US3364477A

US3364477A - Vault alarm system

Info

Publication number: US3364477A
Application number: US432646A
Authority: US
Inventors: Vincent T Mcdonough
Original assignee: American District Telegraph Co
Current assignee: American District Telegraph Co
Priority date: 1965-02-15
Filing date: 1965-02-15
Publication date: 1968-01-16
Anticipated expiration: 1985-01-16
Also published as: BE687478A

Description

Jan. 16, 1968 v. T. MCDONOUGH 3,364,477

VAULT ALARM S YSTEM 4 Sheets-Sheet l Filed F'eb. l5, 1965 Jan. 16, 1968 v T, MCDONOUGH 3,364,477

VAULT ALARM SYSTEM 4 Sheets-Sheet 2 Filed Feb. l5, 1965 Jan. 16, 1968 v. T. MCDONOUGH '3,364,477

VAULT ALARM SYSTEM 4 Sheets-Sheet 5 Filed Feb. l5, 1965 303m mmm Jan. 16, 1968 V. T. MCDONOUGH 3,354,477

VAULT ALARM SYSTEM Filed Feb. l5, 1965 4 Sheets-Sheet 4 D! Lu [E U LL] D E J Z L I I I I I I I I I I I I I I l l U Z u.: D C LLI f u.

.LndlnO FIG. 4

United States Patent Office 3,354,477 Patented Jan. 16, l68v 3,364,477 `VAUL'lF ALARM SYSTEM Vincent T. McDonough, Verona, NJ., assignor to American District Telegraph Company, `Iersey City, NJ., a corporation of New Jersey Fiied- Feb. 15, 1965, Ser. No. 432,646 5 Claims. (Cl. 340-261) ABSTRACT F THE DISCLSURE An alarm sys-tem of the soun-d monitoring type for the protection of vaults affording superior protection against attacks designed to elude the response characteristics of such systems of the prior art and a reduc-tion in the possibility of the production of false alarm signals. The system employs a preamplifier and a peaking circuit, the output of which is supplied to a continuous channel and an impact channel. The latter provides a voltage pulse output proportioned in amplitude and duration to values of the signal voltage passed by a thresh-old control. The output of both channels is applied to an integrating circuit. An alarm is produced when the energy stored in the integrating circuit exceeds a predetermined value.

The present invention relates to burglar alarm systems, and more particularly to burglar alarm system of the type especially adapted for the protection of vaults and similar enclosures.

Burglar alarm systems for vaults and the like operating on` the principle of noise detection have been used for many years, and a variety of such systems have been proposed. One type comprises those alarm systems designed to be responsive primarily to the vibrations produced within the walls and other structure of a Vault by a burglarious attack thereon rather than to airborne noises. Systems of this type, particularly when employed in conjunction with means for integrating or accumulating the effects of detected sounds have been proved effec-tive and U. S. Patent 3,147,467, issued Sept. l, 1964, to Peter Laakmann is an example o-f such a system.

A particular feature of the above mentioned Lankmann patent is the dual channels provided for the integration of detected vibrations. One channel, termed the integrating channel, is designed to detect sounds of low intensity but prolonged duration and, by integrating the total energy represented by such sounds, initiate an alarm indication when the accumulated energy level is deemed to be representative of a genuine attack upon the protected vault. The integration effect, plus frequency discrimination, make it possible for the integrating channel to discriminate successfully between the sounds of a bona fide attack conducted by a relatively quiet tool such as an electric drill wit-h a diamond tipped cutter and the innocuous sounds of ordinary activity in the vicinity. The disturbing effect of false alarms is thereby avoided or minimized without sacrificing responsiveness to real attacks. Since the possibility exists that a high intensity sound of short duration such as a dynamite blast could overdrive the amplifier and there'by fail to produce an alarm signal, an impact channel was provided to bypass the integrating channel and actuate the alarm signal directly upon the occurrence of a high intensity sound.

Systems yof the foregoing type have been placed in service and while found to be generally satisfactory, have suffered from a tendency to produce false alarms upon a dip in the voltage of the power supply and by response of the impact channel to a single sound of moderate intensity as that resulting from a vehicle in a parking lot striking the side of the building containing the protected vault.

