Burglar alarm system

Sept. 6, 1938.

2,129,261

L. H. CHASE ET AL

BURGLAR ALARM SYSTEM.

Filed May 2, 1936

<img file="US2129261A_D0001.tif" />

ATTORNEY

Patented Sept. 6, 1938

2,129,261

UNITED STATES PATENT OFFICE

2,129,261

BURGLAB ALARM SYSTEM

Leland H. Chase, Little Neck, and Clifford L. ^Y” “»*&lt;“&lt;*8 to Holmes Electric Protective Company, New York N Y a corporation of New York ’ *

Application May 2, 1936, 4 Claims.

This invention relates to burglar alarm systems and more particularly to burglar alarm systems which are maintained and controlled from a central office.

B object of this invention is to provide a burglar alarm system which will register an alarm at the central office when an opening is made in the walls of a protected area either by opening a door or window or cutting a hole 10 through the wall to gain entrance to the vault or protected room.

Another object of the Invention is the provision of means controlled from the central office for simulating an opening in the protected area for IS testing purposes.

Heretofore burglar alarm systems have been constructed so that an alarm is transmitted to the central office when electric contacts on doors or windows are opened or closed; and when wires 20 on walls, doors or windows are broken or brought into contact with each other; also various systems have been devised which use photoelectric cells in combination with light beams, the interruption of which closes a circuit to cause an 25 alarm to be sounded.

While the above systems are satisfactory, they do have some drawbacks, are costly to install and need considerable supervision.

The present invention provides a burglar alarm 30 system which may be installed and maintained with a minimum amount of expense and which may be supervised with a minimum amount of effort.

In applicants’ system the air in the protected 35 enclosure is maintained at a pressure different from that in an adjoining area by means of an electric fan or other type of electrically operated air pump, which is mounted in an aperture in the wall of the enclosure. It is not important 40 that the enclosure be air-tight as long as the leakage is constant. This difference in pressure is transmitted from the enclosure to the adjoining area or vice versa through a hollow tube to a metallic bellows. The difference in pressure 45 on the inner and outer surface of the bellows wall will cause it to assume an abnormal length. One end of the bellows is arranged to act as one of the elements of an electric condenser in coniunction with an adjacent fixed parallel plate. This con50 denser is connected across the tuned plate circuit of a high frequency oscillator whose grid ^C«^CUl^t ?^{as an} extremely high Q characteristic afforded by a piezoelectric quartz crystal. The plate circuit of the oscillator is timed by a varia55 ble condenser so that the circuit has a slightly

Serial No. 77,522 (CI. 177—314) lower natural frequency than that of the crystal excitation; that is, the plate circuit is near resonance with the crystal but has a capacity reactance, resulting in an impedance considerably lower than that at resonance and with a very κ steep plate current characteristic relative to slight tuning capacity changes in either direction.

In operation, if the value of pressure difference ^th® ®^ncl°^sure and the adjoining area be 10 changed, either by changing the speed of the fan or by making an opening in the enclosure wall, the length of the bellows will change. Thus the distance between the fixed plate and the bellows is varied, their action changing the 15 impedance of the high frequency oscillator plate circuit with a resultant change in the oscillator mate current. By means of a series of relays change in the oscillator plate current, Whether positive or negative, is transmitted to 20 tne central office over a pair of wires where a zero center galvanometer and additional relays operate visual and audible alarms. This same apparatus constitutes an alarm indicating device for any failure of local power or apparatus. Ad- 25 ditional equipment at the local and central office locations provides means for testing the switching and operation of the entire apparatus.

A signaling key and an alternating current bell at the subscriber’s location, and a switching key 30 provided with a source of alternating current at the central office, provide a means for exchange of signals between the subscriber and the central office operator to afford cooperation at opening and closing times. ₃₍now to the drawing, the enclosure 10 has mounted in the wall thereof an electrically driven fan 11 which will either draw air from the enclosure 10 or pump it in. In either case the action of the fan 11 will cause a change in 40 the air pressure between the interior of the enclosure 10 and the adjoining area which may be in the order of .1 to .3 inch of water level. A hollow tube 12 transfers this pressure condition from the interior of the enclosure to the interior 45 of a flexible metallic bellows 13. Due to the difference in pressure inside and outside of the bellows 13, it will either expand or contract along its axis depending upon whether or not the pressure on the inside is greater or less, respectively than 50 the pressure on the outside. The T connection 14 at the junction of the tube 12 and bellows 13, affords a point whereby the pressure difference may be measured by means of a suitable pressure gauge (not shown). During the normally closed 55

S,129,361 period, that is, when the enclosure 10 is protected, and all the openings into the enclosure are closed the plate circuit of the oscillator 10 is tuned by means of the condenser 16. The plate 5 circuit is so tuned that the capacity in the circuit is slightly greater than the amount necessary for resonance with the piezoelectric crystal 11 in the grid circuit. The exact setting is determined by the plate current characteristic of the oscillator 10 as shown and described in C. L. Loudon Case 1, Serial No. 693,262, filed October 12, 1933.

