Reed switch arrays

#The present application is the U.S. national stage application of International Application PCT/GB2005/050252, filed Dec. 16, 2005, which international application was published on Jun. 22, 2006 as International Publication WO 2006/064292. The International Application claims priority of British Patent Application 04 27886.7, filed Dec. 18, 2004.

#This invention relates to reed switches, and in particular to an array of reed switches and methods of assembling such an array. The invention further relates to a control circuit for controlling the operation of an array of reed switches and yet further to a method of controlling such an array.

#A typical reed switch has two blades of a ferromagnetic structure mounted in a tubular glass envelope, which supports the blades mechanically and provides a hermetic seal for the contacts. The free ends of the blades provide contacts which can come into engagement with each other to make the switch, the reed blades being plated in the contact area to ensure they have good switching characteristics. Most reed switches are manufactured so that the contacts are normally open but when a magnetic field is applied, the blades are drawn together to make contact. The magnetic field can be provided by a permanent magnet which moves relative to the switch to make or break the contact, or the switch may be operated by a wire coil provided around the outside of the glass envelope. Current passing through the coil generates an axial magnetic field which draws the blades together to make contact, so long as the magnetic field strength is high enough to overcome the resistance of the blades to bending. A typical reed switch might require 20 Ampere-turns (AT) for consistent operation, depending upon the switch ratings, reed size and other factors.

#By way of example only, certain specific embodiments of this invention and of methods in accordance with this invention will now be described in detail, reference being made to the accompanying drawings in which:—

#FIG. 1 shows the mounting of a reed switch as a first embodiment of this invention;

#FIG. 2 shows an alternative mounting of a reed switch, as a second embodiment of this invention;

#Referring initially to FIG. 1, there is shown the implementation of a reed switch mounting arrangement permitting the construction of an array of reed switches according to the invention. A printed circuit board (PCB) mother board 10 supports a daughter board 11 to extend parallel to the mother board. The daughter board, which may also be a PCB or may take some other form as will be described below, is supported on pillars or a connector in a manner known in the PCB art. The daughter board 11 has an aperture 12 formed therethrough, which aperture takes the form of a round hole within which is mounted a reed switch 13 comprising a tubular glass envelope 14 supporting a pair of ferromagnetic spring blades 15. The free ends of the blades are formed as contacts which move together in order to make the switch, when the reed switch is subjected to an external magnetic field. The envelope 14 is shown diagrammatically and provides a support for the blades and also hermetically seals the switch contacts from the ambient.

#A wire wound coil 16 is bonded directly to the daughter board in order to surround the envelope 14. The ends (not shown) of the coil are connected by soldering to tracks (also not shown) provided on the daughter board. The connecting leads 17 for the reed switch 13 are wired to the mother board, the lead from the lower end of the envelope 14 connecting directly to the mother board and the lead from the upper end of the envelope being turned through 90° twice and then being fed through a hole formed in the daughter board, also to connect to the mother board.

#As shown in FIG. 1, the reed switch envelope 14 is offset with respect to the daughter board, such that the contacts of the blades 15 lie in the region of the wound coil 16 and so in the area of greatest magnetic field.