In addition, the six-stage amplifier with multiple feedback circuits makes the electronic equipment both complex and costly with attendant difficulty in routine servicing operations.

The principal object of the present invention has been to provide a novel and improved vault alarm system of the acoustical type.

An impor-tant feature of the present invention is the ability t-o resist defeat by an attack consisting of a number of l-ow energy blows delivered at spaced intervals. The earlier system is, a-t least theoretically, susceptible to cornpromise by a series of blows, each of low enough energy to escape detec-tion by the impact channel and sufficiently spaced in delivery time so that the normal discharge time constant of the integrator will prevent the accumulated energy from reaching the alarm level. The present invention, by providing means to integrate the effect of impact sounds, is far less susceptible to such attack and consequently exhibits a greater dynamic range.

A further feature of the invention is the provision of threshold means and pulse generating and amplifying means in the impact channel whereby the evolution of false alarm indications from a single impact of moderate energy are avoided without sacrificing the detection of the impact sounds of a bona fide attack.

Another object of the invention has been the provision of a sensitive vault protection system which is resistant -to spurious alarms due to innocuous sounds but requiring relatively simple electronic circuitry and a minimum number of stages of amplification.

A feature of the invention is the means provided whereby the response characteristics of the amplifier are specially tailored to detector elements in such manner as to maximize the signal-to-noise ratio.

Still another object of the present invention has been the provision of an electronic vault protection system which is not subject to .the evolution of false alarm indications upon the occasion of brief reductions in the voltage of the power supply.

The electrical protection system of the invention detects physical attacks on a vault or strong room intended for the safekeeping of valuables and comprises vibration transducer means physically disposed relative to the structure to be protected so as to produce a signal voltage proportional to the energy expended in an attack upon that structure, integrating means coupled to the transducer means and arranged continuously to average the signal voltage over a predetermined relatively long time interval, and alarm signalling means coupled to the integrating means and arranged t-o produce an alarm signal indication when the integrated voltage exceeds a predetermined level.

Other and further objects, features and advantages of the invention will appear more fully from the following description of the invention taken in connection with the appended drawings, in which:

FIG. l is a block diagram of a system embodying the invention;

FIGS. 2 and 3, when joined together, area schematic illustration of a circuit arrangement embodying the in-vention;and

FIG. 4 is an illustrative plot o-f output versus frequency for use in explaining certain principles of the invention.

Referring now to FIG. l, the system input is derived from the interior surface of the vault to be protected by means of vibration transducers 2G. The vibration transducers, a number of which are preferably connected in parallel, are mounted in direct contact with the internal surface of the vault, preferably in an equally spaced pattern. Typically, twelve transducers might be used, although the number may be varied to suit particular requirements.

The vibration transducers are preferably microphones of the piezoelectric type and may each comprise direct actuated Rochelle salt crystal elements arranged in Bimorph construction and housed in a die-cast metallic casing. To insure reliable sensitivity over large ambient temperature and humidity ranges, the vibration transducer case may be sealed in a suitable potting compound such as an epoxy resin with a hardening agent so that vibratory energy will be transmitted to the case and from the case to the crystal elements within the case. Enclosing the vibration detecting elements also serves to shield these elements from airborne sounds except as the latter may produce vibrations in the vault structure. Vibration transducers other than those of a crystal type may be used, although the crystal type is preferred.

When any one or more of the vibration transducers are subjected to an acceleration, which will occur when the surface to which the transducer is attached vibrates, an alternating voltage will be produced. This alternating voltage is applied to a preamplifier 21 which increases the signal sufiiciently to permit subsequent operations. The signal is then passed to the peaking circuit 22, which will be more fully described hereinafter, for the purpose of maximizing the signal-to-noise ratio. A maximum signalto-noise ratio is important to permit operation with a minimum number of stages of amplification. The optimized signal is then amplified at 23 and passed to the channel divider 24. From this point, the signal is processed either by the continuous channel 25 or the impact channel 26 depending upon the characteristics of the sound detected.

