US2390221A - Alarm system - Google Patents

Description

Dec. 4, 1945 M. H. A. LINDSAY Erm.

ALARM SYSTEM Original Filed April 19,l 1940 F l G.

FIG.2

Patented Dec. 4,- 1945 ALARM SYSTEM Maxwell H. A. Lindsay, Summit, N. J., and Konstantin Woloschak, Port Chester, N. Y., assignors to American District Telegraph Company, Jersey City, N. J., a corporation of New Jersey Original application April 19, 1940, Serial No. 330,464. Divided and this application July 3, 1943, Serial No. 493,330 f 8 Claims. (Cl. 177-314) This invention relates to electric signaling systems generally and more particularly to alarm 1 systems of the type which are sensitive to changes of electrical capacity and which are particularly adapted to detect alarm conditions such as ilres or the approach of an unlawful intruder to an object or place to be protected.

This application is a division of our copending application Ser. No. 330,464, tiled April 19, 1940,

for Alarm systems, Patent No. 2,343,987, issued.

March 14, 1944.

This invention contemplates the'use of what is known as a capacity alarm system for property protection and intruder' detection within a subscribers premises wherein the approach of an unlawful intruder or the occurrence o! any dangerous condition disturbs the balance or some other condition within the protective circuit by means of changes in electrical capacity. The circuit then transmits a signal to an alarm device which may be located Within the subscribers'premises or at a central station. In the capacity alarm system it is only necessary to provide an antenna at the place to be protected, such an antenna being easy to conceal and substantially trouble proof. .Such

systems thereby obviate the necessity of using contact detecting devices or other mechanical and electrical devices which are disturbed by loca-l conditions such as heat, weather or various forms of tampering.

A further advantage of such a system is that a minimum amount of equipment is necessary. In the usual form of burglar alarm systems, a large amount of wiring must be installed, such wiring becoming useless for other purposes, whereas the equipment necessary to carry out this invention requires a minimum amount of wiring and permits numerous successive installations of a single system.

Previous systems of this character have been inherently subject to defeat in various ways and have also had the disadvantage that their sensitivity is affected by temperature and humidity conditions.

Previous capacity alarm systems have also been deficient in that no means has been provided for testing the circuit when it is put into operation whereby the subscriber for alarm service has not been able to ascertain Whether or not his system is operative. Capacity alarm systems can be compensated to overcome small changes in weather conditions or other local conditions which might render the system inoperative or out of balance but the fact still remains that such systems sometimes require slight adlustment in order to obtain high quality operation. Therefore it is not only necessary to provide the central station operator with a testing means at the central station whereby he can supervise the remote detecting apparatus but the unskilled subscriber himself must be provided with a simple and yet effective testing circuit whereby he may be'certain that the detection system will operate, in case of need, in the manner in which it was intended to operate.

Accordingly the principal object of this invention is to provide an improved alarm system.

Another object of this invention is to provide an improved capacity alarm system.

Still another object of this invention is to provide an improved alarm system of the capacity typewhich is extremely sensitive and which cannot be defeated.

A further object of the invention is to provide a system of the capacity type which may be tested by the subscriber and by the central station operator.

Still another object of this invention is to provide an alarm system of the capacity type which is extremely sensitive and which may be tested by an unskilled operator with a simple testing equipment.

Further objects and advantages of this invention will be obvious to those skilled in the art from an inspection o1 the accompanying specification and drawing, wherein Fig. 1 is a circuit diagram of one modication of this invention, and

Fig. 2 is a circuit diagram of a further modification of this invention wherein an extremely sensitive circuit is illustrated.

Referring to Fig. 1 of the drawing, the protected objectis'shown in the form of a safe or cabinet I. This protected object may take the form of any desired device or area which it is desired to protect, the safe being shown for purposes of illustration only. The protective circuit, located in a subscribers premises, is made up of a capacity and inductance bridge and a vacuum tube oscillator lll which is connected to the protected safe I and the bridge circuit by means of the antennae 2 and 3. This connection is shown by way of example only as it is also feasible to protect an object or area by means of a loop antenna. The bridge circuit comprises an adjustable condenser 5 in one arm thereof, an inductance 6 in another arm thereof, and inductance 1 `in the third arm vof the bridge. The fourth arm of the bridge is the capacity 8 which is the capacity to ground of the safe I.

