US3408642A - Burglar alarm systems - Google Patents

Description

1968 R. A. PALLADINO 1 BURGLAR ALARM SYSTEMS 2 Sheets-Sheet 1 Filed March 4, 1966 INVENTOR ROBERT A. PALLADINO ATTORNEY POWER SOURCE R. A. PALLADINO BURGLAR ALARM SYSTEMS Oct. 29, 1968 2 Sheets-Sheet 2 Filed March 4, 1966 INVENTOR ROBERT A. PALLADINO ATTQRNEY CORRECT r 3;- Y

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United States Patent 3,408,642 BURGLAR ALARM SYSTEMS Robert A. Palladino, 34 Aspen Road, West Orange, NJ. 07052 Filed Mar. 4, 1966, Ser. No. 531,677 Claims. (Cl. 340276) This invention relates to burglar alarm systems, and especially to improvements in the type of system which is controlled by a combination or permutation of numbers in a numerical sequence entered into the system, for instance by dialing a multiple-digit sequence, for the purpose of controlling the alarm system.

It is a principal object of the present invention to provide a system in which authorized personnel are provided with a number comprising a series of digits which they can enter into the system by way of a telephone-type dial in order to either test the system, or to turn the system on, or to turn the system off, whereby no keys or other mechanical devices are necessary in order to control the system while avoiding the setting off of needless alarms.

It is another important object of this invention to provide an improved system in which authorized personnel can change the digits comprising the correct combination in a simple and quick manner so as to make it easier to maintain full security, and inrwhich there is no limit to the number of digits which can be used in a correct combination, additional digits being provided at relatively small extra expense.

Another object of the invention is to provide an improved system wherein the dial by which th correct combination of numbers is entered, when controlling the system, is located inside the guarded premises, and in which the system provides two time delays, one of which delays initial activation of the alarm system long enough after dialing the combination to permit authorized personnel to unlock the premises, enter, and dial the correct combination to turn off the system before an alarm is actually sounded.

Another important object of the invention is to provide an improved burglar alarm system in which the burglar sensing wiring around doors and windows is provided with a certain value of ohmic resistance which the alarm system is adjusted to continuously measure, and wherein an alarm will be set off if the value of this resistance is changed. In this way, unauthorized persons cannot gain access, by either clipping the wiring or by short-circuiting it, without setting off the alarm. v

Still another object of the invention is to provide a system having three dial-controlled conditions: Off, Delay with System Checking, and On. An authorized person departing the guarded premises will dial the correct combination to change the system from off to delay, and this delay period is adequate to permit the departing person time during the delay period to observe system indicator means which check certain specific conditions of the system, including continuity of the burglar sensing wires and the condition of a stand-by emergency battery. After this delay period has terminated, the system then automatically moves to on condition where it remains until it is either tripped into action or until the correct combination 'is redialed to change the system to off condition.

Another object of the invention is to provide a system in which the alarm, once set off, can be disabled only by dialing the correct combination again into the system. At any time when a person dials a wrong digit into the system, the alarm is immediately set off.

A further object of the invention is to provide a system in which battery stand-by power is provided so that the system will not be defeated by a general power failure at the guarded premises. The system may be arranged if desired so that a power failure will set off the alarm, or alternatively so that a general power failure will merely change the system over to battery operation.

A further object of the invention is to provide an improved remote alarm system to be installed at a local police station, this alarm system providing different alarm which respectively signify the difference between ,unbalancing of the resistance measuring bridge, as by tampering with the burglar sensing wiring, and power failure at the guarded premises. Cutting or short-circuiting of the wires to the police station will also result in the latter type of alarm being set off.

Other objects and advantages of the present invention will become apparent during the following discussion of the drawings, wherein:

FIGS. 1A and 1B when placed side-by-side together form a schematic illustration of a system according to the present invention.

General description Referring now to the drawings in detail, the present system comprises a burglar alarm which is operated by a dial resembling a telephone dial, by way of which persons can dial numbers into the system. If the person dials the correct number (three digits in the presently illustrated embodiment), the alarm system is advanced into a brief delay period, from which it then automaically moves into an operative or on condition. On the other hand, if the person dials any incorrect digit when the system is in on condition, the alarm is immediately set off and can be turned off only by dialing the correct number sequence into the system.