If the sound is of low intensity but sustained duration as would result from the operation of an electric drill, the signal will pass through the continuous channel 2S. If the sound is of an impact nature such as a sledge hammer blow, the resultant signal will be processed by the impact channel 26.

In the continuous channel the signal first passes to the sensitivity control 27 which controls the amplitude of the signal and consequently the alarm reaction time for a given level of attack signal. The modified signal is then amplified at 28, fed to a rectifier and voltage doubler circuit 29 and thence to an integrator 39.

impact signals pass from the channel divider 24 to a rectifier 31 and then to a threshold control 32 which controls the amplitude of the signal and consequently the intensity of impact sound required to initiate an alarm signal indication. The modied signal is then applied to a pulser-amplifier 33 where the pulse to be fed to integrator 30 is generated.

The integrator 3@ averages the signal inputs over a suitable time interval, preferably about -25 minutes. Thus the output of the integrator is proportional to the average wall vibrations over a selected period regardless of whether the input is derived from the continuous channel or the impact channel. It should be understood that the longer the time interval over which the averaging occurs the less likely it will be that an attack will be undetected, since even an extremely slow attack will eventually produce a substantial integrated output.

The output of integrator 30 is supplied to a voltage sensitive detector 34 which actuates an alarm relay 35 when the voltage reaches a predetermined level. The alarm relay may conveniently be provided with a set of transfer contacts which control the operation of conventional alarm signal indicators, It is highly desirable that the alarm signal be transmitted to a central station, guard station or police headquarters, but a local audible or visual alarm may be provided as is well known in the art.

Interposed between the voltage sensitive detector and the alarm relay is a suppressor circuit 36 whose function is to prevent the initiation of false alarm signal indications upon the occurrence of a decrease in the voltage of the power source.

The detailed operation of a circuit embodying the principles of the present invention will now be described in conjunction with FIGS. 2 and 3 wherein typical values of the resistors and capacitors are shown in ohms and microfarads respectively, within brackets adjacent to the pictorial representation of the circuit component. Vt/hile specific values of the various components as well as operating voltages will be referred to in the following description, they are intended for illustrative purposes only and not as limitations of the present invention.

The vibration transducers 2@ are connected in parallel between the shield and the conductor of a shielded coaxial microphone cable 37. The conductor of the shielded cable is connected to the high side of the primary winding of an impedance matching transformer TF1 While the shield is connected to the other side of the primary winding and to ground at 38. One side of the secondary winding of transformer TF1 is connected vto the base of a transistor T1 which forms the preamplifier 21 while the other side is connected to a point between resistor R1 and the parallel combination of resistor R2 and capacitor C1. Resistor R3, connected across the secondary winding of transformer TF1 helps reduce the noise level while capacitor C2, connected between the low sides of the primary and secondary windings provides a path to ground 38 for unwanted alternating current signals which may be induced in the apparatus by extraneous sources.

The series combination of resistor R1 and resistor R2 is connected at one end to a positive voltage supply conductor 39 and at the other end to a negative voltage supply conductor 40. Conductors 39 and itl are coupled to voltage supply input terminals i1 and 42 through resistors R4, R5, rectifier REC 12 and rectifier circuit 43 which comprises rectiers REC 1, REC 2, REC 3 and REC 4 arranged to maintain the polarities of conductors 39 and itl irrespective of the supply voltage polarity. The supply voltage typically might be 25 volts, D.C. and rectiers REC 1 through REC 4 conveniently might be of the IN 2070 type. Resistors R4, R5 and capacitors C3, C4 comprise a lter network to smooth the supply voltage while a diode REC 5 which might conveniently be of the IN 30248 type is connected between conductors 39 and 4i) for protection in the event of voltage surges in the supply. The potential between

conductors

39 and 40 would be typically 15 volts.

