US3276005A - Capacity intruder alarm having capacitive a.c. coupling and d.c. bias coupling in parallel between a detector and amplifier - Google Patents

Description

p 1956 E. G. QUEST ETAL 3,276,005

CAPACITY INTRUDER ALARM HAVING CAPACITIVE A.C. COUPLING AND D.C. BIAS COUPLING IN PARALLEL BETWEEN A DETECTOR AND AMPLIFIER Filed May 5, 1964 QAJIN .II

INVENTORS United. States Patent CAPACITY rNTRUonii AIZARM HAVING CAPACI- TIVE A.C. COUPLING AND D-C. BIAS COUPLING IN PARALLEL BETWEEN A DETECTOR AND AMPLIFIER Eric G. Quist, Roxbury, Conn, and Richard H. Geis, Hopewell Junction, N.Y., assignors to Mosler Research Products, Inc., Danbury, Conn, a corporation of Delaware Filed May 5, 1964, Ser. No. 365,034 7 Claims. (Cl. 340258) This invention relates to security alarm systems and is particularly directed to a novel alarm system of the capacity type.

The present capacity alarm is particularly adapted to protect small areas against unauthorized intrusion. For example, the alarm can be utilized to protect the ofiice area surrounding file cabinets, safes and the like. In general, the present alarm system comprises an oscillator which is connected to a tuned resonant circuit. The tuned resonant circuit includes an antenna element which may be constituted in part by the files, safes and other elements being protected. The alarm further includes a detector for sensing any changes in capacity in the resonant circuit, such as might be caused by the presence of an intruder in the area. This change in capacity manifests itself as a change in potential of the output of the detector. In the present alarm system the detector is coupled to an amplifier stage which in turn controls the energization of a relay for actuating either a remote alarm, a local alarm or a combination of the two.

It will be appreciated by those skilled in the art that the presence of an intruders body in the electromagnetic field surrounding the antenna results in a relatively small change in capacitance, for example, a change of the order of micromicrofarads. Consequently, the detector and relay amplifier must be quite sensitive. At the same time, while the alarm system must signal the approach of an intruder with the utmost reliability, it is likewise important that the system not give false alarms due to slowly changing environmental conditions such as temperature, humidity and the like. Nevertheless, if these slow changes in the detector output continue beyond a given amount, they indicate a failure in one or more of the alarm components, such as battery voltage or the like. In such a case, the alarm system is not efifective to provide proper surveillance of the area. Consequently, it is desired that an alarm signal be given if the detector output change exceeds a predetermined amount, no matter how slowly this change takes place.

The present invention is predicated upon the concept of providing a fail-safe circuit eifective to cause an alarm under each of these two conditions: a rapid change in detector output corresponding to an intrusion and a change in output in excess of a predetermined amount corresponding to a malfunction. More particularly, the present failsafe system interconnects the detector stage and the amplifier stage of the alarm. The fail-safe circuit comprises a first purely resistive path between the detector and the amplifier, which path provides an operating bias for the amplifier. The fail-safe circuit further comprises a second conductive path including a capacitor and a series connected sensitivity resistor interconnecting the detector and amplifier stage. The amplifier is rendered normally conductive by the applied bias and is connected in the alarm circuit so that the alarm is actuated when the output of the amplifier stage is substantially reduced or is completely cut off.

The first conductive path of the fail-safe circuit, which provides an operating bias for the amplifier, protects the system against component failure no matter how slowly Patented Sept. 27, 1966 that component failure may cause a change in detector voltage. For if the oscillator, tuned resonant circuit or detector are not functioning properly, the detector output will not be sufiicient to apply the requisite bias potential to the amplifier and this amplifier will in turn cease to conduct sufiicient current to prevent actuation of the alarm.

The second conductive path includes a capacitor which is charged up when the alarm unit is first turned on. When a sudden increase occurs in the capacitance of the resonant circuit due to the intrusion of a burglar, or the like, the detector potential drops rapidly and the capacitor discharges in the reverse direction to substantially decrease or stop the output of the amplifier and thereby actuate the alarm.

One important advantage of the fail-safe circuit is that it provides maximum protection against both the intr-usion of the area and malfunction of the unit with very simple and reliable circuitry.

Still another advantage of the present alarm system is that it minimizes the time during which the system is insensitive after it is turned on. More particularly, it Will be appreciated that most alarm systems are utilized only during portions of each twenty-four hour period, for example, at night when an office is empty. Thus, a typical alarm is turned 01f in the morning when the ofli-ce is opened and workers arrive and is turned on again at night when the last worker leaves. In the past, alarm systems of the capacitor type have been subject to the defect that for an appreciable length of time after the alarm was turned on it was relatively insensitive so that an intrusion could occur without setting off an alarm.

