Security Tips and Industry Musings
Doors and windows
Security systems require sensors to tell it in what condition monitored parts of the premises are in. The sensors we use on doors and windows are called contacts. This is a generic term for small switches that let the control panel know a door or window has been opened or closed. There are many different styles of contacts. These contacts can be activated through magnetic or mechanical force. Whether the contact is connected to the control panel by a wire or through a wireless transmitter makes no difference. As part of the "Guided tour of the security system" series, I'll illustrate several of the more common contacts and their uses below.
Recess contacts- These devices are installed inside the door jamb facing the side of the door. The switch is pushed into the jamb so that it is flush with the jamb itself. Directly opposite of the switch, a magnet is pushed into the side of the door so that it is also flush. They are installed similarly into a window, with the switch in the sill and the magnet in the sash. Once installed these switches are not visible until the door or window is opened. When the door is open, the magnetic force is removed, sending a signal to the control panel. These contacts are most commonly 3/8” or 3/4”. The magnet must be within 2” gap (typically) of the switch to maintain “contact”.
Surface contacts- These switches have many applications. When they are used in home security systems they are typically mounted on the exposed interior of the door or window. The switch is installed on the molding surrounding a door and the magnet is attached to the door itself, directly opposite of the switch. The application for windows is very similar. The magnetic gap is the same for these contacts is the same 2” as the recess contacts. Most wireless transmitters have surface contacts built into the circuitry of the transmitter. This allows the transmitter to be used as a contact as well.
Rollerball contacts- The rollerball contacts (or plunger) are switches that use mechanical force to sense the condition of the door. Unlike the other styles of contacts, these switches should only be used on doors that require the contact to be on the side of the door where the hinges are located. They are installed in the door jamb (on the hinge side) in a hole usually 3/4“. Like the recess contact above they are not visible when the door is closed. Unlike the magnetic switches, there are no magnets used with these contacts.
Overhead Door contacts- These switches are much more robust versions of surface switches. They are typically encased in a steel housing and mounted on the floor of the garage or warehouse. A large magnet is attached to the garage door directly opposite the switch. They are typically used in business security systems in an industrial location.