The circuit network of resistor R1, resistor R2 and capacitor C1 furnish a forward bias voltage for the preamplifier transistor T1 which might be conveniently of the TI 495 type. The emitter of transistor T1 is coupled to conductor 40 by the parallel combination of a resistor R6 and a capacitor CS which, together with the preceding network, establish the operating point of transistor T1. The collector of transistor T1 is coupled to conductor 39 by means of a tap on the primary winding of a transformer TF2 and the parallel combination of a resistor R7 and a capacitor C6 connected across the primary winding of transformer TF2. One side of the secondary winding of transformer TF2 is connected to the base of a transistor T2 While the other side is connected to the junction of resistors R1 and R2.

The components in the collector circuit of transistor T1 comprise the peaking circuit 22. Capacitor C6 and transformer TF2 constitute a tuned circuit whose frequency response is designed to match the natural frequency response characteristic of the vibration transducers. The solid line curve of FIG. 4 depicts the typical response of a transducer comprising a major output peak followed by several lesser peaks as the frequency of the input sound increases. The corresponding response characteristic of the peaking circuit 22 is shown as the dashed line curve and it will be noted that capacitor C6 and transformer TF2 have been so selected as to provide peak response at a frequency corresponding to the major peak of the transducer response curve. Furthermore, the peaking circuit curve has been made asymmetric so that the right hand side will embrace the minor peaks of the transducer curve. Still another modification to the peaking circuit response has been the provision of the resistor R7 whose function is to broaden the peak of the peaking circuit curve sufficiently to allow for the normal variations in characteristics to be expected between individuals of a number of vibration transducers. The practical purpose of the shaping of the peaking circuit response is to improve the signal-to-noise ratio which, in turn, permits operation of the entire apparatus at the desired sensitivity level with far fewer stages of amplification than have been required in the prior art. It is desirable that the peaking circuit effectively suppress usual ambient noise frequency components in the transducer outputs. For this purpose the peaking circuit should cut ofi frequency components below about 1000 cycles per second.

The signal output of preamplifier Z1, after shaping by the peaking circuit 22, is applied to the base of a transistor T2, which might conveniently be of the TI 495 type, and which comprises the amplifier 23. The network of resistor R1 and resistor R2 plus capacitor C1 establish the forward bias voltage for transistor T2 and, together with resistor R8 and capacitor C7 in the emitter circuit of transistor T2 determine the operating point of transistor T2. The collector circuit of transistor T2 is coupled to conductor 39 through the primary winding of a transformer TF3, which, with its double wound secondary, constitutes the channel divider 24 and provides for maximum signal transfer.

The high side secondary winding 44 is a portion of the continuous channel 25 and has one end connected through a potentiometer R9 to the base of a transistor T3 while the other end is connected to the junction of resistors R-R11 and to the junction of resistor R9 and capacitor C8. Signals representing sounds of a continuous nature such as those produced by the operation of an electrical drill or prolonged hammering of relatively low intensity are applied, after shaping in the peaking circuit 22, to the base of transistor T3, which is the amplifier 28 of the continuous channel 25, through the potentiometer R9 that comprises the sensitivity control 27. The amplitude of the signal in the continuous channel is controlled by potentiometer R9. The setting of potentiometer R9 establishes the length of time a signal of given amplitude must persist to initiate an alarm indication. Signals representing sounds of an impact nature will also be applied to the continuous channel, but because of their short duration and the fact that their relative amplification in amplifier 28 will be small as compared to the amplification accorded to the lower amplitude signals, these impact signals will not have an undue alarm producing effect on the continuous channel.

The forward bias voltage for transistor T3, which also might be of the TI 495 type, is supplied by the network comprising resistor R10 and resistor R11 plus capacitor C8 which, together with resistor R12 and capacitor C9 in the emitter circuit, establish the operating point for transistor T3. A transformer TF4 whose primary winding connects the collector of transistor T3 to conductor 39 supplies the amplified signal output of transistor T3 to the rectifier-doubler 29. The low side of the secondary winding of transformer TF4 is connected directly to conductor 40 while the high side is coupled through a capacitor C10 to the junction of rectiers REC 6 and REC 7 which both might conveniently be of the IN 2070 type. The cathode of rectifier REC 6 is connected directly to conductor 40 while the anode of rectifier REC 7 is connected to conductor 40 through a capacitor C11. The combination of capacitor C10 and rectifiers REC e and REC 7 comprise the full wave rectifier-voltage doubler 29 whose output is stored in capacitor C11.