In order to accomplish th above purpose, the system shown in the schematic diagram includes a dial 15 which operates a first stepping relay 20 to step the contacts located above the winding on the schematic diagram through a certain number of positions depending upon the digit entered via the dial 15. This dialing sequence sequentially grounds one of a group of wires connected to the terminals of a second stepping relay 30, these terminals being in a vertical column generally indicated by the reference numeral 31. This stepping relay is of a type which goes through its four sequential positions and then begins the sequence again. The first three positions correspond with the three digits to be entered by way of the dial 15, and the fourth digit serves a switching purpose to be hereinafter explained.

The outputs from the row of stepper terminals 31 are connected to the terminals of three IO-pcsition-two-circuit selector switches appearing in columns above the reference character 40, and including the switch 41 which corresponds with the first digit to be dialed, the switch 44 which corresponds with the second switch to be dialed and the switch 47 which corresponds with the third digit to be dialed. Each of the switches includes two sections. One of these sections in each switch is of the type in which all contacts are shorted except the selected one, for instance as illustrated by the switch sections 42, 45, and 48. The other switch section is of the type in which all of the contacts are open except the one which is selected, and these sections are labeled 43, 46, and 49. The wires which represent wrong numbers entered via the dial are respectively selected by the switch sections 42, 45, and 48, and the wires which represent the correct digits in each dialing sequence are selected by the switch sections 43, 46, and 49. The three selector switches 41, 44, and 47 are controlled by separate shafts (not shown) and are located in an inconspicuous or lock-protected place since they control what combination of digits must be dialed for the purpose of controlling the present alarm system.

Whenever a correct digit is dialed, a relay 50 is actuated via the appropriate switch section 43, 46, or 4'9, but whenever an incorrect number is dialed, a different relay labeled 60 is energized through an appropriate switch section 42, 45, or 48. Whenever the relay 60 is energized the alarm is sounded immediately. The appropriate contacts for the relays 50 and 60 are shown to the right of the relay windings.

The alarm system itself includes, for example, two different bells, and also includes time-delay circuitry which provides a brief delay interval before the system sounds an alarm in order to permit authorized personnel, to have time to set the alarm and leave the premises, or to enter and dial a number to turn off the system. The delay and alarm circuits are generally shown to the right in FIG. 1B.

The following detailed discussion supplements the above general description of the layout of the system:

Dialing system The dial 15 and the first stepper 20 cooperate to function in the manner of a conventional telephone dialing system having ten selectible digits for each dial rotation. The stepper 20 has two circuit sections including the lower section 21 and its wiper 22, and the upper section 23 and its wiper 24. The stepper has two solenoid actuators, including a winding 25 operative to advance the wipers 22 and 24 to the next sequential position, and including a reset winding 26, which when energized resets all the wipers to their rest positions in which they appear on the present drawing. The stepper 20 is advanced everytime the switch 16 is closed as the telephone dial returns through one of its digital positions, whereby the switch 16 pulses the winding 25 with a series of pulses whose number is determined by the number to which the dial is manually turned. When the dial 15 reaches its rest position, the switch 17 is mechanically closed, this switch being open Whenever the dial is in any other position.

It will therefore be observed that whenever the dial 15 has stepped the stepper 20 out of its rest position, a circuit is completed through the wire 27 and the bank of contacts 21 to the wire 29 from the power source P so as to place positive voltage upon one end of each of the relay windings 50 and 60. If the other end of either of these relay windings is grounded through stepper section 23 by way of the terminals 31 and the selector switches 40, then that relay, 50 or 60, will become energized.

It is the function of the second stepper 30 via its contacts 31 to join the stepping bank 23 of the dial-operated first stepper 20 with either the selector switch 41, 44 or 47 depending upon which digit in the sequence of threedigit dialing is being dialed. There are ten wipers to the left of the contacts 31 and these wipers are respectively connected to positions 1 through of stepper bank 23. When the wipers are all connected to the lowermost of the four positions in each group of contacts 31, all of the ten positions from stepper bank 23 are then connected to ten positions of the selector switch 41 corresponding with the first operation of the dial. In the presentschematic, it will be noted that the switch 41 is set on position 8 so that if a person dials 8 in the first digit, when the dial returns to rest position closing switch 17, a ground connection will be made through switch 17 and to terminal 8 in stepper bank 23 and to the switch section 43 whichhas been manually set to the number 8 position. A ground connection will thereby be made to the wire 59 and to the upper end of relay winding 50 to energize the correct-number relay 50.

