Removal of resistor R eliminates the battery-charging
feature and allows the use of a dry-cell battery that sits
idle until ac power fails. At that time, S1 pens and S2
closes, connecting the battery to the alarm system.
There are many variations of this basic power
supply design. These variations add such features as
voltage regulation, current limiting, and automatic
high-rate/low-rate charging, controlled by the state of
the battery charge. All designs normally provide
current and voltage meters, pilot lamps, and switches
for manual control of charging rate.
The surface magnetic detector is the most versatile
entry detector for residential alarm systems and should
be considered first as a method of protecting any
movable door or window. These detectors can be
mounted on wood, metal, and even glass, if necessary.
They can be mounted with screws, double-sided tape,
or epoxy. Obviously, the tape and epoxy are useful on
glass, aluminum, or any other surface where screws
cannot be used. However, when applying tape or
epoxy, make certain that the surface is clean, dry,
smooth. and at least 65°F (18°C).
Surface-Mounted Magnetic Contacts on
A switch is mounted on the window casing with a
magnet on the window casing and a magnet on the
window (fig. 8-18). As long as the switch and magnet
are parallel and in close proximity when the window is
Figure 8-18.Double -hung window with surface-mounted
shut, they may be oriented side-to-side, top-to-side, or
Surface-Mounted Magnetic Contacts on Doors
Where appearance is not the most important
consideration, the use of a surface-mounted switch (on
the doorframe) and a magnet (on the door) will simplify
installation (fig. 8-19).
Recessed Magnetic Contacts in Doors and
Where the appearance of surface-mounted systems
is objectionable, recess-mounted magnetic protectors
may be used. These detectors are more difficult to install
and require greater care on the installers part, but few
problems develop if the following precautions are
1. Be careful not to damage or destroy any
weatherproofing seal around windows, doors, or other
2. If a recessed-mounted entry detector is installed
in the windowsill, you must prevent water seepage to
the switch by applying a sealant under the switch flange
and around the switch body.
3. When drilling holes to accept each half of the
detector, be sure the holes line up. Holes are drilled in
the door and in the casing, one directly across from the
other, and a pair of wires from the positive side of the
Figure 8-19.Surface-mounted magnetic contacts on door..