Patent GB-665750-A (GB)
Title: Improvements in or relating to electrical contact operating devices for use in burglar or like alarm systems
IPC: H01H35/00
Filed: 1949-02-22
Abstract: 665,750. Actuation of switches. ERICSSON TELEPHONES Ltd., and STEVENS, J. R. H. Feb. 20, 1950. [Feb. 22, 1949] No. 4746/49. Class 38 (v). [Also in Group XXXVIII] A contact operating device comprises a plurality of contact operating cams fixed or frictionally coupled to a shaft which is spring urged to rotate in one direction and a cable, which is wound round the shaft and carries a link adapted to engage with a fixed bracket, so that any displacement of the cable rotates the shaft. The shaft is rotated in one direction and then in the reverse direction during the engagement of the link with the bracket. The invention is applicable to burglar alarm systems. Figures 1 and 2 show a contact unit enclosed by a cover 2, and attached to one end of a cable 5 which is stretched across a window or other aperture to be protected, the other end of the cable being attached to a link 9 which engages with a fixed bracket 10. Fig. 3 shows the rear view of the contact unit with the cover removed. The cable 5 passes through a conical stopper 4 and is wound round a drum attached to spindle 31, so that any movement of the cable out of the contact unit moves spindle 31 in an anticlockwise direction as shown in Fig. 3. Springs are provided to urge spindle 31 in the clockwise direction. Spindle 31 carries a fixed cam 25, and also frictionally engages a cam 23 by means of a spring washer 24. Movement of cam 23 is limited by stops 26 and 27, and movement of the fixed cam 25 and spindle 31 is restricted to one revolution, when a projecting tooth on the cam engages a pin 44 from either direction. A contact assembly 29 is operated by a lever 45 adapted to be engaged by the tooth on cam 25, and/or a projection on cam 23 which operates the contact assembly when cam 23 engages stop 27. The length of cable 5 is adjusted to the size of the aperture concerned, such that when setting the device, link 9 is pulled to rotate spindle 31 against the spring tension until the centre portion of link 9 can be passed through the narrow entrance portion of the slot in bracket 10, and then the cable restores until the wider end portion of link 9 engages with the wider, inner portion of the slot in bracket 10. After this setting operation the tooth on cam 25 is positioned midway between the two limiting portions caused by pin 44, and moving cam 23 is positioned against stop 26, so that the contact assembly 29 is not operated. Any subsequent movement of the cable 5 out of the contact unit will rotate spindle 31 in an anticlockwise direction, so that the contact assembly 29 will be operated by movement of cam 23, and any movement of the cable into the contact unit will also operate the contact assembly 29 due to the clockwise rotation of cam 25. The contact assembly may be used to control an indicating lamp 3, and may be utilised in various electrical burglar alarm systems such as a multi-wire system in which an alarm is given if one or more conducting circuits is interrupted. Additional cams and contact assemblies may also be provided. A lever 8 is also provided, to transmit any displacement of stopper 4 to cam 23, so that any attempt to remove cover 2 when the device is set, introduces alarm conditions.