When this correct first digit has been dialed, the relay 30 steps all 10 wipers to the next-higher contacts in the groups of contacts 31, and these contacts are connected to the selector switch 44 corresponding with a second operation of the dial. When the dial returns to rest position the second time and the switch 17 closes, if the operator has dial 5, the wiper 24 will be on contact 5 of stepper bank 23, thereby providing a grounded circuit through selector switch section 46 to the wire 59 to again energize the correct-number relay 50. The second closing of the relay 50 advances the stepper 30 to place the Wipers on the third-contacts-up in the stepper contacts 31, thereby connecting the 10 positions of the stepper bank 23 to the 10 positions of the selector switch 47. If the person dials 1 for the third digit, the relay 50 is closed again, thereby advancing the second stepper 30 to the fourth position in the bank of contacts 31, which contacts are all unused in the bank 31 although the fourth contacts are used in the bank 32 located directly beneath the winding of the stepper 30. The bank of contacts 32 then carries out certain functions which will be described hereinafter, culminating in resetting the second stepper 30 to the first (lowermost) position.

On the other hand, whenever a person dials a wrong digit, that is, a number different from 8 in the first digit, 5 in the second digit, or 1 in the third digit, a wire is grounded through the bank 23 to one of the selector switch sections 42, 45, or 48, thereby energizing the wire 69, closing the relay 60 and immediately setting off the alarm. Means is provided by which the alarm continues to operate until it is manually reset by a person dialing the correct sequence of digits, 8-5-1, as will presently be described. Thus, the dialing of just one wrong digit in any sequence immediately sets off the alarm.

Although the illustrated positions of the selector switch 40 show that the correct number to be dialed is 8-51, this number can be changed in the first digit by moving the switch 41, in the second digit by moving the switch 44, or in the third digit by moving the switch 47. In this manner any desired three-digit combination can be selected.

Correct num'ber dialed Whenever a correct digit has been entered in the dial 15 in the manner described above, the wire 59 becomes grounded, thereby energizing the correct-number relay 50 and moving the contacts 51 and 52 to their lower positions. When the contact 51 is in the lower position, it breaks the circuit through the dial switch 17 to the stepper bank 23 until the relay 50 becomes deenergized again, at which time the contact 51 again closes the circuit from the switch 17 to the band 23 to energize the next digit. Moreover, the contact 51 places ground current upon the wire 39 through the switch 17 to pulse the second stepper 30 to its next sequential position. When relay 50 closes it advances stepper 30 and returns stepper 20 to its rest position. When stepper 20 is returned, it breaks the circuit through wire 29, thus releasing relay 50 to drop open in preparation for the next dialing sequence. Whenever the relay 50 is closed, the contact 52 is lowered, thereby connecting ground current to the wire 28 and energizing the reset winding 26 to return the first stepper 20 to its rest position in preparation for the dialing of the next sequential position.

When all three selected digits have been correctly dialed, the closing of the contacts 51 for the third time pulses the second stepper 30 into its fourth position in which the wires 33 and 34 are both grounded at contacts 32. The stepper 30 remains in the fourth position pending the following sequence of events.

