US3462755A - Capacity alarm - Google Patents

Description

19, 1969 D. ELHANSEN 3,462,755

CAPACITY ALARM Filed April 26, 1966 5 Sheets-Sheet 1 mvsm'oa AT TOR N E-YS.

D. E. HANSEN CAPACITY ALARM Aug. 19, 1969 5 Sheets-$heet 2 Filed April 26, 1966 N ENTOR ywfiw AT'TO RN EYS 19, 1969 D. E. HANSEN 3,462,755

CAPACITY ALARM Filed April 26. 1966 5 Sheets-Sheet. 5

V0.4 746;? VOL 7/162 p9 :%ENTOR ATTORNEYS United States Patent 3,462,755 CAPACITY ALARM Donald E. Hansen, Brookfield Center, Conn., assignor to Mosler Research Products, Inc., Danbury, Conn., a corporation of Delaware Filed Apr. 26, 1966, Ser. No. 545,465 Int. Cl. G08b 13/26 U.S. Cl. 34l)258 8 Claims ABSTRACT OF THE DISCLOSURE A capacity type intruder alarm system, including an oscillator and a tuned resonant circuit. The tuned resonant circuit includes an antenna formed by equipment being protected. The detector senses changes in oscillations due to intrusion. The detector normally has a predetermined output signal which rises or drops due to the presence of an intruder. This detector output is amplified and applied to a multivibrator. When the detector output changes in either direction, the multivibrator is flipped to actuate an alarm. High and low fail-safe circuits are provided. The unit also includes a regulated power supply and a variable capacitance coupling between the oscillator and detector.

This invention relates to intrusion alarm systems and is particularly directed to a novel alarm system of the capacity type.

Capacity alarm systems are utilized in many different types of installations to protect small areas or selected equipment against unauthorized intrusion. Thus, in a typical installation, a capacity alarm system can be utilized in a store or office to protect storage units, such as safes, file cabinets and the like.

Conventionally, a capacity alarm system includes an oscillator having a tuned resonant circuit comprising an inductance and a capacitance. This tuned resonant circuit also includes an antenna, a portion of which is preferably constituted by the files or other equipment being pro,- tected. The oscillator thus establishes a radio frequency energy field in the area surrounding the equipment being protected. A detector is provided for sensing changes in the oscillations resulting from an introduction of an object or intruder into the field. In the present alarm system, the output from the detector is applied to an amplifier which in turn controls operation of a one-shot multivibrator. When this multivibrator shifts from one state to another, it actuates an alarm relay resulting in an alarm signal being sent from the protected area to the remote alarm station. The alarm relay can also be used to actuate a local alarm, or both a local and remote alarm.

One of the principal objects of the present invention is to provide an alarm system which is several times more sensitive than previously proposed systems of this type, and which at the same time is very stable. Thus, one of the present alarm units can protect an area or a number of pieces of equipment which previously required several alarm units. This not only results in greater economy, but also eliminates problems inherent in the interaction of a number of oscillators.

A second object of the present invention is to provide a capacity alarm which can readily be set up in a wide variety of environments to have a preselected sensitivity despite differences in power factors due to differing environmental conditions.

Another object of the present invention is to provide 7 3,462,755 Patented Aug. 19, 1969 ice causing an alarm is greatly increased.

A further object of the present invention is to provide a capacity alarm which will be actuated in response to the presence of either a high or a low voltage at the detector output. As a result, the present system is effective to automatically provide an alarm in the event of power failure, the application of an antenna ground, an external signal, component failure or the like.

It is still another object of the present invention to provide a capacity alarm in which drain upon the selfcontained battery, or other power sources, is minimized. Thus, for example, a typical alarm of the present type can be operated from conventional dry cells for a period in excess of one year.