The present invention is in part based upon the concept of minimizing, or substantially eliminating, this period of insensitivity by providing a rectifier in shunt with the sensitivity resistor. This rectifier is connected so as to provide a low impedance circuit to the capacitor enabling the capacitor to charge rapidly when the unit is .turned on; and a high impedance path to the discharge of the capacitor so that the capacitor discharges through the sensitivity resistor which remains effective to prevent false alarms. I

Another advantage of the present alarm circuit is that it facilitates the use of a standard relay eliminating the need for an expensive meter relay of the type previously required. The elimination of a meter type relay not only renders the system more economical, but it also results in a system having a more rapid response than was previously possible or practical.

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

In the drawing:

FIGURE 1 is a diagrammatic view showing the manner in which a capacity alarm system of the present type is installed to protect oflice equipment.

FIGURE 2 is an electrical wiring diagram of the pertinent portion of the present capacity alarm system.

The present alarm system is particularly adapted for the protection of storage units in offices, stores and the like, although it will readily be appreciated that the unit has utility in other installations as well. One typical installation of the present system is shown in FIGURE 1. As is there shown, the objects to be protected include a file cabinet 10 and a safe 11. These units are electrically connected to one another as indicated by the connector strip 12. Both the file cabinet 10 and safe 11 are insulated from ground as indicated diagrammatically by insulation blocks 16, 14 and 15.

The present alarm system includes an oscillator circuit and a detector circuit which are connected through a lead 16 to the safe 11 and file cabinet 10. These latter units, which are connected in series, form an antenna system for the present alarm. The oscillator circuit is effective to radiate radio frequency energy through the resonant antenna into the area surrounding the antenna system. Thus, an electro magnetic field of stored low radio frequency energy is set up in the area of the room surrounding the safe and file cabinet. The electrical circuit also includes a high-Q detector coil which is coupled to the oscillator and tuned to the frequency of the oscillator.

The detector circuit is tuned so that any increase in capacity to ground of the resonant circuit will cause a voltage drop across the coil. Consequently, as a person approaches the protected units, such as cabinet and safe 11, he increases the capacity of the resonant circuit and causes a voltage drop in the detector circuit which is ultimately utilized to actuate relays causing the energization of either a local alarm, a remote alarm or a combination of the two.

In the system illustrated in FIGURE 1, a power supply, oscillator circuit, detector circuit and relay for controlling a remote alarm and local alarm (not shown), are mounted in an electrically conductive housing 17 which itself preferably forms part of the antenna system.

This housing is interconnected through a suitable cable 18 to a remote alarm 19.

More particularly, as is shown in FIGURE 2, the capacity alarm comprises a power source, such as batteries 20, a low radio frequency oscillator circuit 21, a detector stage 22 and a relay amplifier stage 23. The relay amplifier stage 23 controls energization of a relay coil 24 which in turn effects actuation of remote alarm 19. It is to be understood that relay 24 can likewise be used to control energization of a local alarm, such as a bell (not shown), located in or near the area being protected.

As is shown in FIGURE 2, the positive terminal of battery is connected to a grounded common line 25 while the negative terminal of the battery is connected to line 26. A capacitor 27 is shunted across these lines. Line 26 is joined to the movable contact 2 8 of a threeposition switch 30. Switch 30 is a day-night switch and is provided with a night contact 31, an open day" contact 32 and a test contact 33. It is also to be understood that in practice day-night switch 30 is a multi deck switch and that the decks (not shown) are utilized in conjunction with timer circuits, test circuits, and the like, which have been omitted from the present schematic diagram since these circuits constitute no part of the present invention.

Both contacts 31 and 33 of switch 30 are joined to line 34. A lead 35 is connected to line 34. This lead is connected to base 36 of oscillator transistor 37 through resister 38. A capacitor 40 is shunted between lead 35 and common line 25. Transistor 37 also includes an emitter 41 connected to common line 25 and a collector 42 joined to win-ding 43 of oscillator transformer 44. The opposite terminal of winding 43 is connected to base 36 through capacitor 45. The secondary winding 46 and tank capacitor 47 form the oscillator tank circuit and determine the RF output of the oscillator circuit. The output frequency of this circuit is preferably in the low radio frequency range, for example 15 kc.

