【正文】 =(~(1PCIE2))。 ISR (PCINT2_vect) { if (!(PINDamp。 PortGreenLeds |= value。 PortGreenLeds amp。 } }void DisplayGreenLeds (char value){ value amp。 j100。 iduree。s property and have evolved in order to stop increasingly plex methods of theft [1]. The first antitheft devices to be built were all mechanical devices and were generally locks of various kinds. These have evolved over the years and are still in fashion today. These include steering wheel locks, tire locks, hood locks, gear shift locks, ignition/steering wheel column locks, to mention a few. However, in recent years, crime has bee more sophisticated leading to the invention of electronic based car antitheft systems. They can be divided into three main groups: car alarms, vehicle tracking systems and mmobilizers. In this paper we put forward an efficient car antitheft system with improved features to reduce carjacking or automobile from being stolen.2. PREVIOUS WORKSVarious types of car alarm system have been in existence for long. A car alarm system is an electronic device or circuit built to alert car owner in case someone attempts to steal the cars. There are many forms of car protection system including vehicle tracking system, car immobilizers and others. Also the notification system varies as technology grows. Commonly encountered car alarm system operation does involve a hidden switch used to activate the alarm system. Automatically the car is off and the hidden switch put in the ON position, the user is given 15 to 30s to get out of the car and close all the doors. After returning and opening the door, a buzzer will sound and the user will be given some 10s to move the switch back to Off position. Failure to do so will make a siren to sound. In other cases, when the alarm system is activated the siren sound as anything es in contact with the car. In addition to the notification by siren, some recent car alarm systems GSM modems to send SMS or call the user in case of a theft [1][3]. Also vehicle tracking system has been adopted to further locate the car even if it is stolen despite all this protective measures. Another effective car antitheft system is known as car immobilizer. A car immobilizer is any device which when installed in a car would prevent the car from starting without the authorization of the owner [4][5]. It is one of the best ways to prevent thieves from stealing a car even if they gain access to it. In 1993, 144,057 cars were reported to be stolen in Germany. This number was reduced to 57,402 in 2002 after car immobilizers were made pulsory for all new cars in 1998 [6].3. METHODThis section of the paper describes the steps and procedure involved in the design and construction of a semi automatic car antitheft system. To start with, a state machine was used to explain the main concept of the design. This was followed by the implementation of the state machine in the C programming language using the AVRStudio. The program was piled and loaded into the microprocessor with adequate circuitry that enabled the physical implementation. Logical System ImplementationThe car antitheft system will work in four different states namely Idle, Waiting, Alarm and finally Active. To these states, correspond respectively, the digital values 00, 01, 10 and 11. Figure 1 illustrates the state the system will stay in the idle state (00) and in this state the microcontroller will be in the power saving mode until the user opens the car door. Upon opening the door, the system exits the power saving mode and goes into the waiting state (01). A timer is initiated to give a time limit within which an authorization must be received. Authorization may e in the form of a PIN code being entered, a hidden button being pressed or a phone call being placed. If no authorizationis received during the stipulated time, the system changes automatically to the alarm state (10) where the system will activate a siren and call the phone number of the vehicle owner. Also, to prevent the alarm from running down the car International Journal of Computer Applications (0975 – 8887) Volume 63– , February 2013 battery, the system will return to the waiting state (01) after sometime. On the other hand, when an authorization is received whilst in the waiting state, the system will go into the active state (11). Once in the active state, the user is allowed to start the car. The system will return to the idle state in three ways. Firstly, it returns if the user opens the door after turning off the key switch. Secondly, it will return if the user presses the distress button even if the key switch is not turned off. Lastly, it will return if a phone call is placed to the GSM modem in the system Hardware ImplementationThe first step in the hardware implementation will consist of assigning to various entities, the corresponding electronic ponents that best describe their distress button is a manual override switch used to trigger the system from the active state to the idle state. When pressed, the microcontroller initiates a countdown timer to arm the system. It could also serve as a means of providing authorization. A pushtomake switch is the most appropriate for such an application. The car key switch was represented by a SinglePole SingleThrow (SPST) switch. Also a magnetic field sensor switch was used to determine the opening of the car door. GSM modem serves as both an input and an output entity. A Sony Ericsson K700i mobile phone was used. The siren was represented by a standard 12V car siren. Both the ignition circuitry and fuel pump were represented by 12VDC motors. This allows displaying what happens to those parts of the car when the system engages them. The final circuit diagram of the system is shown in figure the circuit of figure 2, the relays controlling the ignition circuitry, the siren and the key switch have their contacts normally opened while the one controlling the fu