Actuating a Reed Switch
In all reed switch actuation applications, a magnet and a reed switch must be brought to within a specific distance of each other. This distance will vary in accordance with the magnetic sensitivity of the reed switch and the strength and size of the magnet. When the magnet is close enough, the Normally Open contacts will close or operate. When the magnet is taken away, the contacts will open or release. The relative distance for an operate is always less than the distance for a release. There are various ways in which a reed switch is actuated with a magnet and some examples of magnet motion are shown below.
When the magnet is moving parallel to the reed switch, it provides as many as three closures with maximum magnet travel and one closure with minimum magnet travel. It should be kept in mind that approaching a reed switch with a single pole of magnet is not advisable.
Rotating the magnet or reed switch, normal to their axes, reverses magnetic polarity resulting in two closures per revolution. When these axes are parallel, the switch closes. When the axes are perpendicular, the switch opens. Although the poles reverse, they still induce the opposite poles that close the reed switch. This is because a reed switch is an omni-polar device.
A biasing effect is produced by placing a stationary magnet permanently near the reed switch, to keep it in a Normally Closed state. The approach of another magnet with reversed polarity cancels the magnetic lines of force, and the reed switch opens. Care should be taken not to bring the actuating magnet too close to the biased reed switch, as it could close again (re-closure). Form A type reed switches meant for this kind of application should be selected from a release AT group instead of from an Operate AT group.
In this type of actuation, magnet and reed switch are permanently fixed in such a position that the reed switch contacts are closed. A piece of Ferro-magnetic material is passed between the magnet and the reed switch, to cause a drop out. The magnetic field that is keeping reed switch closed, is shunted by the external ferro-magnetic material, eliminating the attraction between the reeds. When the shield is removed, the reed switch closes once again.