One terminal of secondary winding 46 is tied to common line 25 through lead 48, while the opposite terminal of this winding is connected through coupling capacitor 50 to the antenna tunning circuit which includes tuning capacitor 51 connected in parallel with the primary winding 52 of detector transformer 53. Specifically, one lead of winding 52 is connected to common line 25 while the other lead is connected through lea-d 54 to capacitor 50. The antenna system includes cabinets 10 and '11 which are connected through lead 16 to an intermediate tap 55 of primary winding 52 and housing 17 of the alarm unit which is connected to primary winding 52 through lead 56 and capacitor 57. Thus, the alarm circuitry and the local alarm (if mounted within housing 17) are protected since the housing 17 forms part of the antenna system.

Detector transformer 53 further includes a secondary winding 58 which is connected to common line 25 and through inductance 60 to base 61 of detector transistor 62. A capacitor 63 is shunted between base 61 and common line 25. Detector transistor 62 includes a collector 64 connected through lead 65 to line 34. The emitter 66 of transistor 62 is connected through resistor 67 to a milliammeter 68, the other terminal of the milliammeter being connected to common line 25. Emitter 66 is connected through resistor 70 to 'base 7.1 of relay amplifier transistor 72. A diode rectifier 80 and coupling capacitor 81 are connected in series with one another and are shunted across resistor 70. A sensitivity'resistor 82 is in turn shunted across diode rectifier 89. In one circuit, resistor 70 is 22K ohms, resistor 82 is 5.1K ohms and capacitor 81 is 3000 microfarads. A filter capacitor 73 is shunted between emitter 66 and common line 25. Emitter 74 of transistor 72 is connected through lead 75 to common line 25. The collector 76 of transistor 72 is connected through one lead of relay coil 24. The opposite end of this relay coil is joined to line 34 and the relay coil is shunted by a capacitor 77.

Relay 24 includes a movable contact 83 engageable with either fixed contact point 84- or 85. The relay is shown in an energized condition in FIGURE 2. With the relay in this condition, movable contact 83 is in en gagement with lower stationary contact 85. When the relay is 'deenergized, movable contact 83 is shifted into engagement with upper stationary contact 84, completing a circuit to actuate remote alarm 19 and a local alarm (not shown).

In operation, the detector coil 52 is tuned so that any increase in capacity to ground in the vicinity of the antenna system, i.e. cabinets 10 and 11 and casing 17, will cause a drop in voltage developed across winding 58. This is accomplished by tuning the coil from minimum capacity through the resonant peak and slightly beyond it. Consequently, if a person approaches the protected objects, he causes an increase in the capacity relative to earth ground of the tuning circuit with the resultant drop of voltage across winding 58. across the detector winding is rectified and is A.C. coupled to the transistor relay stage 23. Consequently, any drop in rectified voltage will cause relay 24 to drop out, energizing the remote and local alarms.

Specifically, the rectified -D.C. voltage, which is of the order of .70 v. with respect to ground, appears at the emitter 66 of transistor 62. Capacitor 73 acts as a filter for. this voltage. This voltage is A.C. coupled through resistor 82 to the relay amplifier stage 23, L6. to the base of transistor 72. This transistor is in a saturated or full on state so that practically all of the supply voltage is developed across the relay coil 24 in the collector circuit of the transistor 72.

It will be appreciated that relay amplifier transistor 72 receives its DC. bias through resistor 70 interconnecting base 71 of transistor 72 and emitter 66 of transistor 62. If this bias voltage is insufiicient, as for example because of the excessive tuning of the detector coil or low supply voltage, transistor 72 will not be saturatedrand relay 24 will not be energized to cause an alarm. Thus, this portion of the circuit functions as a fail-safe circuit which has the efiect of being a constant monitor of the detector tuning, oscillator voltage, battery voltage and of the circuit components which might be defective. It will readily be appreciated that if transistor 72 were biased directly from battery 20, a slow drop in detector voltage to an insensitive state could happen Without triggering an alarm. Moreover, turning on the alarm with the detector detuned or the oscillator inoperative would also fail to cause an alarm.

Capacitor 81 is a coupling capacitor which is effec- This voltage developed tive to provide a low impedance coupling path for shortterm changes. Specifically, when the capacitor alarm is first turned on and properly balanced, a D.C. voltage appears at the emitter 66 of transistor 62. Transistor 72 being biased from this point begins to draw bias current through a circuit from its emitter 74 through its base 71, and resistor 70 to the emitter 66 of transistor 62.

Since capacitor 81 must also be charged to approximately -.70 volt, additional current flows from the emitter of transistor 72 through its base, rectifier 80, and resistor 82 to the capacitor 81.

