A plunger switch is a mechanical device that mounts in the jamb on the hinge side of swinging or sliding doors. When a door is opened, a spring-operated plunger extends out of the center of the mechanism. When a door is closed, it pushes the plunger back in the switch mechanism. These mechanical switches sometimes are constructed with metal strips and center contacts that travel up and down when a door is opened and closed. Reed switches have replaced some of these older mechanisms, because reed switches are impervious to moisture, dust and corrosive environments.
Once installed, plunger switches are hidden from view and do not detract from a decor. However, a potential tampering problem exists when they are installed on out-swinging doors. Doors that swing outward usually are hinged on the outside of a structure, with the weather seal on the inside of the structure. This type of installation can expose an otherwise hidden plunger switch, and make it susceptible to corrosion. If the door is not frequently used the switch can remain stuck closed even when the door is opened, thus not creating an alarm situation. For this reason, another switch should be selected and installed elsewhere.
Plunger switches usually are 3/4 inch diameter and 1 inch long. When installing plunger switches in existing homes, they should be installed close to the floor on the hinge side of the jamb. After drilling a 3/4 inch hole at the bottom of a jamb, use a 1/4 or 3/8 inch bell hanger bit to drill a hole into the basement or crawlspace. A wire then is installed between the switch cavity and the basement using a rod, coat hanger, fish tape, or stiff piece of wire with a two-conductor cable attached.
Plunger switches do not work in every situation. The only should be placed on the hinge side of doors. When installing a switch on the side of a door frame opposite a hinge, or at the top of a door frame, plunger switches will not work. Roller switches should be used for these situations. A roller switch is similar to a plunger switch. However, instead of a solid rod that travels in and out of the switch body, it contains a roller mechanism. Some roller switches use ball-bearings and others use wheel mechanisms. In either case, when the edge of a door or window comes in contact with a roller, it is depressed into the switch body.
Roller switches are available in many shapes and sizes. Some are equipped with small screw terminals and others have insulated wires that protrude from the rear of the switch bodies. Most of these inner mechanisms are sealed. However, if the conditions under which a switch is to operate are corrosive or humid a sealed magnetic switch is recommended. Any type of magnetic contact switch that protects a door can be used to secure a window. In addition, there are sensors designed specifically to detect intrusion through windows, usually by detecting glass breakage.