More particularly, a preferred form of capacity alarm constructed in accordance with the principles of the present invention is powered from an unregulated dry cell or battery, the output of which is applied to a voltage regulator. This regulator maintains the battery output voltage at a predetermined fraction; for example, 60% of the maximum battery potential. The regulated voltage is applied to a voltage limited oscillator circuit. The oscillator applies an AC voltage to a resonant tank circuit. The provision of a regulated voltage driving the oscillator is not only effective to increase battery life, but also to maintain a constant voltage across the resonant circuit. This in turn stabilizes the sensitivity of the entire alarm system.

The present alarm system further includes a variable capacitor coupling the oscillator and detector. This variable coupling capacitor provides means for controlling the sensitivity of the alarm from unit to unit so that the same sensitivity can be maintained in all units despite changes in power factor due to differing environmental conditions.

More particularly, the variable coupling capacitor is set initially so that the meter of the alarm unit reads full scale under the worst power factor conditions likely to be encountered. The unit is then further adjusted by means of a trimmer capacity so that it operates at a point approximately 70% of the full scale meter voltage. As is explained in detail below, when the unit is operated in an installation presenting more favorable power factor conditions, the trimmer capacity is readjusted to return the meter to the same voltage reading. This results in an operation in which the change in voltage for a given change in capacitance is'substantially the same as it was under the worst power factor conditions.

The present alarm system further includes a-one-shot multivibrator adapted to be actuated by an amplified signal taken from the detector output. The one-shot multivibrator is effective to cause an alarm in the event that there is either an increase or decrease in capacity in the oscillator tuned resonant circuit. This multivibrator is also effective to provide both a high and a low fail-safe. Thus, an alarm is actuated if there is a potential either above or below predetermined limits in the detector output circuit. As a result, the present alarm system is effective to provide an alarm not only in the event of intrusion, but also in the event of battery, or other components, failure or attempted compromise, e.g. by the application of an external potential to the alarm circuit or by grounding of the antenna.

These and other objects and advantages of the present invention will be more readily apparent from a consideration of the following detailed description of the drawings illustrating a preferred embodiment of the invention.

In the drawings:

FIGURE 1 is a semi-diagrammatic perspective view of a typical installation of the present alarm system.

FIGURE 2 is a schematic circuit diagram of the present alarm system.

FIGURES 3A, 3B and 3C are diagrammatic views showing the relationship between meter readings and voltage versus capacity curves for this system at the worst power factor and at a more optimum power factor.

As shown diagrammatically in FIGURE 1, in one typical installation an alarm system is utilized to protect a bank of storage equipment, including a safe 11 and a file 12. It will be appreciated, however, that the present alarm system can be utilized to protect many different types of equipment in stores, ofiices and the like. The units being protected are insulated from ground as indicated diagrammatically by strips of dielectric material 13 and are electrically tied together as by means of conductive strips 14. The alarm system 10 comprises a local unit 15 mounted within the area being protected and a remote unit 16. The remote alarm unit 16 may be located at a central guard station or the like.

The local unit 15 and remote unit 16 are interconnected by means of a suitable conductive cable arrangement 17. The local unit 15 is interconnected to the safe and file being protected as by means of conductor 18. As a result, the safe and file unit form part of the antenna system of the alarm unit. The local unit 15 is also provided with a ground line indicated at 20.

The details of the alarm circuit are best shown in FIGURE 2. As there shown, the alarm includes a DC voltage regulator which supplies regulated voltage to a transistorized oscillator circuit 26. The transistorized oscillator circuit includes a parallel resonant tank circuit indicated at 27. The oscillator is connected to an antenna system 28 including the apparatus, such as safe 11 and file 12, being protected. Oscillator 26 is effective to radiate low radio frequency energy through this antenna system into the area surrounding the file and safe unit. As a result, an electrostatic field of stored low radio frequency energy is established surrounding this equipment. The alarm circuit further includes a detector 30 including a high Q-detector coil which is coupled to the oscillator through a variable capacitor 29. This coil is also tied to the antenna system and is tuned to the frequency of the oscillator. Any increase in capacity to ground in the resonant circuit is effective to cause a voltage drop across the coil, while a decrease in capacitance will cause an increase in voltage across the coil.

