This is a door alarm system which can be easily built and installed. It uses the basic principle outlined by the Arduino Uno Designers that is to use the Sensor, Micro-controller and the Actuator. In this design the sensor is the Hall-effect device (A1324) together with a permanent magnet (or you can use your own electromagnet), the micro-controller is the ATmega 328 embedded in the Arduino Uno and lastly the actuator is a 8ohm speaker used with a 555 timer and a optocoupler. It can be powered using the5V power supply of the Arduino Uno. It is highly sensitive, low cost and easy to construct and install.