. The grid circuit of the tube Il) is connected to the antenna 2 through coupling resistor II and condenser I2 while the filament of tube I0 is connected to the grid circuit through resistor I4.

The bridge circuit is connected to the filament of tube I by means of conductor I6 and is coupled to the input of vacuum tube I8 by means pears across the coil is rectified lin the grid circuit of tube I8 by the grid leak 24'and condenser whereby a direct current potential biases the grid circuit of tube I8.

The output of tube I8 is connected to a local alarm relay 26 which controls the alarm device 28 at a central ofiice by means of contact 2l and transmission line 48. A day`and night switch 49 `is shown in the night" position but during business hours it is thrown to the ground contact to provide test facilities. In view of changing weather conditions and changing electrical conditions in the vicinity of the detecting apparatus, a testing circuitis provided having such characteristics that it can be operated satisfactorily by the person subscribing to the protection service. Such persons are usually unskilled in the signaling art and therefore it is necessary to provide simple, yet adequate, testing circuits for indicating whether or not the circuit is in operative condition or whether it is too sensitive or not sensitive enough. Therefore, the buzzer 30 is provided in the subscribers premises for indicating the condition of relay 26, or in other words, for indicating whether or not the alarm device 28 is operating. This indicating buzzer 30 may be used for indicating an alarm condition or for indicating lack of sensitivity, as explained later. A condenser 32 which is the equivalent of the capacity of a person approaching the protected safe or other object I is included in the test circuit. Contacts 33 of switch 34 connect the condenser 32 across the bridge circuit. Lamp 36 is provided for indicating too much sensitivity, contacts of i switch 34 connecting the signal lamp 36 in series with a source of alternating current 38 and the contacts 4I of relay 40 which is connected in series with the relay 26 and battery 43.

It is also important that the central station v operator be enabled to test the circuit and therefore a switch 45 'in series with battery 46 establishes a circuit through the transmission line 43 to relay 50 which connects condenser 32 in the bridge circuit through contact 5I.

The circuit described above is intended for installation in stores or other commercial establishments which are under the supervision of persons unskilled in the art of signaling and it is important that they be enabled to determine positively whether or not their burglar protection system, is operating properly. This type of burglar alarm system is normally inactive or disconnected throughout the normal business hours of the protected establishment and therefore at closing time the proprietor operates switch 34 to close contacts 33 which connect the condenser 32 to ground to simulate thereby the approach of a person to the protected object I. If the condenser 5 is properly adjusted, the bridge circuit will be normally unbalanced and there is a potential difference between the grid and cathode tube I6, this potential being 180 outvof phase with respect to the p0- tential between cathode and plate which is kept at ground potential by condenser 22. Therefore, oscillations are maintained between grid and cathode of the tube I0 which appear across coils 6 and 1 and a potential is built up across the coil 20 whereby the tube I8 is biased by its grid in 'such a manner that its output is substantially zero. When condenser 32 is inserted across the bridge circuit, the bridge will become balanced and there will not be any oscillating potential between the grid and cathode of tube I0. There will be no feedback from the plate to the grid circuit and thus the tube I0 will no longer oscillate and the bias on tube I8 is reduced to zero. When the bias of tube I8 is reduced to zero the current in its plate circuit increases to such a point that relay 26 is operated.' Contacts 29 of relay 26 close `and energize buzzer 30 to inform the proprietor that the alarm relay has operated. During such test procedure switch 49 is in its ground connection position so ythat alarm device 28 is not energized.

It may be possible that the condenser 5 is not properly adjusted and therefore closure of switch contacts 33 does not produce a signal on buzzer 30, in which case condenser 5 is adjusted until an alarm condition is indicated by the buzzer. This test alone is not suiflcient, however, as the circuit might be too sensitive and therefore condenser 5 must be adjusted in sucha manner that the bridge becomes balanced when a person approaches the Aprotected object within a predetermined distance and unbalanced by a certain definite amount when no person is close to the protected object I. Therefore it is necessary to test for the 'proper operating point of the protective circuit. Switch 34 is operated to close the contacts 35 and insert lamp 36 in circuit with the alternating current source 438 and contacts 4I of relay 40.