A resistor R13 is connected across the capacitor C11 to form, with capacitor C11, the integrator 30. All incoming signals from the continuous channel 25 are integrated over an interval determined by the time constant of resistor R13 and capacitor C11 (4.7 meg. 250

mf.=19.5 minutes). Thus the output of integrator 30 represents the average of all signals received during that period. While for practical purposes an integrating time interval of about 20 minutes is preferred, an integration time interval of as little as one-half minute will provide improved results over systems without this feature. On the other hand, an integrating time interval of as much as several hours might be used to advantage in special cases where an extremely slow rate of attack was possible.

A violent attack, as for example a dynamic blast, while of high intensity, may also be of such short duration as to provide insufficient signal energy to raise the integrator circuit output to the alarm level. As noted above, a high intensity signal will not receive the same relative amplification as a continuous signal. In this regard it should be noted that a typical continuous attack signal at the transducer output might be of the order of 20 microvolts while an impact alarm signal at the transducer output typically might be of the order of 2 volts. To guard against loss of impact alarms of short duration, an impact channel 26 has been provided that represents distinct advantages and improvements over similar features of the prior art. The impact channel of the previously identified Laakmann patent was subject to false alarms resulting from single impact blows of innocuous nature as caused by a workman dropping a tool or a vehicle striking the exterior of the building containing the protected vault. The present invention overcomes the tendency toward false alarms without sacrificing the ability to detect genuine attack by the provision of a threshold device, a pulse circuit and routing of impact signals through the integrator as explained below.

The impact channel 26 begins with the low side secondary winding 45 of transformer TF3 whose upper side is connected to conductor 40 through -a rectifier REC 8 which might be of the 1N 2070 type. The lower side is connected to the base of a transistor T4 'through a rectifier REC 9 which also might be of the IN 2070 type. Rectifier REC 9 and capacitor O12, which is connected across secondary winding 45, comprise the rectifier network 31 which converts signals representing impact sounds to direct current. The rectifier network 3'1 in conjunction vvith the variable resistor R14, which connects the -emitter of transistor T4 to conductor 40, determines the collector current of transistor T4. Transistor T4, which might conveniently be of the TI 495 type, together with rectifier REC 9 and resistor R14, constitute the threshold control 312.

The function of the threshold control is to establish a signal level which must be exceeded before energy can be passed to the integrator so that random, individual sounds may be prevented from producing false alarms. Since the desired signal level will vary from one installation to another and, indeed, from time to time in a particular installation, 'the variable resistor R114 is provided so that suitable adjustments may be made. The series combination of resistor R15 and rectifier REC 8 which connects the collector circuit of transistor T4 to conductor `40 tends to reduce the threshold but is primarily provided to compensate for the effect of temperature variation-s on the base-emitter junction of transistor T4. Resistor R116 connected across the secondary winding 45 is intended to minimize 4the etiect of temperature induced variations in the collector-'base current of transistor T4 on the collector current. A typical threshold voltage (baseemitter of transistor T4) might be in the range of Z50 to 300 microvolts. Considered in another way, the threshold level for the impact channel typically might be 25 db above the usual alarm signal level for the impact channel.

Resistors R17 and RdS are in series connection between the collector of transistor T4 and a conductor 46 which connects their terminal to the positive voltage supply conductor 39. Resistors R17 and R18 provide the forward bias voltage for a transistor T5 whose base is connected to the junction of resistors R17 and Rl. Thus whena signaly of suliicient amplitude to pass the threshold occurs, the transistor T is caused to become conductive through its emitter-collector circuit.

The pulse-amplier 33 comprises transistors T4 and T5 together with their respective collector and emitter circuits. Transistor TS, which might be conveniently of the'ZN 328A type, has its emitter coupled to conductor 46 (the positive voltage supply) through a resistor R19v delivers amplified pulses to capacitor Clit.

