APPARATUS FOR THE PREVENTION OF THEFT OF ELECTRICALLY OPERATED ARTICLES
United States Patent 3765008
Apparatus for the prevention of theft of electrically operated articles includes a detection system and an alarm system. The detection system is activated by interruption of electrical power to the gate of a programmable unijunction transistor which then becomes conducting and completes a circuit to the gate of a thyristor. The triggered thyristor is thus converted from an off position to an on position, thereby allowing current to flow therethrough, which current activates an alarm system. The alarm system may be visual or audible or a combination of both. Interruption of the electrical power to the detection system is caused by unplugging the input power cord to the electrically operated article to be protected, or by cutting said cord or by interrupting or interfering with the input power cord in any way such as in an attempted theft of the article.
US Patent References:
THEFT-PREVENTING ALARM DEVICE
Tellerman - January 1969 - 3425050
OFF-DELAY SOLID-STATE TIMER SYSTEMS
Elliot - February 1972 - 3641397
/3553674.html
Head - January 1971 - 3553674
ALARM SYSTEM FOR DETECTING PHYSICAL DISTURBANCES OF ONE OR MORE ELECTRICAL APPLIANCES AND THE LIKE FROM A LOCATION REMOTE FROM SAID APPLIANCES
Hanson - December 1970 - 3544984
APPLIANCE THEFT ALARM USING VOLTAGE MAGNITUDE SWITCH
Daniel - October 1972 - 3696378
THEFT-PREVENTING ALARM DEVICE
Tellerman - January 1969 - 3425050
OFF-DELAY SOLID-STATE TIMER SYSTEMS
Elliot - February 1972 - 3641397
/3553674.html
Head - January 1971 - 3553674
ALARM SYSTEM FOR DETECTING PHYSICAL DISTURBANCES OF ONE OR MORE ELECTRICAL APPLIANCES AND THE LIKE FROM A LOCATION REMOTE FROM SAID APPLIANCES
Hanson - December 1970 - 3544984
APPLIANCE THEFT ALARM USING VOLTAGE MAGNITUDE SWITCH
Daniel - October 1972 - 3696378
Application Number:
05/265608
Publication Date:
10/09/1973
Filing Date:
06/23/1972
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
327/466, 340/652
International Classes:
G08B13/14; G08B13/00
Field of Search:
340/280,256,276 307/252F
US Patent References:
| 3679912 | OVERVOLTAGE-UNDERVOLTAGE SENSOR | July 1972 | Tenenbaum |
Primary Examiner:
Caldwell, John W.
Assistant Examiner:
Partridge, Scott F.
Claims:
We claim
1. Apparatus for the prevention of theft of electrical appliances of the type having a power cord connectible to an electric outlet, said apparatus comprising: a voltage source disposed at said electrical appliance, said source capable of supplying a continuous voltage output as long as said power cord is connected to an energized AC electric outlet; a detection system and an alarm system; said detection system comprising an input, a rectifier, a programmable unijunction transistor, a thyristor, a capacitor, means for applying a positive voltage to the anode of said transistor and means for maintaining a charge on said capacitor, the output of said voltage source being connected to the input of said detection system, said rectifier being so electrically disposed between said input and the anode gate of said transistor that a positive DC voltage is applied to said anode gate as long as said voltage source supplies a continuous voltage to said input of said detection system, the relationship between the positive voltages on said anode gate and said anode being such that said transistor is in an off condition as long as said continuous voltage is supplied, said capacitor being so electrically connected that upon cessation of said continuous voltage said capacitor will discharge through said transistor and apply a triggering current to the gate of said thyristor and whereupon said thyristor will activate said alarm system.
2. The apparatus of claim 1 wherein said voltage source is a step-down transformer, the primary of which is connected to said power cord and the secondary of which supplies said continuous voltage.
3. The apparatus of claim 1 wherein said means for applying a positive voltage to the anode of said transistor is a battery, which is also the means for maintaining said charge on said capacitor.
4. The apparatus of claim 3 wherein said alarm system includes an indicator light and said battery supplies the power to energize said light.
5. The apparatus of claim 1 wherein said alarm system includes an audible alarm in series with a silicon controlled rectifier, said silicon controlled rectifier being so electrically connected to said detection system that it is fired when said thyristor is triggered.