After capacitor 81 becomes charged, the only current continuing to flow between stages 22 and 23 will be bias current flowing through resistor 70. Any slow, long-term changes in the detector voltage appearing at collector 66 will cause a very slow charge or discharge of coupling capacitor 81, but this will not cause any appreciable change in the saturated state of transistor 23. However, any rapid decrease in the detector voltage causes a likewise rapid discharge of capacitor 81. This capacitor discharges through resistor 82 and through the base emitter circuit of transistor 72. Since this discharge current is in the opposite direction from the bias current flow from the emitter 74 of transistor 72 to the emitter 66 of transistor 62, the net bias current is reduced depending upon the rate at which capacitor 81 discharges. If the bias current is reduced by a suflicient amount, transistor 72 will no longer be saturated and will not pass enough current through its collector circuit to keep relay 24 energized. When this relay is deenergized, contact 83 shifts into engagement with stationary contact 84 causing actuation of remote alarm 19 and the local alarm.

Resistor 82 serves as a desensitizing resistor. Specifically, this resistor reduces the coupling between detector transistor 62 and amplifier transistor 72 and limits the frequency response of the alarm. This resistor further inherently tends to limit the charge up rate of ca pacitor 81 which adversely affects the sensitivity of the alarm unit when it is first turned on. Specifically, as long as capacitor 81 is charging, it is drawing bias current through transistor 72. Consequently, a large drop in detector voltage is required to cause the capacitor to start discharging through transistor 72 to render it nonconductive.

In accordance with the present invention, a prolonged loss in sensitivity is avoided and the time of less than maximum sensitivity is minimized by diode 80. Diode 80 effectively short circuits the resistor 82 during charge up because it is forward conducting, i.e. provides a low impedance path to capacitor 81. Capacitor 81 thus charges up to maximum potential very rapidly and provides a fast set up time for the alarm as far as sensitivity is concerned. By way of example, the present alarm is set up in approximately one second as compared to the forty seconds required by previous units. Nevertheless, when the unit is in operation, resistor 82 is still effective to limit the sensitivity of the device as it was intended. More particularly, an alarm is caused by the discharge of capacitor 81 in a reverse direction in which rectifier 80 is nonconductive. Thus, the entire discharge current of capacitor 81 must flow through resistor 82.

From the foregoing disclosure of the general principles of the present invention and the above description of a preferred embodiment, those skilled in the art will readily comprehend the various modifications to which the invention is susceptible. Therefore, we desire to be limited only by the scope of the following claims.

Having described our invention, we 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 and developing a D.C. potential in response thereto, amplifier means, fail-safe circuit means interconnecting said detector and said amplifier means, said fail-safe circuit means including means providing a conductive path for D.C. operating bias from said detector for said amplifier means, whereby said amplifier output is substantially reduced in response to a gradual change in detector output in excess of a predetermined magnitude, said fail-safe circuit also including a capacitor providing an A.C. coupling between said detector and amplifier means, said capacitor being discharged in response to a rapid change in detector output and being effective to substantially reduce the output of said amplifier means, and alarm means energized by said amplifier means and adapted for actuation in response to said substantial reduction in the output of said amplifier means.

2. 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 antenna circuit and developing a D.C. potential in response thereto, amplifier means for controlling actuation of an alarm, and fail-safe circuit means interconnecting said detector and said amplifier means, said failsafe circuit comprising a resistor interconnecting said detector and said amplifier means to provide a D.C. operating bias for said amplifier means, and parallel circuit means including a capacitor effective to cause current flow in a reverse direction to said amplifier in response to an increase in capacity in said resonant circuit and a decrease in potential of said detector.

3. 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 antenna circuit and developing a D.C. potential in response thereto, amplifier means for controlling actuation of an alarm, and fail-safe circuit means interconnecting said detector and said amplifier means, said fail-safe circuit comprising a resistor interconnecting said detector and said amplifier means to provide a D.C. operating bias for said amplifier means, and parallel circuit means including a capacitor and series connected sensitivity resistor effective to cause current flow in a reverse direction to said amplifier in response to an increase in capacity in said resonant circuit and a decrease in potential of said detector.

4. A capacity alarm system comprising an oscillator, 21 tuned resonant circuit connected to said oscillator, said resonant circuit including antenna means, a detector for sensing changes in capacitance in said tuned resonant antenna circuit and developing a D.C. potential in response thereto, amplifier means for controlling actuation of an alarm, and fail-safe circuit means interconnecting said detector and said amplifier means, said fail-safe circuit comprising a resistor interconnecting said detector and said amplifier means to provide a D.C. operating bias for said amplifier means, and parallel circuit means including a capacitor and series connected sensitivity resistor effective to cause current flow in a reverse direction to said amplifier in response to an increase in capacity in said resonant circuit and a decrease in potential of said detector, and a diode shunting said sensitivity resistor, said diode being connected to provide a high impedance path to current discharged by said capacitor.

