Key less entry has been a luxury whose availability is confined primarily to vehicles. The cell phone security system takes this idea of key less entry and transforms it into a convenient, versatile security system that utilizes cell phone technology and the landline telephone network. By taking advantage of caller identification and dual-tone multi-frequency signaling, the security system has the ability to introduce two-levels of security.
The first level will be decoding the calling party’s identification information while the second level would consist of the user attempting a password entry over the phone. The system also has the ability to provide feedback to the user regarding the state of the system through a special user mode.
By combining the mobility of this telecommunication medium with micro controllers, the system achieves a secure, convenient, and automated form of security for a place of residence.
DESIGN PROBLEM AND REQUIREMENTS
The fundamental requirements for the cell phone security system remained fixed throughout the design process. My goal was to design a system which would allow the user automated and convenient access to their home security system through a telephone network.
The fundamental objectives of the system include:
- Correctly decode DTMF signals from the use.
- Correctly decode caller identification information from the phone line.
- Allow the user to automatically lock the entryway.
- Allow the user to automatically unlock the entryway.
- Allow the user to check the status of the entryway.
- Conform to all telephone standards.
In addition to meeting these goals, I also intended to create a design that would come as close to a real-world product as possible. This meant creating an interface with a live telephone line that a user could just take the design and plug it into a phone line and the system would be ready to go. This interface would have to conform to the federal communication commission’s (FCC) standards for use of their telephone networks.
The telephone lines in the United States are a federally regulated network that includes countless protocols and standards. These protocols define every aspect of their telephone lines from the impedance of the interface to the signals allowed on the lines. The following is a brief overview of some of the protocols that are applicable to this project.
RANGE OF SOLUTIONS
In every design, there is always a range of alternatives that could be utilized in arriving at a solution. These alternatives can be in the form of components, approaches, or algorithms. Some design alternatives for the cell phone security system include the following:
Physical Locking Mechanism:
A simple interface using LEDs was used to inform the user whether the system was in the locked state or unlocked state. I chose the most intuitive colors to represent each state, red for locked and green for unlocked. The following is a picture of this interface:
There exists a wide variety of choices for the actual locking mechanism. These options range from magnetic, solenoid, or motor driven locks. There are many advantages and drawbacks to each choice. However, the deciding factor is both price and importance. I decided against using electric or magnetic driven locks after doing research and discovering they cost in the hundreds of dollars.
DESIGN AND IMPLEMENTATION
Simulated Phone Line:
Due to constraints on resources available to me, I did not have access to a live telephone line that I could test my design on. As a result, the requirements for my project evolved to include the need to design and simulate a telephone line that would be as close to a real phone line as possible. This simulated phone system would have to provide identical signals to a real phone line in order to be convincing of an accurate simulation.
The Atmel Mega32 microcontroller was used as the main component in this lab. The mcu was set up and soldered onto the standard PCB layout offered by Professor Land for ECE 476. Using the dremel, excess parts of the PCB board were cut off in an effort to allow packaging.
I utilized the Atmel Mega32 microcontroller for this design. I took advantage of the 8-bit timer 0, 3 data ports, and serial USART (universal synchronous and asynchronous serial receiver and transmitter).
When I thought of this project, I wanted to design the system such that I would be able to enclose it in a well-made package instead of on a circuit board with components everywhere. I obtained an old corded phone and took out all the parts inside.
I was very satisfied with the results I obtained. I set out to design a system that would decode caller ID information as well as DTMF signals from a phone line, and I accomplished just that.
Despite the fact that I was not able to test on a real phone line, I was very confident in the fact that my simulations mirrored a real phone line exactly. The DTMF signals I generated were clean and virtually free of noise. I verified this by using the oscilloscope to capture the signal generated and checking for noise spikes.
My cell phone security system was successful in meeting the goals I intended for it. It allowed for a more convenient and automated form of security for a residential setting. The user mode with system feedback proved to be a very useful feature that I was very happy with the telephone system within the United States is a very complicated network. Before this project, I was not aware of all the restrictions and FCC regulations placed on interfacing with the phone line.
They place a lot of effort into isolating their lines fromany users in order to protect their infrastructure.It is an understandable measure and they go to great lengths to enforce these rules. The principles that were utilized in this project were also very interesting. DTMF is a very clever algorithm of transmitting data across the telephone line without much signal degradation. The frequencies chosen for the DTMF system are also clever since they prevent accidental generation from human voice. Caller ID is a relatively new feature on phone systems.
It was interesting to learn how well the data transmission worked with the existing infrastructure. By transmitting the data between ring bursts, no extra hardware or changes to the phone network had to be made, saving them the cost of new hardware and gaining a valuable feature. The security system will hopefully fulfill its other primary goal which is to influence a change within the market a change towards a more convenient, responsive, automated security system such as the cell phone security system.
Source: Cornell University
Author: Jason Chiang