Relay 40 is adjusted to operate at a predetermined critical current value which if exceeded indicates that the system as a whole is too sensitive. Therefore if the output of tube I8 does not. exceed a predetermined value, lamp 36 will be lighted when switch 34 isoperated to close contacts 35. If the critical value is exceeded due to the fact that the condenser 5 is improperly adjusted, the test lamp will not light because relay 40 will have operated and opened its contacts 4I.

Thus if the test lamp lights but buzzer 30 does not sound, condenser 5 may be adjusted until'V the buzzer 30 is operated. If switch 34 will then light the test lamp it is evident that the system is properly adjusted to initiate an alarm when a person approaches the protected object within a given distance. Similarly, if the test lamp d'oes not light on the first test, it is again evident that the circuit is not properly adjusted and the capacity 5 must be increased until the lamp does light. Here again if the buzzer and the lamp both operate the subscriber receives a positive indication that his system is operating properly.

This is accomplished by the use of relay 50 which I is connected in series with the central station Switch 45, battery 46 and transmission line 48.

Relay 50 is operable only on a comparatively high test voltage applied when switch 45 connects battery 48 to the relay. l Operation of switch 45 causes relay 50 to close contact 5l thereby grounding condenser 32. This operation performs the same function as switch 34 and the operator l receives an alarm if condenser is properly adjusted and if the remainder of the system is in operative condition.

After the subscriber has tested his protective system and has adjusted it to its proper operating condition, the approach of an unlawful intruder to the projected object or area will operate the system by balancing the bridge circuit in the same manner as described above in connection with the circuitv test involving the substitution of condenser 32. The bridge circuit becomes balanced thereby causing oscillations to cease. The potential across coils 6 and 1 decreases thereby decreasing the potential across inductance '20 to the point where the bias on the grid of tube I8l becomes zero. This allows the output of tube I8 to' increase sufficiently tooperate alarm relay l26 and actuate the alarm device 28 through contacts 21 and transmission line 48', since switch 48 is moved to its position completing the circuit to device 28 after all testing has ended.

It should be noted that the antennae 2 and 3 are connected between the bridge circuit, the projected object I and the oscillator tube I0. This particular feature of the invention provides a means of obtaining an alarm signal even though an attempt is made to defeat the system by cutting or otherwise tampering with the antenna leads. If an intruder approaches the antenna he changes the characteristics of the bridge circuit to such an extent that it becomes 4balanced in the same manner as it would if he approached the protected object itself. If he cuts either antenna 2 or 3, the metallic connection between grid and ground is destroyed and tube I8 ceases to oscillate. If only one wire were used for the antenna connection, as is the usual practice, cutting the antenna would not necessarily cause an'alarm, Ibecause the bridge would become more unbalanced than in the normal condition. But with this arrangement an alarm is always initiated ln spite of attempts at tampering.

Y Fig. 2 of the drawing illustrates another type of capacity circuit wherein supervision of the antenna as well as the protected object itself is obtained also. This modification of the invention also illustrates a means of obtaining much greater sensitivity to alarm conditions than is possible in previous types of capacity alarm circuits. In this modification of the invention the antenna 64 is disposed in the form of a loop around or within the object or area which it is desired to protect. The circuit consists of oscillator tubes 60 and 6I, together with their respective tuned circuits which comprise condensers 62 and 63, together withthe inductances 65fand 66. Normally, the variable condenser 62 is adjusted in such a manner that the frequency of oscillator tube 68 is almost equal to the frequency of tube 6| and therefore when the outputs of tubes 60 and 6l are fed into the input of mixer tube 68 its plate circuit contains the audio beat note of the two frequencies generated in tubes 60 and 6 I. The output voltage of the beat frequency which occurs in the output of tube 68 appears across the choke coil 12 whi-ch is connected between plate battery and the radio frequency filter consisting of coil 69 and condensers 18 and 1|. 'I'his filter eliminates substantially all of any other frequency `the system as a whole.

components which may appear in the output of tube 68. The coil 12 is coupled to the rectifier 15 Iby a network comprising the coupling condenser 13 and resistor 14 which is frequency discriminating over the range of audio frequencies normally obtained and thus as the constant potential energy available across coil 12 varies in frequency the voltage across resistor 14 varies in proportion thereto. Varying frequency being changed to varying amplitude, the diode rectifier 15 applies a rectified voltage across the resistor 18 and condenser 18. This voltage is applied to tube as a bias'which normally prevents emission in this tube and in turn prevents energlzation of the alarm relay 82. Relay 82 controls the alarm 84 through the contact 83 and battery 85. With this arrangement, the alarm system can be regulated to operate upon a change in direct current voltage at any point within a predetermined range.