As hereinbefore described, the integrator 39 accepts signal energy from thetcontinuous channel in the form of a voltage which is averaged over a time period. Vfhen the integrator output voltage, which represents the integrated input signal voltage and appears across the terminals of capacitor Cil, reaches a predetermined value, the voltage sensitive detector 34 will respond and actuate alarm relay 35. Thus the hounds of an electric drill or the scraping away of the mortar between the bricks of a` vault wall will build up, even though interrupted at intervals, until the alarm level is attained. The impact channel', on the other hand, will respond instantly through the integrator to produce an alarm signalfupon the occur-r rence of a single violent impact sound but will not produce a false alarm upon a single light impact sound due to the suppression effect of the threshold. Furthermore, impact sounds which are above the threshold but below the alarm level are integrated with the result that signals are stored in the integrator so that repetition thereof will produce an alarm. An additional feature of the threshold rcontrol is the provision of the variable resistor RM which permits adjustment of the circuit so that an alarm will occur upon a discrete number of blows of a given intensity.

The voltage sensitive detector 34 comprises a unijunction transistor (or double base diode) T6, which may be of the 2N-490` type. The base Bl of transistor T6 ls coupled to conductor 39 through a resistor R21 and the base B2 is connected directly to conductor 4d. The emitter is also coupled to conductor 46 through the series combination of the coil of the alarm relay 35 and resistor R13, while the junction of the alarm relay 35 and resistor R13 is connected to the positive terminal of capacitor C711.

Unijunction transistor T6 will not conduct until the voltage between its emitter and base Bl equals or eX- ceeds a predetermined percentage of the voltage between bases B1 and B2 thereof, e.g. 60%. ln other words, the unijunction transistor will not conduct until a predetermined signal voltage is supplied to the emitter of transistor T6 from capacitor Clit of the integrator 30. Resistor R21 reduces the normal volt potential between

conductors

39 and 40 to approximately 9 volts at the base B1. Thus, when the voltage on capacitor Cll reaches 60% of 9 volts (approximately 5.5 volts), the transistor T6 will become conductive.

When transistor T6 becomes conductive, an energizing circuit for the alarm relay 3S is momentarily completed and the relay is energized thereby operating the transfer contacts 46 which in turn actuate conventional alarm annunciating devices (not shown) as is well known in the art.

Practical experience with unijunction transistors when utilized as described above has shown that a false alarm hazard exists when a dip in the voltage supply occurs at f falls to a G a time when there is a charge on capacitor C11.Since the transistor becomes conductive when the emitter-base Bl voltage reaches a certain percentage of base Bit-base B2 voltage, this condition may be attained by a decrease of the Bl-BZ voltage as readily as by increase of the emitter-base El voltage as is intended. Consequently, at n some time when random sounds have caused capacitor Cil to become charged to a voltage less than the alarm level, say 4 volts, a dip in the Bil-BZ: voltage from the normal i9 volts to about 6.5 volts will result in the production of a false alarm indication.

yThe present invention is protected against such contingency by the suppressor circuit 35 comprising the conductor 47 which connects the collector of transistor T5 through rectifiers REC l@ and REC il, which both might be of the iN 2070 type, to the junction of resistors R22 and R23twhich act as a voltage divider for the potential supplied at the terminals 4l and 42. The purpose of this connection is to allow the charge on capacitor C11 to be dissipated to ground when the power supply voltage predetermined value via the pathof conductor 57 and resistor R23 to the ground conductor 40. Resistors R22 and R23 are selected so that the voltage at their junction is somewhat above the 5.5 firing voltage of the unijunction transistor T5, say 7 volts, and rectilier REC ll. will conduct in the forward direction when the voltage at the junction of resistors R22 and R23 is less than the voltage on capacitor Cil'. Thus, when a dip in the supply voltage reduces the potential at the resistor B22-R23 junction to a value less than that atthe capacitor, the capacitor Clt ywill discharge through conductor i7 thereby preventing a false alarm indication.

