Group 2: Double Deuce Alarm System
Design Constraint Analysis and Component Selection Rationale
Author: Brian Stratman
Introduction:
Double Deuce Alarm group’s project is a home security system. This security system will feature a variety of features. The main feature of the system will be the web based configuration and monitoring interface. The owner/monitoring company will be able to watch the system from anywhere in the world as long as the system has an external Internet connection. Additional features will include fully expandable module based system. Each type of sensor (smoke, window break, intrusion detection and motion) will be able to be utilized by the system. Each module will be able to control a number of each type of sensor and the system supports up to 7 modules. The system of modules makes the system super customizable. Keypads will be used to arm and disarm the system; the passwords will be set via the web interface. All wait times that the system uses will also be fully customizable.
Other options that the alarm system will sport are battery backup and field swappable modules. As discussed before the modules are interchangeable but the key is that they may be changed out in the field and are not “hardwired” into the system. The battery backup system will have a built in trickle charge to keep the battery at maximum power. All sensors will be wired normally closed. This will allow for error signals to be flagged if a communication with a sensor becomes severed. The main part of the system will be built into a steal case with lock to prevent tampering.
Constraint Analysis:
Physical Constraints (Size and Cost):
The case for the project will be contained in a custom sheet metal case. The design is fairly straightforward and can be built to suit the size needs of the final circuit design. The budget was set as low as possible. Original budgeting was set at $500 for the whole project. However, picking up donations to the project has cut that number down to almost $0.
Main Microcontroller:
The project requires a main CPU that will control the bulk of the systems functionality. This processor will have to have Ethernet connectivity as well as support for IIC bus system. The Ethernet connectivity is required for the web interface that is a center point to the project. Speed is not so much of an issue. If the CPU is capable of running a basic web server it will have plenty of power. The I2C bus is used to poll the sensor modules at regular intervals to determine the status of each sensor on that module. As one can see the processor requirements are fairly straightforward.
Module Microcontroller:
The module controller has only a few constraints on it. The most important is that is runs at the same voltage as the primary CPU. The next is that is has a built in IIC interface like the main CPU. The last is that it has enough ram and flash to build a program to keep basic statistics about what the sensors are doing between update poles from the primary microcontroller.
Sensors:
The system will be able to receive information from 4 different types of sensors. These sensors will include intrusion detection, motion detection, glass break detection, and smoke detection. All of these devices need to operate on DC current preferably less than 12 volts. The sensors will also need to use a relay type trigger that is normally closed for safety reasons.
Power Supply:
The power supply will need to provide 2 different voltages. The first voltage will be 3.3 volts for the microcontrollers. The second voltage will be somewhere around 12 volts to power the sensors. The supply should be able to provide battery backup in case of a power outage. The supply should also include a trickle charger to keep the battery at peak power.
Selection Rationale:
Main Microcontroller:
Two microcontrollers were compared: The Rabbit Semiconductor RCM3200 and the Freescale MC9S12NE64. Both were able to support our needs however the Rabbit required external chip for Ethernet control, adding extra parts to the design requirements. The Freescale processor contained a single chip implementation of Ethernet connectivity. The big decision maker for this part of the design was past experience. Both our experience and experience that other have had with the chips. The rabbit has been used by many teams in the past, however there was very little good things said about the “dynamic C” compiler and documentation for it. The Freescale CPU was a new one, but it was based on the HC12 architecture, which we have prior experience with. The NE64 chip also used code that was well documented and standardized. The MC9S12NE64 processor was chosen.
Module Microcontroller:
A variety of options were investigated for this processor. The first option that was ruled out was the use of a PLD. A PLD would be a good solution if an IIC bus was not going to be used, however we have decided for simplicity to use such a bus system. Next option was a microcontroller. There were 3 key players: HC12 variants, Atmel ATMega series, and microchip PICs. The HC12 variants were decided to be over kill for this particular functionality as all the extra I/O was not necessary. The PIC and ATMega’s where practically identical. Both had similar number of I/O, similar speed, ran at the required 3.3 Volts, and had similar memory specifications. It came down to which had a better IDE and programming capabilities. The Atmels seemed like they had a nicer more compact C compiler that was provided for free. Also the programming cable for the Atmels was reasonably priced. So the Atmel ATMega88 was chosen.
Sensors:
Sensors from a variety of different brands were researched, however, many required a supply voltage of 120VAC. We did not want to have to deal with such requirements so they were quickly ruled out. The only company that was found that consistently used low voltage DC current to power their devices was GE Security Systems. All of the sensors we plan to use GE Security have available. All operate on a 9VDC power supply. This makes it very easy to interface with the system. A simple line level converter and an opto-coupler and we have the 3.3 I/O voltage.
Power Supply:
Now that we know what voltages we are going to be working with, 3.3V and 9V, we can go ahead and finalize the power supply specifications. A dual coil system, one coil providing the 3.3V and another to provide the 9V. A simple filter cap and regulator will do the rest. Texas Instruments regulators have been chosen to do the job.
Development costs:
Part / Vendor / Part Number / Quantity / Unit CostFreescale Dev Board / / EVAL9S12NE64 / 1 / $250.00
Atmel ATMega88 / / ATMega88-20PJ / 4 / $0.00*
USB BDM / / USB-ML-12 / 1 / $100.00
AVR ISP / / AVRISP / 1 / $30.00
HC12 Compiler / / CWS-H12-C64K-CX / 1 / $1000.00
Smoke Detector / / 541C / 2 / $50.00
Shatter Detector / / 5812EZ-W / 2 / $60.00
Intrusion Detector / / 2505 / 2 / $3.00
Motion Sensor / / RCA-A / 2 / $50.00
Horn / / MPI-47 / 1 / $40.00
*Samples