GB735766A - Improvements in or relating to burglar alarm and like warning systems - Google Patents

Abstract

735,766. Pressure gauges. LEVY, J. P., and MEEKS, L. E. Dec. 21, 1953 [Oct. 23, 1952], No. 26681/52. Class 106 (2). [Also in Groups XXXVII and XXXVIII] A pressure-sensitive device for use in a burglar alarm comprises two compartments 20, 21 separated by a diaphragm 22, the compartment 20 having an orifice 23. The device is placed within the chamber, e.g. a room, to be protected, the pressure within the chamber being maintained at a value different from that existing outside the chamber by means such as a blower or fan, and a pipe is taken from the nipple 44 communicating with the compartment 21 to the outside atmosphere; alternatively the device is placed outside the chamber and a pipe led into the chamber from the nipple 44. The device is so adjusted, in particular by adjustment of the position of tension springs 40, that, when the desired pressure differential is maintained, the contact arm 35 is held in a central position with respect to contacts 38, 39. When a change in the pressure differential occurs, for example due to the opening of a door in the chamber or a person making an entrance through an alreadyexisting aperture, the arm 35 makes contact with one of the contacts 38, 39 thus short-circuiting the resistor 51 forming one arm of a normally balanced Wheatstone bridge. The resistor 51 is connected to the remaining arms of the Wheatstone bridge, which are associated with the alarm and may be located at a remote point, through a pair of leads 18. Upon the bridge becoming unbalanced, the relay 79 is energized, closing contacts 82, thus energizing the self-holding relay 83 having contacts 87, 88 and 90 and causing the alarm, shown as a bell 89, to be operated. The Wheatstone bridge and relay circuits are normally supplied from the A.C. mains through a bridge-rectifier 60 ; in the event of failure of the mains supply, relay 73 is de-energized and battery 72 is switched into the circuit. Opening of switch 69 releases the selfholding relay 83 and allows the circuit to be re-set. In order to prevent the system being rendered inoperative by blocking of the orifice 23, a small aperture 22a is made in the diaphragm 22, whereby the pressures in the two compartments become equalized should the orifice 23 be closed.