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Door
and Window Switches
Doors
and windows are protected using magnetic switches. There are
two parts to the switch, a small magnet that is installed
in the door or window, and a magnetic "reed" switch
that is installed in the frame. The reed switch is constructed
of 2 pieces of very thin metal ("reeds") separated
by a tiny gap and sealed inside a small glass tube. The switch
is surrounded by a plastic housing and can be as small as
3/8" x 1/2" in size. When the magnet is brought
close to the switch one of the reeds bends and comes into
contact with the other reed thereby forming an electronic
switch. In security systems the switched is "closed"
when the magnet is brought in line with the switch by closing
the door or window. If the door or window is opened, or the
wire is cut or damaged, the circuit is opened and the alarm
is triggered.
Standard
reed switches allow for a gap of about 1/2" between the
door and door frame making them very reliable, even on doors
that do not fit tightly. There are many specialized switches
available for steel doors, overhead doors, surface mounting,
concealed mounting, high security applications, and even wide
gap switches which allow for a space of 2" to 3"
between the magnet and switch.
On
most wood, plastic or aluminum framed doors small concealed
switches are used. A small hole is drilled in the edge or
top of the door and frame and the switch and magnet are pressed
in making them almost invisible. For sliding doors and windows
2 sets of magnets can be placed a few inches apart (usually
3" to 6") allowing the system to be armed while
a small opening is left for air flow. If someone tries to
enter by pushing the window open the alarm is immediately
triggered. This feature is particularly useful for bedroom
windows and sliding doors.
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