There is a third sequential stepper which controls the on-otf condition of the alarm system and operates four sets of contacts 81 sequentially through three positions, the cycle being continuous and progressing through three contact positions as the winding 80 is progressively grounded at its upper end. Assuming that the contacts 81 are in their off position as shown in the drawing, when the contacts 32 move into the fourth position and ground the wire 34, the upper end of the winding 80 is grounded, thereby moving the contacts 81 into their intermediate position, labeled delay. In this position of the contacts 81, the wiper 82 connects a voltmeter V temporarily across a battery B, which serves the purpose of sustaining the alarm in spite of a power failure, and this voltmeter provides a momentary showing of the condition of the battery. Also in this delay position of the stepper 80 the wipers 83 and 84 connect the wires 86 and 87 in series between a light bulb 88 and the plus 12 volt supply so that if the light bulb lights, it provides an indication that the burgler sensing wiring W which is attached to the premise around its doors and windows is continuous. This wiring of the building is connected to terminals 87a and 86a, and comprises one or another well known type of resistance tape, for instance having a resistance of about 50 ohms per foot. There is a normally-closed switch contact 64 connected in series between the wire 86 and the contact 8611 when the system is alert, and the reason for this contact will be discussed hereinafter.

When the third stepper 80 was energized via the wire 34, it also closed contacts 89 and connected grounded wire 33 to wire 38 and wire 39, thereby energizing the winding of the second stepper 30 to step it back to its first position (the one shown in the drawing).

The contact 85 is grounded, and when the stepper 30 is in the intermediate or delay position the contact 85 energizes a time delay relay 90 having a single normallyopen contact 91. The interval of delay can be selected to suit the desires of the owner, but a delay of approximately 90 seconds is desirable to give the operator time in the delay condition of the system to observe that the lamp 88 is lighted to confirm continuity of the sensing wiring W, that the voltage measured by the voltmeter V across the alarm battery B is adequate, and then to leave the premises after locking the door and before the 90 second delay is over. When the delay ends, the relay 90 closes the contacts 91, again grounding the coil of the third stepper 80 and stepping the contacts 81 into the third or on position. The stepper 80 then remains in this position until someone enters the correct sequence of digits into the dial 15.

As mentioned above, when the time delay relay 90 closes, it advances the third stepper 80 to move the contacts 81 to the third or on position, thereby deenergizing the time delay relay 90, taking the voltmeter off of the battery B, and moving the contacts 83- and 84- to their uppermost positions in which the wires 86 and 87 coming from the resistive burglar wiring W are connected across the open leg L of the Wheatstone bridge 92. The resistors 93, 94, and 95 are selected to operate with the ohmic value of the burglar wiring W in order to balance the Wheatstone bridge 92. The resistance 95 is made adjustable to effect exact balance of the bridge so that no voltage appears across its terminals a and b when the terminals 0 and d are connected respectively between the 12 volt supply and ground through the wiper 85.

The circuit remains quiescent in the on position of the stepper 80 so long as the bridge 92 remains balanced, and so long as nobody dials a wrong number into the dial 15. However, if someone either short-circuits the burglar wiring W connected to the terminals 86a and 87a, or alternatively breaks this wiring, the Wheatstone bridge 92 becomes unbalanced, and current flowing through the wires 92a and 92b closes the sensitive relay 96, thereby moving the contacts 97 and 98 into their lowered positions. The making of contact between the wires 92a and 97a keeps the relay 96 energized because it short-circuits part of the bridge 92 and thereby maintains it in unbalanced condition even though the burglar wiring may be restored. The closing of the contact 98 places ground potential upon the wire 99, and therefore energizes the time delay relay 100, which relay waits a certain number of seconds and then closes, setting off the alarm. The delay, of for instance 30 seconds, interposed by the relay prevents the sounding of the alarm long enough to make it possible for authorized personnel to open a door, enter the building, and take steps to prevent the alarm from going off by redialing the correct sequence of digits, in the manner to be presently described. If the unbalance of the bridge is not counteracted by an authorized person who knows how to do it within the 30 second time-delay assumed for the relay 100, at the end of this delay the relay 100 will operate to open its normally closed contact 101, which results in setting off the alarm.

Alarm The alarm system is shown in the upper right-hand corner of FIG. 1B and includes the battery B, a bell connected so that it can be sounded by the battery despite a commercial power failure which disables the power source P, and further includes a second bell labeled 70 which is connected on one side to the power source P through a wire 72 so that it can be powered independently of the battery B. The alarm system also includes two other relays 73 and 111 whose windings are connected in series between ground at one end through a fuse 109, and the plus 12 volt supply line through the normally-closed contacts 101 of the time delay relay 100. As long as the contacts 101 are closed (the relay 100 deenergized) the relay windings 73 and 111 are both energized so that the contacts 74, 75, 76, and 112 are in their lower positions as illustrated in dashed lines, whereby the wire 71 is held disconnected from ground so that the alarm 70 is disabled, and also so that the contact 112 is open and the bell 110 is disconnected from the battery B.