When a person approaches the antenna the capacity to ground is changed and the frequency of oscillator 60 decreases thus changing the beat note between the frequencies generated by tubes 60 and 8|. Thus if the original frequency of oscillator 60 was higher than the frequency of tube 6i the beat note decreases in frequency and a lower frequency audio voltage appears across the coil 12. A lower frequency audio voltage also appears across the series combination of condenser 13 and resistor 14 and because of the discrimination by condenser 13 against the lower frequency, a smaller audio voltage is applied across resistor 14. Therefore a similar rectified voltage is impressed on the grid of tube 88 allowing the plate current of this tube to increase and energize the alarm relay 82 causing contacts 83 to close and the alarm 84 to operate.

As described above the two oscillator tubes 68 and 8l are adjusted to be almost equal in respect to their output frequencies but this arrangement does not provide the ideal in sensitivity and therefore it is desirable to increase the sensitivity of In this system as so far described the beat note changes exactly the same number of cycles per second as does the frequency of tube 60 and it is this change in frequency that determines thesensitivity of-the system. Obvi- 'ously ifv the'antenna capacity is large, a small change in it will cause only a small change in the frequency of tube 60, especially if this frequency is comparatively low, and the beat note will like- Wise change by ,only a few cycles per second.

Therefore, in accordance with this invention, the frequency of the tube 6I is made equal to a higher frequency than that of tube 88 and has an approximately integral relationship to the frequency of tube 68. In other words it is 1t times the frequency of tube 60, where n is any integer. Under this condition, the frequency of tube 68 can no longer beat with that of tube 6| to produce an audio beat note, but the nth harmonic of the first frequency can, and does, beat with the fundamental of the second frequency to produce the required audio beat note.

It is evident that when a variation of the antenna capacity causes the frequency of tube 68 to change f cycles per second, the nth harmonic will change nf cycles, and the audio beat note in the plate circuit of tube B8 will likewise change nf cycles. Thus the sensitivity of the system is raised n times by adjusting the frequency of tube 8| to be n times the frequency of tube 60, and then using the nth harmonic of the second frequency to beat with the fundamental of the flrst frequency.

The present arrangement possesses advantages in that the variable oscillator may be set at any of several fundamental frequencies and selection made, after modulation or demodulation, of the beat frequency desired. For example, if the system is designed to operate on an audio beat frequency of five hundred cycles, the constant frequency oscillator could be set to oscillate at one hundred thousand five hundred cycles and the variable oscillator could be set to oscillate at any of a number of fundamental frequencies. Should this oscillator |be set to operate at fifty thousand cycles, such fundamental frequency and the harmonies thereof would be imposed upon the demodulator along with the fundamental from the fixed oscillator. The selecting apparatus would then select the beat obtained from the fundamental of the fixed oscillator and the second harmonic of the variable oscillator, namely, one hundred thousand cycles. Should, however, this second harmonic of fty thousand cycles possess characteristics unfavorable to operation in the present system, the variable oscillator could very well be set to oscillate at twenty thousand cycles, in which event the selecting equipment would select the beat frequency between the fundamental of the constant oscillator and the fifth harmonic of the variable oscillator,. again one hundred thousand cycles. Still further, the variable oscillator could be set at twenty-five thousand cycles in which event the selection would be made of the beat frequency obtained by the fundamental of the xed oscillator and the fourth harmonic of the variable oscillator, namely, one hundred thousand cycles. Thus it will be seen that the present arrangement can be operated over a wide range f variable oscillator conditions.

It should also be noted that it is also possible in this circuit as in that illustrated in Fig. 1 of the drawing to supervise the antenna in the same manner and with the same advantageous results.

It is also possible to test the circuit' shown in Fig. 2 in the same manner as illustrated in Fig. l since condenser 62 in Fig. 2 can be grounded by a switching device such as 34 shown in Fig. 1 to simulate a person approaching the antenna. Obviously the remainder of the test circuit can also be applied to the circuit shown in Fig. 2 and can be operated in the same fashion.