Rectifier REC l@ is provided to prevent discharge of the capacitor Cil into'the collector of transistor T5 while rectifier REC 12 prevents the discharge of capacitors C3 and C2i into resistors R22 and R23. While the presence of capacitors C3 and C4 provide a delaying effect and thus help prevent a supply voltage dip from arriving at tne unijunction transistor Tdbeforethe suppressor circuit has had time to discharge capacitor C11, another capacitor C13 is connected between the bases B1 and B2 of transistor T6 to further slow the effect of voltage dips upon the transistor.

While the invention has been described in connection with a specific embodiment thereof and in a specific use, various modifications thereof will occur to those skilled in the art without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

ll. An electrical protection system for detecting physical attacks on a vault or like structure, comprising:

(l) vibration transducer means disposed in close proximity to a surface of the structure to be protected so as to be subject to acceleration resulting from vibratory energy in the structure walls and arranged to produce an alternating output voltage proportional to the magnitude of the acceleration to which it is subjected;

(2) a first amplifier coupled to the output of said transducer means;

(3) a continuous signal Channel comprising:

(a) a second amplifier having a dynamic range selected to accommodate input signals corresponding to signal voltages characteristic of relatively low-noisedevcl attacks on said structure;

(b) a sensitivity control for adjusting the input level to said second amplifier;

(c) means to supply the output of said first amplifier to the input of said second amplifier; and

(d) rectifier means coupled to the output of said second amplifier for producing a irst direct voltage proportional to the amplitude of said signal voltage;

(4) an impact signal channel comprising:

(a) a rectifier circuit;

(b) means to supply the output of said first amplifier to said rectifier circuit to produce a second direct voltage proportional to the amplitude of said signal voltage;

(c) a threshold control coupled to the output of said rectifier circuit to suppress values of said second direct voltage below a preselected threshold value, said threshold value being substantially greater than the input voltage resulting from a signal voltage level characteristic of a relatively low-noise-level attack on said structure; and

(d) means coupled to said threshold control to produce direct voltage pulses proportional in amplitude and duration to values of said second direct voltage passed by said threshold control;

() an integrating circuit having a relatively long time constant;

(6) means to apply said first direct voltage and said direct voltage pulses to said integrating circuit whereby the latter integrates said first direct voltage and said direct voltage pulses; and

(7) alarm signalling means coupled to said integrating circuit and arranged to produce an alarm signal indication When the energy stored in said integrating circuit exceeds a predetermined value.

2. An electrical protection system for detecting physical attacks on a vault or like structure, comprising:

(l) a plurality of vibration transducers disposed in close proximity to the inside surfaces of the structure to be protected so as to be Subject to acceleration resulting from vibratory energy in the structure Walls and each arranged to produce an alternating output voltage proportional to the magnitude of the acceleration to which it is subjected;

(2) means coupled to said transducers to combine said output voltages into a composite signal voltage proportional to the intensity of vibrations in said walls;

(3) a peaking circuit coupled to the output of said last mentioned means and arranged to provide a peak response at approximately the natural frequency of said transducers, a smoothed response at frequencies in the outputs of said transducers above said natural frequency and a sharply attenuated response at frequencies in the outputs of said transducers substantially below said natural frequency;

(4) a first amplifier coupled to the output of said peaking circuit;

(5) a continuous signal channel comprising:

(a) a second amplifier having a dynamic range selected to accommodate input signals corresponding to composite signal voltages characteristic of relatively low-noise-level attacks on said structure;

(b) a sensitivity control for adjusting the input level to said second amplier;

(d) a voltage doubler-rectifier circuit coupled to the output of said second amplifier for producing a first direct voltage proportional to the amplitude of said composite signal voltage;

(6) an impact signal channel comprising:

(a) a rectifier circuit;

(b) means to supply the output of said first amplifier to said rectifier circuit to produce a second direct voltage proportional to the amplitude of said composite signal voltage;

(c) a threshold control coupled to the output of said rectifier circuit to suppress values of said second direct voltage below a preselected threshold value, said threshold value being substantially greater than the input voltage resulting from a combined signal voltage level characteristic of a relatively low-noise-level attack on said structure;

(d) means to adjust said threshold value; and

(e) means coupled to said threshold control to produce direct voltage pulses proportional in amplitude and duration to values of said second direct voltage passed by said threshold control;

(7) an integrating circuit having a relatively long time constant;

(8) means to apply said first direct voltage and said direct voltage pulses to said integrating circuit Whereby the latter integrates said first direct Voltage and said direct voltage pulses; and

(9) alarm signalling means coupled to said integrating circuit and arranged to produce an alarm signal indication when the energy stored in said integrating circuit exceeds a predetermined value.