However, when the 30 second time delay has ended and the relay 100 closes due to unbalance of the bridge 92, the contact 101 opens, thereby deenergizing windings 73 and 111 and permitting the contacts 74, 75, 76 and 112 to return to their normal positions illustrated in solid lines, in which positions the bell 70 is connected to ground via the wire 71 so that it is caused to continuously ring, and so that the bell 110 is connected across the battery B via the contact 112, thereby also causing it to continuously ring. As pointed out above, the relay contact 97 throws a permanent unbalance on the bridge 92 so that the relays 96 and 100 both remain energized to main tain the alarm continuously operative.

The only way to stop the alarm is to dial the correct sequence of numbers into the system again by actuating the dial 15. If a wrong number is dialed, nothing happens and the alarm continues to sound, but if the right digits are dialed, each correctly dialed digit advances the second stepper 30 in accordance with the sequence set forth above. The first and second numbers which are dialed merely advance the second stepper 30 to receive another digit, but the third correct number advances the stepper 30 to its fourth position, thereby grounding wire 34 at bank 32 and advancing the third stepper 80 from the on position back to the OE position and thus removing power from the bridge 92 and the relays 96 and 100 so that the contacts 101 resume their normally closed condition. When the relays 73 and 111 are reenergized, they close to break the contacts 74 and 112 which were causing the bells to ring.

The present system includes one additional alarm feature provided by the wires 115 and 116, which may comprise telephone lines leading to a local police station. At the far ends of the lines, are located two oppositely poled diodes 117 and 118, and each of these diodes is connected in series with the winding of a relay 120 and- 121 respectively. It will be seen that in the on condition of the system (relays 73 and 111 energized and contacts 75 and 76 down) the line 115 is positive with respect to line 116 and the relay 120 will be energized to close 7 the contacts 122 and disable the alarm 124. The contacts 123 will remain open so that the alarm 125 is also disabled. Recalling that the alarm is set off whenever the relays 73 and 111 become deenergized, it will be noted that deenergizing of relay 73 moves the contacts 75 and 76 to their uppermost positions so that the wire 116 becomes positive with respect to the wire 115. Therefore, the relay 120 will open and the relay 121 will close and only the alarm 125 will be connected across the battery A if unbalance of the bridge caused the alarm situation. But, if this change was caused by a general failure of power at the source P, then only alarm 124 will ring since neither relay 120 nor 121 will be energized.

Wrong number dialed As stated above, whenever a wrong number is dialed in any digit, the relay 60 is immediately actuated thereby moving the four wipers 61, 62, 63, and 64 from the upper (rest) positions shown in the drawing to their lower positions; When this occurs, the lowermost contact 61 grounds the wire 71 setting off the alarm device 70, for instance illustrated as a bell, which continues to ring so long as the relay 60 is energized. When the relay contact 62 moves downwardly, it breaks the circuit between the normally closed switch 17 at the dial and the selector bank 23 so that even if another digit is dialed it will not have any effect on the system once the alarm has gone off. When the wiper 63 is moved to the down position, it grounds the wire 28, thereby energizing the reset winding 26 of the first stepper 20, and returning both of its banks to the rest position shown in the drawing. Inasmuch as the first stepper has been returned to rest position, the ground current path is removed from the wire 69, and the relay 60 drops open, thereby returning the wipers 61, 62, 63, and 64 to their up positions as shown in the drawing. However, when the contact 64 of the relay 60 was moved to the down position as a result of dialing a wrong number, it broke the circuit to the external burglar wiring W, thereby unbalancing the bridge 92 just as though the building were being burglarized, and when the bridge 92 was unbalanced, it closed the relay 96, and applied a continuous short-circuit across the bridge via the wire 97 to maintain the bridge unbalanced. Therefore, at the end of 30 seconds, the time delay relay 100 closes and deenergizes relays 73 and 111, causing continuous sounding of the alarms 70 and 110, as well as alarm 125, until a person equipped with the correct number dials this number into the system by way of the dial 15, thereby resetting the system to off condition.