The detector signal is in turn fed to a DC amplifier 31. Amplifier 31 is connected to a one-shot multivibrator indicated at 32. This multivibrator in turn controls energization of an alarm relay 33. When this latter relay is dropped out, an alarm is energized at the central, or remote, unit 16. Alternatively, an alarm can be actuated at the local unit 15, or an alarm can be energized at both the local and remote units.

Considering the circuit in more detail, the voltage regulator portion of the circuit 25 includes terminals 34 and 35 adapted to be connected to an unregulated threevolt battery or dry cell. Positive terminal 34 is connected to the emitter of transistor 41. Transistor 41 and transistor 42 form a series voltage regulator with Zener diode 43 providing the voltage reference element. This Zener diode is connected between the collector 44 of transistor 41 and the emitter 45 of transistor 42. The base 46 of transistor 41 is interconnected to the collector 47 of transistor 42. Base 46 is also connected through resistor 50 and lead 48 to battery terminal 34. This resistor is provided to reduce the effect of leakage current of transistor 41. A second resistor 51 interconnects lead 48 and a lead 52, lead 52 in turn being joined to collector 44 and diode 43. Resistor 51 is provided to insure starting of the regulator.

Negative battery terminal 35 is connected to a common line 53 which is grounded. A resistor 54 interconnects this line and emitter 45 of transistor 42. A potentiometer winding 55 is shunted between lines 52 and 53. The tap 56 of this potentiometer is tied to base 57 of transistor 42. Potentiometer 55 is adjusted to provide a voltage substantially less than the maximum voltage of the dry cells or battery connected between terminals 34 and 35. For example, in one preferred embodiment, the dry cells connected between these terminals have three-volt maximum potential, while potentiometer 55 is adjusted to maintain a 1.8 volt potential at collector 44. A capacitor 58 is also shunted between lines 52 and 53.

The regulated output voltage from regulator 25 is applied to the oscillator circuit 26. This circuit includes a transistor 60, the emitter 61 of which is tied to line 53. The collector 62 of transistor 60 is joined to primary winding 63 of transformer 64. Winding 63 is also connected to lead 52. A second primary winding 65 of transformer 64 is interconnected to the base 66 of transistor 60 and to a lead 69. Lead 69 is interconnected to line 52 through resistor 67 to provide bias for transistor 60. Lead 69 is also interconnected to line 53 through R.F. capacitor 68. The secondary winding 70 of transformer 64 is shunted by capacitor 71. These two components constitute the parallel resonant tank circuit of the oscillator. Secondary winding 70 and capacitor 71 determine the R.F. output of the oscillator circuit. Preferably, the output frequency is in the low radio frequency range; for example, 15 kc.

Oscillator 26 operates as a voltage limited oscillator, i.e. with the oscillator collector 62 swinging through the full applied voltage at the loads which are applied. Consequently, the AC voltage across the parallel resonant circuit 27 remains constant at the value determined by the voltage regulator 25.

The oscillator output is coupled to the detector circuit 30 through variable coupling capacitor 29. A trimmer capacitor 72 interconnects ground line 53 and lead 73 which is joined to the coupling capacitor.

This trimmer capacitor is connected in parallel with primary winding 74 of detector transformer 75. One leadof winding 74 is tied to common line 53, while the other lead of the transformer is connected through lead 73 to coupling capacitor 29. The antenna system 28, including files 12 and safe 11, is connected through lead 18 to an intermediate tap on primary winding 74. The casing 76 of the alarm unit 15 is preferably also connected to lead 18 through capacitor 77. As a result, all of the circuitry of the local alarm unit mounted within local unit 15 is protected since casing 76 in effect forms part of the antenna system.