6. The apparatus of claim 1 wherein said alarm system includes a switch, said switch being normally closed and being so electrically connected that activation of said alarm system requires opening of said switch to deactivate said alarm system.
1. Apparatus for the prevention of theft of electrical appliances of the type having a power cord connectible to an electric outlet, said apparatus comprising: a voltage source disposed at said electrical appliance, said source capable of supplying a continuous voltage output as long as said power cord is connected to an energized AC electric outlet; a detection system and an alarm system; said detection system comprising an input, a rectifier, a programmable unijunction transistor, a thyristor, a capacitor, means for applying a positive voltage to the anode of said transistor and means for maintaining a charge on said capacitor, the output of said voltage source being connected to the input of said detection system, said rectifier being so electrically disposed between said input and the anode gate of said transistor that a positive DC voltage is applied to said anode gate as long as said voltage source supplies a continuous voltage to said input of said detection system, the relationship between the positive voltages on said anode gate and said anode being such that said transistor is in an off condition as long as said continuous voltage is supplied, said capacitor being so electrically connected that upon cessation of said continuous voltage said capacitor will discharge through said transistor and apply a triggering current to the gate of said thyristor and whereupon said thyristor will activate said alarm system.
2. The apparatus of claim 1 wherein said voltage source is a step-down transformer, the primary of which is connected to said power cord and the secondary of which supplies said continuous voltage.
3. The apparatus of claim 1 wherein said means for applying a positive voltage to the anode of said transistor is a battery, which is also the means for maintaining said charge on said capacitor.
4. The apparatus of claim 3 wherein said alarm system includes an indicator light and said battery supplies the power to energize said light.
5. The apparatus of claim 1 wherein said alarm system includes an audible alarm in series with a silicon controlled rectifier, said silicon controlled rectifier being so electrically connected to said detection system that it is fired when said thyristor is triggered.
6. The apparatus of claim 1 wherein said alarm system includes a switch, said switch being normally closed and being so electrically connected that activation of said alarm system requires opening of said switch to deactivate said alarm system.
Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is in the field of theft prevention apparatus that is automatically responsive to attempts to remove protected electrically operated articles.
2. Description of the Prior Art
Various methods have been employed to prevent the theft of electrically operated applicances but have so far been relatively unsuccessful. One such method which has been employed, for example, to prevent the theft of television sets from motels where such thefts are prevalent, consists in affixing a self-contained alarm system to the inner portion of the television chassis. Although variations of this method have resulted in quite complex circuit concepts, the device mechanism usually comprised a bell, a battery, and a self-activated switch. When a television set was moved, the switch, usually a mercury relay, closed, causing current to flow from the battery to the bell sounding the alarm.
This method could be easily by-passed by removing the alarm from the set or by cutting the wire to the bell, thereby preventing completion of the circuit.
Another method for preventing such thefts included a concealed electrical signal generator that transmitted one or more RF signals throughout the room where the appliance to be guarded was positioned. The appliance had a special reflector concealed within its structure which continuously reflected the signal back to the source. When the appliance was removed from the room, the signal was no longer reflected back to the generator and a complex alarm system was activated.
This method, besides being very expensive, has not been too successful since the reflector could be interrupted in other ways besides removal of the set and spurious alarms were sometimes sounded, to the great distress of the occupant of the room and to the embarrassment of the innkeeper when such appliance was a television set in a hotel or motel room, as was quite often the case.
My invention overcomes all the above mentioned obstacles, is inexpensive, and cannot be by-passed or caused to become non-operative by any known means.
It is an object of this invention to provide a theft detection system for electrically connected appliances wherein an alarm is activated when the electrical power supply to said appliances is interrupted, either by unplugging or severing of the power input cord or even by a power failure.
It is a further object of this invention to describe a theft prevention system for electrically connected appliances wherein the power input cord to said appliances also serves to supply a voltage and/or to a programmable unijunction transistor in order to keep the transistor in an off state of conduction until the signal is interrupted by disconnecting the power cord.
