One of the most frequently employed switches in alarm systems is the
magnetic reed switch shown in Fig. 1. As shown by the figure, there
are two components of the reed switch-a permanent magnet embedded
in one unit that is normally connected to the movable element (door, window, and so on) and a reed switch in the other unit that is connected to the electrical control circuit.
Fig. 1: Magnetic reed switch.
The reed switch is constructed of two iron-alloy (ferromagnetic) reeds in a hermetically sealed capsule. The cantilevered ends of the two reeds do not touch but are in very close proximity to one another. In the absence of a magnetic field the reeds remain separated. However, if a magnetic field is introduced, the reeds will be
drawn to each other because flux lines seek the path of least reluctance
and, if possible, exercise every alternative to establish the path of least
reluctance. It is similar to placing a ferromagnetic bar close to the ends
of a U-shaped magnet. The bar is drawn to the poles of the magnet, establishing a magnetic flux path without air gaps and with minimum reluctance. In the open-circuit state the resistance between reeds is in excess
of 100 MΩ, while in the on state it drops to less than 1 Ω.
Fig. 2: Using a magnetic reed switch to monitor the
state of a window.
In Fig. 2 a reed switch has been placed on the fixed frame of a
window and a magnet on the movable window unit. When the window is
closed as shown in Fig. 2, the magnet and reed switch are sufficiently close to establish contact between the reeds, and a current is established through the reed switch to the control panel. In the armed
state the alarm system accepts the resulting current flow as a normal
secure response. If the window is opened, the magnet will leave the
vicinity of the reed switch, and the switch will open. The current
through the switch will be interrupted, and the alarm will react appropriately.
One of the distinct advantages of the magnetic reed switch is that the
proper operation of any switch can be checked with a portable magnetic
element. Simply bring the magnet to the switch and note the output
response. There is no need to continually open and close windows and
doors. In addition, the reed switch is hermetically enclosed so that oxidation and foreign objects cannot damage it, and the result is a unit that
can last indefinitely. Magnetic reed switches are also available in other
shapes and sizes, allowing them to be concealed from obvious view.
One is a circular variety that can be set into the edge of a door and door
jam, resulting in only two small visible disks when the door is open.