It is obvious from the above description that this invention provides three very import-ant features, namely (l) antenna supervision, (2) a testing circuit, and (3) greater sensitivity by making use of frequency multiplication without unduly complicating the circuit. Further modifications and advantages of this-system will be apparent to those skilled in the art and therefore it is desired that this invention shall not be limited to the modifications described and illustrated herein but shall be limited only to the extent required by the prior art and the appended claims.l

audio beat frequency, means for varying the frev quency of the second source of oscillations whereby the frequency of the output of said modulator varies, means for converting said modulator output to a direct. current, means between the modulator and the converting means whereby variations in the frequency of the modulated output vary the amplitude of the audio signals'imposed upon said converting means to cause variations in the converted direct current, and alarm means controlled by said direct current.

3. In a signaling system, a projected object having a variable capacity to ground, a constant frequency source of oscillations, a variable frequency source of oscillations, a modulator for adding the outputs of said sources and obtaining an audio Ibeat frequency, means controlled by said capacity for varying the frequencyof thel second source of oscillations whereby the frequency of the output-of said modulator varies, means for converting said modulator output to a direct current, means between the modulator and the converting means whereby variations in the frequency of the modulated output vary theamplitude of the audio signals imposed upon said converting means to cause variations in the converted direct current, and alarm means controlled by said direct current.

4. In a signaling system, a variable source of oscillations comprising an oscillator tube, a tuned circuit, and an antenna connecting the tuned circuit to the oscillator tube whereby approach to the antenna or tampering therewith will cause variation of the frequency of oscillation of said source, a source of constant frequency oscillations substantially equal to n times the frequency of the variable source where n is any integer, means for modulating the outputs of said sources to obtain beat frequencies between the fundamental of the constant frequency and the fundamental and harmonics of the variable frequency, whereby variations in the variable frequency source cause variations of the beat frequencies, means for selecting an audio beat frequency between the fundamental of the constant frequency and the nth harmonic of the variable frequency, and means responsive to variations of the selected audio beat frequency for actuating an alarm.

5. In a signaling system, a variable frequency source of oscillations, a source of oscillations of constant frequency bearing substantially integral means for selecting an audio beat frequency be-` tween the fundamental of the constant frequency and a harmonic of the variable frequency, means for converting the selected audio frequency to a variable direct current, and alarm means controlled by-vsaid direct current.

6. In a signaling system, a constant frequency source of oscillations, a variable source of oscillations comprising an oscillator tube, a tuned circuit, and an antenna connecting the tuned circuit tothe oscillator tube whereby approach to the antenna or tampering therewith will cause variation of the frequency of oscillation of said source, means for mixing the outputs of said sources to obtain beat frequencies between the fundamental of the constant frequency and the fundamental and harmonics of the variable frequency whereby variations in the variable frequency source cause variations of the beat frequencies, means for selecting an audio beat frequency between the fundamental of the constant frequency and a harmonic of the variable frequency means responsive to variations of the selected audio beat frequency for converting said variations to a variable direct current and means responsive thereto for actuating an alarm.

'1. In a signaling system, a constant frequency source of oscillations, a variable frequency source of oscillations, a modulator for adding the outputs of said sources and obtaining beat frequencies between the fundamental of the constant frequency and the fundamental and harmonics of the variable frequency, means for selectingA an audio beat frequency between a. predetermined harmonic of the variable frequency and the fundamental of the constant frequency, means for varying the frequency of the second source of oscillations whereby the beat frequencies of the output of said modulator vary, means responsive to the selected audio variable ybeat frequency for actuating an alarm.

8. In a signaling system, a constant frequency source of oscillations, a variablesource of oscillations comprising an oscillator tube, a tuned circuit, and an antenna connecting the tuned circuit to the oscillator tube whereby approach to the antenna or tampering therewith will cause variation of the frequency of oscillation of said source, means for modulating the outputs of said sources to obtain beat frequencies between the fundamental of the constant frequency and the fundamental -and harmonics of the variable frequency, whereby variation in the variable frequency source causes variations of the |beat frequencies over a range greater than the range of variations of frequency in the variable source, means for selecting an audio beat frequency between the fundamental of the constant frequency and a harmonic of the variable frequency and means responsive to variations of the selected audio beat frequency for actuating an alarm.

MAXWELL H. A. LINDSAY.

KONSTANTIN WOLOSCHAK.

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