Another object of this invention is to describe a theft detection system where the principal components comprise a gated transistor and one or more silicon controlled rectifiers whereby the power input source to the electrically connected appliances to be protected also serves to energize the detection system and whereby an alarm is activated when the power input to the appliance is removed.
A further object of this invention is to describe a theft detection system for electrically connected devices comprising a visual light source and an audible alarm, wherein both audible and visual alarms are activated by disconnecting the power input supply cord to said elements to be protected.
SUMMARY OF THE INVENTION
Theft prevention apparatus, in accordance with this invention, comprises an electrical circuit which contains both a detection system and an alarm system.
The detection system includes a programmable unijunction transistor which is in an off position as long as a suitable DC voltage is applied to the gate thereof. Means for supplying said DC voltage includes the power input cord of an electrical appliance protected by the theft prevention apparatus. Thus, as long as the appliance is connected to a power source, such as by having the cord plugged into the usual 110 volt AC wall outlet, said DC voltage is applied to the gate of the transistor. Upon electrical disconnection of the cord, the gate voltage drops toward zero and the transistor is converted to the on position, thereby permitting discharge of a capacitor through the transistor to the gate of a normally off thyristor which, in turn, fires and stays in the on position. The thyristor completes the circuit to a suitable alarm system which becomes activated thereby.
BRIEF DESCRIPTION OF THE DRAWING
The Drawing shows an electrical circuit of theft prevention apparatus in accordance with this invention.
DESCRIPTION OF THE PREFEFFED EMBODIMENT
As shown in the drawing, theft prevention apparatus in accordance with this invention requires an input voltage, shown as "Input" in the drawing, to the detection system of the apparatus. This voltage may be AC or DC; if AC, however, it is rectified to DC before it is applied to the anode gate of programmable unijunction transistor T 1 . Means for supplying the input voltage must be so associated with the line cord of an electrical appliance to be protected that interruption of electrical power through the cord will interrupt the input voltage to the detection system. As a result, the voltage on the anode gate of transistor T 1 will decrease toward zero. When this occurs, a positive voltage at the anode of transistor T 1 permits T 1 to "fire," or switch to the on position, thereby permitting capacitor C 2 , which is charged to +V 1 volts, to discharge through T 1 and apply a current to the gate of thyristor T 2 . This current triggers thyristor T 2 and switches it to the on position, thereby completing a circuit from DC voltage supply V 1 , through switch S (normally closed) to a visual alarm system, such as an indicator light L. Activation of the alarm indicates that the electrical appliance is no longer plugged into an energized electrical outlet. Once thyristor T 2 is triggered, it remains in the on position and, consequently, the alarm stays on, until switch S is manually opened to shut the alarm.
In addition to visual indicator light L, or in the alternative, an audible alarm B (a bell or buzzer or the like) may be activated by the triggering of thyristor T 2 , which results in application of current to the gate of thyristor T 3 and the triggering of T 3 , thereby completing the circuit to, and activating, audible alarm B.
In one particular application, where it was desired to protect from theft TV sets in the rooms of a motel, a small step-down transformer was installed in the back of each TV set. The primary leads of the transformer were connected to the power input cord of the TV set, so that aslong as the cord was plugged into the usual 110 volt AC wall outlet, the transfomer was energized. The secondary voltage of the transformer was 12 volts and was the input to the detection system. The detection and alarm systems were installed in the office of the motel and a twisted pair of 20 guage plastic insulated wires was connected between the transformer secondary and the input of the detection system. For convenience sake, said twisted pair was taped to the TV set cord and extended into and through the junction box containing the wall outlet into which the cord was plugged.
The 12 volt AC input was rectified into DC by diode D 1 with resistor R 1 in series therewith in order to limit the current through diode D 1 . Resistor R 1 also prevented excess current in the transformer secondary in the event of a short circuit. In this particular application, diode D 1 was type IN 914 and resistor R 1 had a value of 100 ohms.
Capacitor C 1 was in series with diode D 1 in order to limit AC ripple to an acceptable level for the purpose of maintaining the voltage at the anode gate of transistor T 1 at a higher voltage than the firing level. Here, capacitor C 1 had a value of 2 microfarads.
