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Figure 0 - This security system works as both a permanent or portable alarm
This security system works as both a permanent or portable alarm

This project details a simple yet effective security system that is perfect for temporarily protecting an area or building. This portable alarm system functions much the same way as a typical hard wired home security system, allowing for a timed entry and deactivation via numerical keypad. The alarm system will respond to practically any type of sensor input such as a motion switch, magnetic door switch, window break tape, infrared motions sensor, and any other sensor that acts like a simple switch. Because the main unit contains all of the electronics, alarm, keypad, and battery, the alarm trigger sensor can be remotely located, making it difficult to disarm the system once it has been activated by an intruder.



The goal of this project is to demonstrate how to connect a keypad matrix to a microcontroller to scan the rows and columns for key presses and to create a simple security system that can be used as the framework to create your own security system, leaving a lot of room for modifications and improvements. Practically any type of matrix keypad can be used and the alarm siren used is a common and inexpensive pocket alarm that has been modified to allow switching through a relay. The microcontroller used is an 8 bit ATMega88, but just about any microcontroller can be used as only a few IO pins are needed. The programming is done in C for portability and clarity.



Figure 1 - Arming and disarming the alarm is done by entering a code on a keypad
Figure 1 - Arming and disarming the alarm is done by entering a code on a keypad

The alarm is activated or deactivated by entering a secret code on a 12 digit keypad like the one shown in Figure 1. This keypad is from SparkFun.com, part number COM-08653, but any keypad will work as long as you know the pinout. You could even make this alarm system with no keypad at all and just use the microcontroller as a simple time delay to allow arming and disarming the system, using the power switch to turn the alarm on or off.

Unlike a permanent home security system where the keypad is right at the door and the alarm is hidden, this system places the keypad and alarm in the same box, so the goal is to locate it deep inside a building and run a wire to the actual trigger sensor. In portable mode, the sensor can be affixed to the alarm box, allowing it to be propped up against a door or placed in a drawer or even a suitcase. Having the alarm, battery, and main electronics located in the same box means that this particular alarm system design is probably not the best solution for a permanent installation, but it does work very well as a portable system or when you just need temporary security for a building.

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