Detector transformer 75 is provided with a secondary winding 78 having one lead connected to common line 53. The other lead of this winding is connected through choke 80 to the base 81 of detector transistor 82. Base 81 of transistor 82 is also connected to common line 53 through a capacitor 83. The choke 80 and capacitor 83 function to suppress high frequency signals which might be picked up in the antenna circuit. The output current flowing through emitter 84 is filtered by a capacitor 85 and is fed to a meter 86. One terminal of meter 86 is connected to emitter 84 while the second terminal is connected through resistance 87 to the common line 53. Meter 86 is a DC current meter, the scale 88 of which is illustrated in FIGURE 3A. It will be noted that the collector 90 of transistor 82 is connected to positive terminal 34 through lead 48.

The detector output signal appears at junction 91. This output is coupled through capacitor 92 to the base 93 of transistor 94. Transistor 94 together with transistors 95 and 96 form a direct coupled stabilized amplifier. Transistor 94 is a high impedance input stage allowing a reduction in size of the coupling capacitor 92, while retaining an adequate time constant. The collector 97 of this transistor is connected to line 52, while the emitter 98 is connectedthro'ugha' diode 100 and resistor 101 to common lead 53. Transistor 95 'is a"'commor'1 emitter stage stabilized frorna feedback through resistor 102, this resistor being interconnected-between the collector 103 of transistor 95 and the base 93 of' transistor 94. The collector-103 of transistor'95is also connected through resistor 104 to line 52.'Emitter 105 of this transmitter is connected to commonline 153'. t

Transistor 96 provides a low impedance output for the amplifier and a stabilized voltageto the input of diodes 106 and 107. Two other diodes'108and 109 are joined to the emitter 111 of transistor 96. These diodes are connected in parallel and are joined to emitter 98 of transistor 94 through potentiometer 11 2. Potentiometer 112 controlsthe' gainof the amplifier and consequently the sensitivity of the alarm system. Diodes 108 and 109 in the gain circuit compensate for the change in voltage drop with temperature of diodes 106 and 107.

The output of the amplifier is taken from emitter 1-11 of transistor 96. This emitter is tiedto coupling lead 114. A resistor 115 is'connected between this lead and common line 53. Lead 114 is connected'to the one-shot multivibrator formed by transistors'116 and'117. The base 118 of transistor 117 normally-receives bias voltage through a low voltage fail-safe circuit. Specifically, this circuit includes lead 120 and resistors 121 and 122 which are effective to interconnect base 118 to junction 91 (the detector output). As a result, transistor 117 is normally biased on. a I

The other transistor 116 of the multivibrator includes a collector 123 which is joined to lead 52 through a resistor 139 and an emitter 124 connected to common line 53. A capacitor 125 is interconnected betweencollector123 and lead 120. This capacitor is normally charged with the polarity shown in FIGURE 2. The base 126 of transistor 116 is connected through the'parallel combination of resistor 127 and capacitor 128 and. through lead 129 to the collector 130 of transistor 117. Rectifiers 106 and 107 interconnect the base 118 of transistor 117 with the base126 of transistor 116. I

It is to be understood that transistor 116 is normally biased off and transistor 117 is normally biased on. The collector circuit of transistor 117 includes the,coil of relay 33 and a bypass capacitor'131. It is to be understood that the relay is provided with suitable contacts arranged so that as long as the relay is held energized, no alarm is is given. However, when the relay drops out in response to the turn-off of transistor 117, an alarm signal is given. This may cause a local alarm to be sounded, a remote alarm to be sounded, or both. The circuits actuated by the contacts of relay 33 are conventional and no detailed explanation is considered necessary.

In addition to these components, the alarm circuit includes a high voltage fail-safe circuit including a diode 134, one side of which is connected between resistors 121 and 122, and the other side of which is connected to base 93 of transistor 94. In operation, if the alarm unit is operating normally, diode 134 is non-conductive. However, if the voltage rises, diode 134 becomes conductive. This causes the amplifier output voltage (114) to drop and if this condition persists, the alarm unit will cycle in and out of alarm.

