In the security system the micro controllers is programmed in such a way that if somebody had entered in home with out permission the sensors will detect and give an interrupt to the microcontroller, if the controller is interrupted it will give commands and user number to the modem to sent the alert SMS to the owner mobile. LCD is connected to microcontroller as 4 bit data mode, before displaying anything in LCD Initialization have to do ,so microcontroller will control the LCD initialization and select the data register and command register according to the purpose. Memory is connected to microcontroller using two pins, it is communicating with the microcontroller through I2C communication.Relay and buzzer is controlled by the microcontroller using single pins, Ie giving high means device will switch on and vice versa. Sometimes it may be interchange according to the transistor used to drive the device.