Door-Windows Sensors
This sensors are used to monitor doors or windows. They are compact and smaller magnetic swtiches. The switches are done with magnetic reed switch. The reed switch is in a sealed glass capsule usually with a special inert gas that prevents the switch contacts from oxidizing (rusting). When a magnet is brought within a specific distance, the reed blades become opposite magnetic poles which attract. This closes the circuit and allows current to flow. When the magnet is taken away, the reed blades lose their induced magnetism and separate thereby opening the circuit. Because the switch is sealed it can be used in harsh environments. Reed switches respond to both magnetic poles (North and South).
Internal picture of a Reed Switch (Magnetic switch)
So as long as a magnet is place near to this magnetic switch the contacts will be in closed condition and if some one moves the magnet then the contact gets open and the microcontroller will sense it and inform the mobile.
The picture shows the magnetic sensor with magnet
This picture shows the installation of Sensor and magnet on the door.