Inasmuch as the capacity between the fixed plate It and the metallic bellows is connected in parallel across the tuned plate circuit through IS condensers 20 and 21, it is a part of the tuned circuit, and any change in its value will cause a change in the plate current of the oscillator II. In making the initial adjustments the distance between the fixed plate and bellows is adjusted to M give the desired plate current change for any pressure difference change. Any fixed plate adjustment is followed by a re-adjustment of condenser 16 to restore the proper electrical capacity condition.

2S The overload relay 22 is so adjusted that its contacts will close when the current through its winding increases to 8 milliamperes or more, and the underload relay 23 is so adjusted that its contacts remain closed as long as the current 90 through its winding is 3.5 milliamperes or more, but will open when the current falls below 3.5 milliamperes.

The resistances 24 and 25 are so adjusted that the direct current from the central office through M the alternating current bell 26, the changer magnet 21, the differential relay 28, the resistances 24 and 25 and the galvanometer 29 is 12 milliamperes and will rise to 15 milliamperes when the resistance 24 is short-circuited.

The galvanometer 29 is so adjusted that at 12 milliamperes the needle will be in the center of the scale and at 15 milliamperes it will be halfway between the center and the right-hand end of the scale. The resistance 30 is adjusted to 40 such a value that when the short circuit is removed therefrom the speed of the fan 11 will be reduced by an amount which will give the same pressure difference change as the minimum sized hole in the enclosure 10, which it is desired to ¢0 protect; that is, if an alarm is required from an opening of 16 square inches the fan speed must reduce to give the same pressure difference change as an opening of that area would give.

Under the above conditions a pressure change M inside the enclosure 19 will cause the bellows 13 to increase or decrease in length depending on the direction of the pressure change. For example, when the bellows 13 increases in elongation, the electrical capacity of the tuned plate 00 circuit will be increased due to the decrease in distance between the end of the bellows 13 and the fixed plate 18. This results in a lowering of the impedance of the plate load of the tube 31, which causes a decrease in the high frequency 05 voltage fed back to the grid of the tube 3i through inter-electrode capacities. This action in turn causes a decrease in the negative bias on the tube 31 due to a diminishing of rectified grid current through the grid leak 32 and will 70 cause an increase in the plate current of the oscillator.

When the displacement of the bellows 13 has progressed far enough to increase the oscillator plate current to 8 milliamperes the contacts of 75 the relay 22 wifi close, short-circuiting resistance and winding 28» of the differential relay 28. The current through its other winding 28^b being unopposed by the winding 28* will close the relay contacts, short-circuiting the resistance 21, thereby causing a large increase of current to 5 flow through the galvanometer 29, which causes contact 29» to close, and energize relay 33 which through its contacts will Jight the lamp 34 as a visual signal, and the lamp current through the relay 38 will operate the audible signal 36. 1&lt; The above condition is maintained by the armature and contact of the differential relay 28 providing a locking circuit which maintains the short circuit on the resistances 24 and 25 and on the winding 28», even though the pressure 1! difference, bellows 13, the plate current of the oscillator 15, and the relay 22 return to their original state.

In the condition where a change in the pressure difference between the interior of the en- ²ⁱclosure 10 and the adjacent area causes the bellows 13 to contract along its axis, the effective capacity in the tuned plate circuit of the oscillator 15 will become less, thereby causing the capacity in excess of the grid circuit resonance -> value to decrease. This will increase the plate circuit impedance, the voltage transferred to the grid and consequently the negative bias on the grid. The net result will be a decrease in the oscillator plate current. When this plate current ³⁰has dropped below 3.5 milliamperes the contacts of relay 23 will open, opening the line circuit through the galvanometer 29 and causing the contact 29^b to close, operating relay 31, which lights the lamp 38 and operates the buzzer 36. ³·’

In order that the system may be properly tested, a changer and contact assembly 39 have been provided. This assembly includes pairs of contacts 40, a cam 41 opening and closing said contacts, a pawl and stop 42 and the operating mag- 40 net 21. The characteristics of the magnet 21 are such that a current of about 100 milliamperes is necessary to pull up the armature 43 as compared with the normal line current of 12 milliamperes. This value of current is provided 45 by operating section A of the key 44, which brings In the high voltage battery 45. The impulses imparted to the changer magnet 21 by means of the key 44 causes the fiber cam 41 to rotate in steps by means of the pawl and ratchet C 3 42, so that three contact combinations may be obtained; namely, both pairs of contacts 40 open as shown, the short pair closed or the long pair closed. Due to the relative width of the cam 41 and the spacing of the contacts 40, both pairs ·^Γ·~> of contacts are never closed at the same time.