The structures of the stepping relays 20, 30 and 80 have been simplified by omitting the intermittent interrupter contacts which are part of standard stepping relays such as Universal Relay Corporation Model R-977 or Type 26. The capacitors across the windings reduce inductive kicks and line noises.

The present invention is not to be limited to the exact illustrative embodiment, for obviously changes may be made therein within the scope of the following claims.

I claim:

1. An alarm system for protecting a guarded area having intrusion sensing wiring connected in series, comprising:

(a) a source of power;

(b) alarm means;

(c) first relay means connected in circuit between the source and the alarm means and holding the circuit open when the first relay means is energized;

(d) measuring means coupled to said sensing wiring and sensitive to changes in the resistance of the latter;

(e) second relay means connected to the measuring means to be actuated by the latter in response to a change in said resistance, and including normallyclosed contacts connected to energize the first relay means from said power source unless the, second r'elay means is energized; (f) digital switching means connected to control the on-otf condition of the system by controlling said digital means including means for preselecting a plural-digit numerical sequence and renderingth'e switching means responsive to the selected sequence to alternately change its condition between ona;n'd

Off; I

(g) digital entry means connected to said switching means to enter a sequence of digits thereinto; and V (h) means connected to said switching means for cou; pling thealarm means directly to the power source whenever a nonselected digit is entered.-

2. In a system as set forth in claim 1, said digitalswitching and entry means comprising: f I,

(a) first stepping switch means having first contacts for each numeral which, can be entered in a digit, and being sequentially controlled by the entry means to connect one of said contacts to the power'sourcep (b) a manual selector switch corresponding with each digit in said sequence, and each switch having a wrong-number bank of contacts and having a correct-number bank of contacts, one contact in each bank corresponding with each numeral which "can be entered in a digit;

(c) second stepping-switch means having contacts representing each numeralwhich can be entered and these contacts being connected between corresponding contacts of the first stepping switch meansand the respective contacts'of all of said manual switch banks; I H

(d) means for advancing the second stepping switch means to connect the first contacts to a different manual selector switch each time a digit is entered;

(e) third stepping switch means having a continuous sequence of positions including contacts for establishing said on and off conditions; and

(f) means controlled by said second stepping switc means after the last digit in the numerical sequence is entered for advancing the position of the third stepping switch means.

3. In a system as set forth in claim 2,

(a) a third relay means connected to the correct-number banks of the selector switches and energized when a correct digit is entered to advance the second stepping switch means; V

(b) fourth relay means connected to the wrong-number banks of the selector switches and energized when a wrong digit is entered to couple the alarm to the power source;

(c) means responsive to energizing of either the third or fourth relay means to return the first stepping switch means to a rest" position; and

(d) means connected between the measuring means and the fourth relay means and responsive to energizing of the latter to simulate a change in the resistance being measured.

4. In a system as set forth in claim 1, said measuring 60 means comprising:

(a) a plural-leg bridge circuit connected across said power source and including said sensing wiring in one leg, and balanced to provide no'output' across two of its junctions unless said resistance changes;

(b) fifth relay means connected to said junctions and actuated when the bridge is unbalanced, and including contacts connected to maintain bridge-unbalance after the relay is actuated.

70 5. In a system as set forth in claim 4,

(a) said second relay means comprising a delayedaction relay having a first time delay, and connected to be energized by said fifth relay means whenthe latter is actuated, the second relay means opening said normally-closed contacts after said delay.

6. In a system as set forth in claim 1, said digital switching means having contacts arranged to be switched in sequential positions including an off position, a delay position, and an on position; and the system further including:

(a) means connected with the contacts in the delay position for checking the continuity of the sensing wiring;

(b) means connected with the contacts in the on position for energizing the measuring means from the power source; and

(c) second time delay means connected to the contacts in the delay position and energized thereby to advance the digital switching means to the on position after the period of delay.

