementation of voice combination function in a single-chip microcomputer system. Electronic Technology. 2001, (9): 5-7
[3] Guo Yaohua, Yao Minglin. Remote temperature communication monitoring system based on fieldbus. Measurement and Control Technology, 2003, (1): 43-47
[4] Li Xuemei, Li Qiuhong. Application of AT89C51 microcontroller in telephone remote controller. Modern Electronics Technology. 2006, 235(20): 36-37
About the author:
Du Liufeng (1980–), male, from Xinxiang, Henan Province, is a teacher at Henan University of Science and Technology, mainly engaged in automatic control, communication and signal processing technology.
Shao Feng (1981–), male, from Shangqiu, Henan Province, is a teacher at Henan University of Science and Technology, mainly engaged in microcontroller applications and mechatronics technology.
Mailing Address: Du Liufeng, Signal Teaching and Research Section, School of Mechanical and Electrical Engineering, Henan University of Science and Technology, Hualan Avenue, Xinxiang City, Henan Province
Postal code: 453003
Telephone: 0373-3714058, 13782556856
E-mail: [email protected]