Using the above equipment two tests can be made at the central office at any time, which will give an indication of the alarm condition. For instance when the subscriber wishes to put the alarm into service he operates his key 46 which breaks the line circuit and lights the lamp 38 at the central office. By prearrangement the central office operator considers this as a closing signal. With the changer 39 in the position as 65 shown and with circuit conditions correct, the needle of the galvanometer 29 will be in its center position. The operator by operating section A of the key 44 transfers the high voltage battery 45 to the line momentarily. Due to the un- 70 balance between the windings 28^a and 28^b of the differential relay 28, caused by the shunt 41 across the winding 28», the increased current through both windings will cause a large enough resultant magnetic field to close the relay con- 75

3,128,861 tacts thereby providing a short path through the changer magnet 27. The changer moves the cam 41 to the point where the short inner pair of contacts 40 close. This short circuits the resistance 24 causing the line current to become 15 milliamperes and the galvanometer needle will move to the right of the center of the scale. This test is to guard against the possibility of an abnormal resistance in the line being compensated by a leakage across the circuit. Since the resistance 24 is at the end of the line circuit, such a condition would give a change on the galvanometer of different magnitude than normal.

For a second test the operator again transfers the battery 45 to the line momentarily, moving the cam 41 to the point where the long outer contacts 40 close, energizing relay 47' which in turn energizes relay 48. This latter step removes the short-circuit from the resistance 30 and will cause the speed of the fan I f to decrease by a predetermined amount to give a change in pressure equivalent to any chosen opening in the enclosure 10. The whole system then functions as on an alarm condition. The movement of the bellows 13 causes a rise in the oscillator current which finally results in the lighting of the lamp 34 and the operation of the buzzer 36 as heretofore described. If the above result is not achieved it indicates that the system Is not capable of giving an alarm under the required conditions.

A third impulse to the line from the battery 45 rotates cam 4( to the position where both pairs of contacts 40 are open (separated by the fibre cam). In this position the differential relay 28 will still be electrically locked in the operated position since the fan 11 does not pick up speed as quickly as the key 44 is operated and it is necessary to break the line circuit by a partial operation of the key 44 to release the lock on the relay 28 after which the circuit will again be normal and the needle of the galvanometer 29 will again be in its central position. Movement of the key 44 to its B position then rings the alternating current bell 26 and informs the subscriber by a prearranged code signal of the condition of the alarm. Although the high current impulse for operating changer assembly 39 operates differential relay 28 on each impulse the momentary break in the line circuit caused by restoring key 44 breaks the lock on relay 28 allowing the circuit conditions as controlled by changer contacts 40 to be shown on galvanometer 29.

Automatic supervision is afforded by the relays 22 and 23. If for any reason the plate circuit of the oscillator 15 should become detuned, the wiring or circuit elements become defective, or the po^T.,er supply fail, one of these relays will operate, showing an alarm condition at the central office.

Loss of sensitivity will be accompanied by a decrease in the oscillator plate current, caused by the detuning of the oscillator plate circuit, which may be caused by a change in pressure which increases the distance between the bellows 13 and the fixed plate 18. The release current value of underload relay 23 affords a means for predetermining the amount of sensitivity which will give an alarm condition.

A bimetallic plate 49 near the plates of condenser 16 compensates for expansion and contraction in the plate circuit elements caused by temperature changes. Since the piezoelectric quartz crystal Π in the grid circuit has an extremely low temperature coefficient, the temperature effect on the whole system is negligible.

The bellows housing 19, with its adjustable vent 50, provides an adjustable damping chamber for 5 the bellows. Its purpose is to give a slight delay to the bellows action so that drafts causing transient changes in the fan speed will not register as alarm signals. This adjustment is obtained by choice of length of tubing 12. 10

A switch 51 is provided to open the local relay battery circuit. Thus, if the changer mechanism should be left in the condition where the long outer contacts 40 are closed, the battery 52 will not become exhausted. 15

A switch 53 is provided to shut off the fan 11. The operation of the above two switches is part of the subscribers’ opening and closing routine.