7. In a system as set forth in claim 1, said power source comprising commercial power lines, and said alarm means comprising:

(-a) auxiliary battery means;

(b) two alarm indicating means; and

(c) said first relay means comprising two relays having contacts respectively connecting one indicating means with said power lines and connecting the other indicating means with said battery means, and said relay means when energized holding the contacts open, and both relays being energized from said power lines.

8. In a system as set forth in claim 7, said power source delivering DC. power having a constant polarity, and said first relay means including two contact members connected to said DC. power;

(a) a remote alarm station joined by wires to said first relay means so that there is a first polarity on the wires when the first relay means is energized, and a reverse polarity on said wires when the relay means is deenergized;

(b) battery means at the remote station;

(c) two alarm sounding means at the remote station and each connected on one side to the battery means;

(d) third and fourth relay means at the station each having actuating windings and having contacts respectively interposed between one sounding means and the other side of the battery means such that the 10 third relay means breaks these contacts when energized, and the fourth relay breaks these contacts when deenergized; and

(e) oppositely-poled diode means respectively connecting the windings of the relay means so that the third relay is energized only by said one polarity and so that the fourth relay is energized only by said reverse polarity.

9. In a system as set forth in claim 1,

(a) said measuring means comprising a Wheatstone bridge having said sensing wiring connecting one of its legs, the bridge being connected to receive power from said digital switching means which is connected across two opposite bridge terminals, and the other two bridge terminals being connected to said second relay means; and

(b) the bridge being normally balanced so that no power flows to the second relay means unless the resistance of the sensing wires is substantially changed.

10. In a system as set forth in claim 1, said digital switching means including means for interposing time delays in its response While being controlled to change between on and off conditions whenever :a selected sequence of digits is entered into the system.

References Cited UNITED STATES PATENTS 791,961 6/1905 Weatherby 340285 1,290,644 1/1919 lNelson 340276 1,923,968 8/1933 Chase 340276 X 2,606,237 8/ 1952 Chase 340274 X 2,843,843 7/1958 Davis 340276 2,855,588 10/1958 Allen 340276 2,964,733 12/1960 Raju 340276 X 3,024,452 3/ 1962 Leonard 340274 3,029,420 4/ 1962 Bagno et a1. 340-276 X 3,128,414 4/1964 Miehle 340164 X JOHN W. CALDWELL, Primary Examiner.

D. L. TRAFTON, Assistant Examiner.

Claims (1)

1. AN ALARM SYSTEM FOR PROTECTING A GUARDED AREA HAVING INTRUSION SENSING WIRING CONNECTED IN SERIES, COMPRISING: (A) A SOURCE OF POWER; (B) ALARM MEANS; (C) FIRST RELAY MEANS CONNECTED IN CIRCUIT BETWEEN THE SOURCE AND THE ALARM MEANS AND HOLDING THE CIRCUIT OPEN WHEN THE FIRST RELAY MEANS IS ENERGIZED; (D) MEASURING MEANS COUPLED OT SAID SENSING WIRING AND SENSITIVE TO CHANGES IN THE RESISTANCE OF THE LATTER; (E) SECOND RELAY MEANS CONNECTED TO THE MEASURING MEANS TO BE ACTUATED BY THE LATTER IN RESPONSE TO A CHANGE IN SAID RESISTANCE, AND INCLUDING NORMALLYCLOSED CONTACTS CONNECTED TO ENERGIZE THE FIRST RELAY MEANS FROM SAID POWER SOURCE UNLESS THE SECOND RELAY MEANS IS ENERGIZED; (F) DIGITAL SWITCHING MEANS CONNECTED TO CONTROL THE "ON"-"OFF" CONDITION OF THE SYSTEM BY CONTROLLING SAID DIGITAL MEANS INCLUDING MEANS FOR PRESELECTING A PLURAL-DIGIT NUMERICAL SEQUENCE AND RENDERING THE SWITCHING MEANS RESPONSIVE TO THE SELECTED SEQUENCE TO ALTERNATELY CHANGE ITS CONDITION BETWEEN "ON" AND "OFF"; (G) DIGITAL ENTRY MEANS CONNECTED TO SAID SWITCHING MEANS TO ENTER A SEQUENCE OF DIGITS THEREINTO; AND

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