The initial set-up and manner of operation of the present alarm unit can best be appreciated from a consideration of FIGURE 3A, 3B and BC in conjunction with FIG- URE 2. More particularly, prior to shipment of the present alarm system, coupling capacitor 29 is normally adjusted so that the needle of meter 86 reaches, full scale with a full antenna load and a shunt resistor representing the worst power factor which should be encountered using reasonable care in installing the system. With this loading the trimmer capacitor 72 is then increased to cause the meter needle to be brought within the operating range.

FIGURE 3B represents a Q-curve at the worst possible power factor. The operating range is chosen so that it preferably lies at about 70% of the full scale voltage on the voltage capacity curve of FIGURE 3B.

It will, of course, be appreciated that most installations are not 'made'under load conditions which give the worst power factor. Rather,the installations are made under more favorable power factors. The Q-curve of a system operating at a better power factor is illustrated in FIG- URE 3C. When the alarm system is actually installed under conditions resulting in the better power factor represented by FIGURE 3C, the trimmer capacitor 72 is adjusted to bring the meter needle into the operating range which is still approximately 70% of the maximum voltage at the worst power factor. It will be'seen that the alarm is thus operating in a substantially lower portion of the Q-curve than would be the case if the unit were set to operate at 70% of the maximum voltage of the curve in FIGURE 3C.

At this lower operating point which is actually obtained through the initialadjustment of variable capacitor 29, a much lower rate of change of voltage with capacity, i.e. vV/dC, is obtained. This rate of change of voltage with capacity more nearly approximates that obtained under the original condition as shown in FIGURE 3B. Thus, it is relatively easy to install a number of alarm systems of the present type in various environments having quite different power factors so that the alarms will have generally the same sensitivity. In other words, a given increase in capacity caused by an intrusion will result in approximately the same change in detector voltage in each of the alarm systems.

After an alarm system has been installed and adjusted, the multivibrator is normally in a condition in which transistor 117 is turned on and transistor 116 is turned off. Capacitor is charged to the polarity shown. If a person intrudes upon a protected area and approaches the protected units, such as the filing cabinets or safe, he causes an increase in capacity of the system relative to ground of the tuning circuit. This causes a drop in the voltage across winding 78 and a drop in the rectified voltage at junction 91. The voltage in lead 114 at the output of the amplifier rises. When the voltage at lead 114 rises to a certain point, diode 106 begins to conduct, turning on transistor 116. When transistor 116 turns on, the capacitor 125 begins to discharge, turning off transistor 117. This causes relay 33 to drop out, establishing an alarm signal. If the voltage in lead 114 has returned to normal when the capacitor 125 is fully reverse charged to approproximately one-half volt, transistor 117 turns on, turning off transistor 116. Relay 33 is again energized and the system is ready for another alarm. (It will, of course, be appreciated that if desired, the circuit energized by the contacts of relay 33 can be provided with a hold-in arrangement effective to maintain the alarm signal despite recycling of the multivibrator and reenergization of relay 33.)

The present alarm system is also effective to detect and cause an alarm in response to a decrease in capacitance such as that which would accompany a movement by an intruder away from the protected area. In the case of such a withdrawal, there is a decrease in capacitance and an increase in voltage at junction 91. This in turn causes a decrease in voltage in the amplifier output lead 114. In such a case, diode 107 becomes conductive. This again results in the turning off of transistor 117 and the turning on of transistor 116. When transistor 117 is turned off, it drops out relay 33 causing an alarm signal as explained above.

Capacitance 125 again goes through a discharge and reverse charging cycle. If as before when capacitor 125 is completely reverse charged to one-half of a volt, the voltage at lead 114 is returned to normal, transistor 117 turns on and transistor 116 turns ofi so that the system is ready for a subsequent alarm.

