Reed Switch Assembly Employing A Magnetic Screen

Abstract

A reed switch (for example, for a keyboard controlling a transmitter) is controlled by a reciprocable high-permeability collar which can be slid axially onto the reed to screen the latter from a stationary magnet. This achieves low crosstalk and permits high-packing density.

Description

This invention relates to a reed switch assembly.

According to the present invention there is provided a reed switch assembly comprising a plurality of reed contact units, a magnet system fixed relative to the contact units for closing their contacts, and a respective high permeability screen associated with each contact unit, each screen being movable between a first position at which it screens the associated contacts from the magnetic field of the magnet system thereby allowing the contacts to remain open, and a second position at which it allows the magnetic field to close the contacts.

FIG. 1 shows a conventional prior art reed switch assembly, and

FIG. 2 shows an embodiment of a reed switch assembly according to the invention.

Referring to FIG. 1, a prior art assembly comprises a plurality of reed contact units 1 each having an associated magnet 2. With the magnets in the position shown the magnetic field across the reed contacts is sufficient to close them. However, each magnet is movable relative to its associated contact unit to a position at which its magnetic field is not sufficient to close the respective reed contacts. This position is indicated by the broken line for the center magnet of FIG. 1. Thus each contact unit can be selectively controlled by movement of its associated magnet.

A disadvantage of this arrangement is that if a high packing density of reed contact units is desired, crosstalk between adjacent magnet contact unit pairs becomes sufficiently great for the reliable controlling of the contact units to be impaired.

FIG. 2 shows schematically a reed switch assembly which to some extent overcomes this disadvantage. The assembly comprises a plurality of parallel reed contact units 1, and a magnet system 3 fixed relative to the contact units for closing their contacts. In this embodiment the magnet system consists of a single permanent magnet magnetized as shown but it could comprise fixed individual magnets or electromagnets which could be deenergized when the assembly is not in use. Each contact unit 1 is surrounded by a respective high permeability hollow cylindrical screen 4, and when these are in the positions shown they screen their associated reed contacts from the magnetic field of the permanent magnet 3 thereby allowing the contacts to remain open despite the magnetic field.

However, each screen 4 is movable in a direction parallel to the axis of its associated contact unit to a position at which it allows the magnetic field of the magnet system to close the contacts. This position is shown for the center screen by the broken lines. Thus the contact units are selectively controllable by moving the screens 4. The axial movement of the screen is greatly exaggerated in the drawing; in practice a movement of a few millimeters would be sufficient.

A practical application of the assembly described with reference to FIG. 2 is in an electronic keyboard controlling a transmitter of, say, a teleprinter. In this case the screens are selectively movable in response to the depression of selected keys. Each screen can be moved by a stiff wire or strip attached at one end to the screen and extending in the direction of movement of the screen. It is not necessary for the wire or strip to support the entire weight of the screen, since the latter can be at least partly supported by the associated contact unit.

We prefer to form the screens from initially flat rectangular portions of high permeability material which are bent into a cylindrical shape, but screens of other configuration are possible. Furthermore, in the case of screens which do not substantially surround their contact units, their direction of movement need not be parallel to the axis of the contact units.

Claims

We claim:

1. A reed switch assembly comprising a plurality of reed contact units, a magnet system fixed relative to the contact units for closing their contacts, and a respective high permeability screen associated with each contact unit, each screen being formed as a hollow cylinder surrounding the associated contact unit, each screen being movable in a direction substantially parallel to the axis of its associated contact unit between a first position at which it screens the associated contacts from the magnetic field of the magnet system thereby allowing the contacts to remain open and a second position at which it allows the magnetic field to close the contacts.