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.

ENTRY DETECTORS

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 Double-Hung Windows

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 shut, they may be oriented side-to-side, top-to-side, or top-to-top.

Figure 8-18. - Double-hung window with surface-mounted magnetic contacts.

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 Casement Windows

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 installer's part, but few problems develop if the following precautions are taken.

1. Be careful not to damage or destroy any weatherproofing seal around windows, doors, or other openings.

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.