Here is a very useful project of a door security alarm circuit. The circuit can be used for security purposes for example it can be used at the door and when a burglar tries to open the door the circuit will become switch on and activate the peizo buzzer.
The circuit is easy to build and can be fitted in a very small place.
small place. The heart of the circuit is a reed switch. A reed switch is a device, which sense the magnet. There are two types of reed switches, which are normally open and normally closed switches. The circuit mentioned here is using normally closed reed switch. A small piece of magnet will be required at the other part of the door to make the reed switch in open condition when the door is closed, and when the door will be opened the distance between the reed switch and magnet will increase due to which the reed switch will become in its original state (normally closed) and the circuit will become activated. The operating voltage of the circuit is 6 to 12V DC.  

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Door Security Alarm Circuit Diagram