5. A capacity alarm system comprising an oscillator, a tuned resonant circuit connected to said oscillator, said resonant circuit including antenna means and an inductive winding, a first transistor connected to said inductive winding for sensing changes in capacitance in said tuned resonant antenna circuit, said transistor including a collector having a D.C. potential in variable response to said change in capacitance, a second transistor for controlling actuation of an alarm, and fail-safe circuit means interconnecting said first transistor and said second transistor, said fail-safe circuit comprising a resistor interconnecting said collector and said second transistor to provide a D.C.

operating bias for said second transistor, and parallel circuit means including a capacitor effective to cause current flow in a reverse direction to said second transistor in response to an increase in capacity in said resonant circuit and a decrease in potential of said collector.

6. A capacity alarm system comprising an oscillator, a tuned resonant circuit connected to said oscillator, said resonant circuit including antenna means and an inductive winding, a first transistor connected to said inductive winding for sensing changes in capacitance in said tuned resonant antenna circuit, said transistor including a collector having a D.C. potential in variable response to said change in capacitance, a second transistor for controlling actuation of an alarm, and fail-safe circuit means interconnecting said first transistor and said second transistor, said fail-safe circuit comprising a resistor interconnecting said collector and said second transistor to provide a D.C. operating bias for said second transistor, and parallel circuit means including a capacitor and series connected sensitivity resistor effective to cause current flow in a reverse direction to said second transistor in response to an increase in capacity in said resonant circuit and a decrease in potential of said collector.

7. A capacity alarm system comprising an oscillator, a tune-d resonant circuit connected to said oscillator, said resonant circuit including antenna means and an inductive winding, a first transistor connected to said inductive winding for sensing changes in capacitance in said tuned resonant antenna circuit, said transistor including a collector having a D.C. potential in variable response to said change in capacitance, a second transistor for controlling actuation of an alarm, and fail-safe circuit means interconnecting said first transistor and said second transistor, said fail-safe circuit comprising a resistor interconnecting said collector and said second transistor to provide a D.C. operating bias for said second transistor, and parallel circuit means including a capacitor and series connected sensitivity resistor effective to cause current flow in a reverse direction to said second transistor in response to an increase in capacity in said resonant circuit and a decrease in potential of said collector, and a diode shunting said sensitivity resistor, said diode being connected to provide a high impedance path to current discharged by said capacitor.

References Cited by the Examiner FOREIGN PATENTS 668,374 3/1952 Great Britain.

NEIL C. READ, Primary Examiner.

R. GOLDMAN, Assistant Examiner.

Claims (1)

1. 7. A CAPACITY ALARM SYSTEM COMPRISING AN OSCILLATOR, A TUNED RESONANT CIRCUIT CONNECTED TO SAID OSCILLATOR, SAID RESONANT CIRCUIT INCLUDING ANTENNA MEANS AND AN INDUCTIVE WINDING, A FIRST TRANSISTOR CONNECTED TO SAID INDUCTIVE WINDING FOR SENSING CHANGES IN CAPACITANCE IN SAID TUNED RESONANT ANTENNA CIRCUIT, SAID TRANSISTOR INCLUDING A COLLECTOR HAVING A D.C. POTENTIAL IN VARIABLE RESPONSE TO SAID CHANGE IN CAPACITANCE, A SECOND TRANSISTOR FOR CONTROLLING ACTUATION OF AN ALARM, AND FAIL-SAFE CIRCUIT MEANS INTERCONNECTING SAID FIRST TRANSISTOR AND SAID SECOND TRANSISTOR SAID FAIL-SAFE CIRCUIT COMPRISING A RESISTOR INTERCONNECTING SAID COLLECTOR AND SAID SECOND TRANSISTOR, AND A PARALLEL OPERATING BIAS FOR SAID SECOND TRANSISTOR, AND A PARALLEL CIRCUIT MEANS INCLUDING A CAPACITOR AND SERIES CONNECTED SENSITIVITY RESISTOR EFFECTIVE TO CAUSE CURRENT FLOW IN A REVERSE DIRECTION TO SAID SECOND TRANSISTOR IN RESPONSE TO AN INCREASE IN CAPACITY IN SAID RESONANT CIRCUIT AND A DECREASE IN POTENTIAL OF SAID COLLECTOR, AND A DIODE SHUNING SAID SENSITIVITY RESISTOR, SAID DIODE BEING CONNECTED TO PROVIDE A HIGH IMPEDANCE PATH TO CURRENT DISCHARGED BY SAID CAPACITOR.

Images