3. An electrical protection system for detecting physical attacks on a vault or like structure, comprising:

(l) a plurality of vibration transducers disposed in close proximity to the inside surface of the structure to be protected so as to be subject to acceleration resulting from vibratory energy in the structure walls and each arranged to produce an alternating output voltage proportional to the magnitude of the acceleration to which it is subjected;

(2) means coupled to said transducers to combine said output voltages into a composite signal voltage proportional to the intensity of vibrations in said walls:

(4) a first amplifier coupled to the output of said peaking circuit;

(5) a continuous signal channel comprising:

(c) means to supply the output of said first amplifier to the input of said second amplifier; and (d) a voltage doubler-rectifier circuit coupled to the output of said second amplifier Afor producing a first direct voltage proportional to the amplitude of said composite signal voltage;

an impact signal channel comprising:

(a) a rectifier circuit;

(d) means to adjust said threshold value; and

(7 an integrating circuit including a capacitor and a 11 resistive element connected so as to discharge said capacitor, said integrating circuit having a relatively long time constant of the order of 15-25 minutes; (8) means to apply said iirst direct voltage and said direct voltage pulses to said capacitor to charge the latter whereby said integrating circuit integrates said first direct voltage and said direct voltage pulses; (9) a source of direct voltage operating potential; and (10) alarm signalling means coupled to said integrating circuit and arranged to produce an alarm signal indication when the voltage across said capacitor exceeds a predetermined value, said alarm signalling means comprising:

(a) a unijunction transistor;

(b) an alarm relay having an energizing coil and signalling contacts;

(c) means to apply said operating potential to the respective bases of said unijunction transistor; and

(d) means including the coil of said alarm relay to couple said capacitor between the emitter and one of the bases of said unijunction transistor whereby the latter will conduct and energize said alarm relay when the voltage across said capacitor exceeds said predetermined value.

4. An electrical protection system as set forth in claim 3 comprising voltage sensitive means coupled to said capacitor and to said source of direct operating potential and arranged to provide a discharge path for said capacitor when said operating potential drops to a selected proportion of its normal value thereby to prevent conduction of said unijunction transistor and energization of said alarm relay upon occurrence of a substantial dip in said operating potential with a partial charge on said capacitor such that the voltage thereacross is substantially below said predetermined value.

5. An electrical protection system as set forth in claim 4 comprising an additional capacitor coupled between the bases of said unijunction transistor.

References Cited UNITED STATES PATENTS 2,799,015 7/1957 Bell 340-261 2,806,082 9/1957 Woods 340-261 X 3,016,457 1/1962 Brown et al. 340-261 X 3,069,672 12/1962 Rau 340-261 3,134,970 5/1964 Kelly et al 340-261 3,147,467 9/1964 Laakmann 340-261 THOMAS B. HABECKER, Acting Primary Examiner.

NEIL C. READ, Examiner.

D. L. TRAFTON, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N6. 3,364,477 January 16, 1968 Vincent T. McDonough It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below Column l, line Z8, for "system" read systems column 3, lines 3 and 4, for "Bmorph" read bimorph n; column 6, line l0, for "dynamic" read dynamite column 7, line 28, for "hounds" read sounds column 8, line l0, for "19 volts" read 9 volts U; column 9, line 32, for "surfaces" read surface column 10, line 50, for "walls:" read walls;

Signed and sealed this 4th day of March 1969.

(SEAL) Attest:

Edward M. Fletcher, J r. EDWARD J. BRENNER Attesting Officer Commissioner of Patents