Magnetic switches are the most common sensing devices for
sensing opening and closing of doors and windows. They are cheap and reliable.
Such sensors consist of two parts - A contact switch that is installed on the door /
window frame and a Activating magnet that is mounted on the door. The magnetic switch is
held in no-alarm contact position when door / window is kept closed. These are installed
on a door or window in such a way that opening the door or window causes the magnet to
move away from the contact switch which activates the alarm. They have proved to be a
tried and tested method of monitoring any of the operable openings in the houses /
apartments. These contacts are available in variety of finishes & materials. Heavy
duty versions are also available for heavy doors, shutters and gates. The magnetic
switches normally have both 'Normally open' & 'Normally Closed' contacts for easier
integration with existing systems. While installing care should be taken that alarm should
go off when the door just opens. If the sensor is wrongly placed then it is feasible that
a person may be able to squeeze in without activating the sensor. A series of such sensors
can cover the whole house including every door / window / ventilator. The Magnetic
switches need to be connected to Control unit (26), which senses
the change in the position of door and activates an alarm.
In Wireless Systems (29), the principle is the same except no wires
are required to connect the sensor to the Control Unit (26).
In cases a window has to be left a little open for ventilation, it is made possible by
using 2 magnets and 1 switch. In this configuration the first magnet is mounted in the
window closed position and the second is mounted in a location where it will align with
the switch when the window is open a few inches, typically 3 or 4 inches. This allows for
ventilation but will sound the alarm if the window is fully opened. Window or wall mounted
air conditioners and Coolers may also require protection if they can be easily removed
from the opening. The standard method used to protect these units is to install a sensor
on the unit.
Advanced Version
For more sensitive control of the door, there is an
advanced version called Balanced Magnetic Switch, in which the electrical contact of the
switch is held in no-alarm condition by two magnets - one is fixed to the door frame and
the other magnet is fixed to the corresponding position on to the door. As long as the
magnetic field remains balanced with the door firmly closed, the contacts within the
switch would be in no-alarm condition. Even with a slight movement of the door from the
door frame the magnetic field gets unbalanced causing the Control Unit to sound an alarm.
The balanced magnetic switch is more tamper-resistant than a simple Magnetic switch but
requires higher skill and time for installation. |