In addition to providing for an alarm in the event of an increase or decrease in capacitance, the present system also provides an alarm in the event of an increase or decrease in detector output signal. Specifically, the system is protected against a low detector output signal by a low fail-safe circuit directly interconnecting the base of transistor 117 with the detector output at junction 91. This circuit includes resistors 121 and 122 and lead 120. Should the detector voltage drop below a predetermined limit (for example to one-third the full scale voltage of meter 86), the base current of transistor 117 will be reduced to a point at which the relay 33 is dropped out causing an alarm signal. This low fail-safe circuit is thus effective to supervise the oscillator and antenna circuits against power failure, component failure, antenna grounds and the like.

The system is also protected against a high detector output voltage by a high fail-safe circuit including voltage divider resistors 122 and 121 and diode 134. When the alarm unit is operating in the normal range, diode 134 is non-conductive. However, if the detector output voltage at junction 91 rises, diode 134 becomes conductive. This will cause the voltage in lead 114 to drop, deenergizing transistor 117 and dropping out relay 33. If the high voltage persists, the alarm unit will cycle in and out of alarm as explained above.

Having described my invention, I claim:

1. A capacity alarm system comprising an oscillator, a tuned resonant circuit connected to said oscillator, said resonant circuit including antenna means, a detector for sensing changes in capacitance in said tuned resonant circuit, said detector normally having a predetermined DC output potential, and developing changes in said DC potential in response to changes in said capacitance, amplifier means for amplifying said changes of DC potential, a multivibrator having an input lead connected to said amplifier means and having an output circuit effective to energize an alarm relay, said multivibrator normally being in a first state when said detector output signal corresponds to said predetermined DC potential, said multivibrator being shifted from said first to a second state to cause actuation of said relay in response to either a rise or fall in the voltage output of said amplifier means, said changes of DC potential corresponding to an increase or decrease in capacitance in said tuned resonant circuit.

2. The capacity alarm system of claim 1 including a low voltage fail-safe circuit interconnecting said detector and said multivibrator, said low voltage fail-safe circuit being effective to cause said multivibrator to shift to said second state when said DC potential of said detector drops below a predetermined level.

3. The capacity alarm system of claim 2 in which said low voltage fail-safe circuit comprises a resistive connection between said detector and said multivibrator, said fail-safe circuit normally providing operating bias for holding said multivibrator in said first state.

4. The capacity alarm system of claim 1 in which the connection between said oscillator and detector comprises a variable capacitance and in which said detector circuit includes a DC meter adapted to be set at full scale reading by adjustment of said variable capacitance.

5. The capacity alarm system of claim 1 in which said oscillator is a voltage limited oscillator and in which voltage regulator means are provided for applying power to said oscillator, said detector, said amplifier and said multivibrator.

6. The capacity alarm system of claim 1 including a high voltage fail-safe circuit interconnecting said detector and said amplifier, said high voltage fail-safe circuit being effective to cause said multivibrator to shift to said second state when said DC potential of said detector rises above a predetermined level.

7. The capacity alarm system of claim 6 in which said high voltage fail-safe circuit comprises a voltage divider network interconnecting said detector and said multivibrator, a normally non-conductive diode connected to the voltage divider network, said diode becoming conductive upon a rise of detector output above a preselected level and being effective to cause said multivibrator to shift to said second state.

8. The capacity alarm system of claim 2 further including a high voltage fail-safe circuit interconnecting said detector and said amplifier, said high voltage fail-safe circuit being effective to cause said multivibrator to shift to said second state when said DC potential rises above a predetermined level.

References Cited UNITED STATES PATENTS 3,041,592 6/1962 Schmidt 340258 3,235,857 2/1966 Bagno 340258 X 3,255,380 6/1966 Atkins et al 340 258 X 3,309,689 3/1967 Keeney 340258 JOHN W. CALDWELL, Primary Examiner D. L. TRAFI ON, Assistant Examiner US. Cl. X.R.

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