Resistors R 2 and R 3 were in series with diode D 1 and acted as a voltage divider in order to provide the desired DC voltage on the anode gate of transistor T 1 in order to maintain T 1 in the off condition as long as input voltage was applied to the detection system. The values of R 2 and R 3 were 22 kilohms and 33 kilohms, respectively, which resulted in a voltage greater than 6 volts on said anode gate.
Transistor T 1 was a programmable unijunction transistor, Type 2N 6027, for example, Transistor T 1 was of the type that will remain in an off condition until the anode voltage exceeds the gate voltage by one diode forward voltage drop, about 0.2 to 0.6 volts.
The anode of T 1 was connected through current limiting resistor R 4 to the positive terminal of voltage supply V 1 which was, in this case, a 6 volt battery. Resistor R 4 was in series with capacitor C 2 and provided a charging path therefor. The value of resistor R 4 had to be small compared to the sum of the leakage resistances for T 1 and C 2 in order that C 2 remain charged as long as T 1 was in the off condition: R 4 had a value of 68 kilohms.
Upon triggering of transistor T 1 , capacitor C 2 would discharge therethrough and apply a current to the gate of thyristor T 2 which, in turn, would trigger T 2 and switch it to the on position. This, in turn, completed an electrical circuit consisting of thyristor T 2 , switch S (normally closed) and indicator light L, and light L would be lit by electrical power from voltage source V 1 . Thyristor T 2 could be a silicon controlled rectifier or triac or similar switching device; here, T 2 was a silicon controlled rectifier, Type ID 200.
Resistor R 5 and capacitor C 3 , in parallel, provided a network for stabilizing thyristor T 2 , the network being in series between the cathode of T 1 and switch S, and also being in series between the gate of T 2 and switch S. Resistor R 5 provided stability to T 1 by providing a leakage path therefor; R 5 had a value of 100 ohms. Capacitor C 3 provided stability by preventing dV/dt (spurious noise) triggering; C 3 had a value of 0.1 microfarads.
The motel operator, upon seeing light L energized, would immediately known that the cord to a TV set had been unplugged or severed and could immediately investigate as to the cause. Of course, there was a separate detection system and indicator light for each room in the motel, with suitable identification matching each indicator light to its room.
A power failure in the motel would also activate all the detection systems and indicator lights, since voltage source V 1 was independent of the power lines. In such a case, all the switches S could be opened to turn off all the indicator lights until power was restored; the switches would then be closed. The fact that the detection systems would be activated by a power failure is a minor inconvenience when compared to the reliability of the system in preventing thefts of TV sets; furthermore power failures are generally quite rare.
Another advantage of theft prevention apparatus in accordance with this invention is that all the components comprising the detection system are quite small and can be mounted on a relatively small printed circuit board. Thus, a moderately sized console could contain enough detection systems and indicator lights for a 50 room or 100 room or even larger motel without requiring much space. In addition, a single voltage source, such as a battery, could serve as the V 1 supply for all systems.
As an adjunct to the visual alarm system, an audible alarm system was also installed in order to be sure to obtain the attention of the motel operator. An audible alarm system is shown in the drawing and it includes a diode D 2 connected to the detection circuit at a point between switch S and light L.
The triggering of thyristor T 2 results in application of a voltage at diode D 2 , which results in a current through resistor R 6 to the gate of thyristor T 3 , thereby triggering T 3 and converting it to the on position. This completes a circuit from a separate voltage supply V 2 through bell B and thyristor T 3 , thereby energizing bell B. Resistor R 6 was a current limiting resistor similar to that of R 1 in the detection system and had the same value as R 1 . The purpose of resistor R 1 and capacitor C 4 was to stabilize thyristor T 3 in the same manner as resistor R 5 and capacitor C 3 stabilized thyristor T 2 ; resistor R 5 had a value of 270 ohms and capacitor C 3 had a value of 0.1 microfarad. Thyristor T 3 could be any suitable switching device and, in this case, was a silicon controlled rectifier, Type ID 200. V 2 was a separate voltage source and was a 6 volt battery. If desired, a single battery can serve as both V 1 and V 2 .
Since only one audible alarm system was necessary to gain the motel operator's attention, the outputs of all the detection systems was connected, through suitable diodes, to point A of the audible alarm system, as shown in the drawing. Thus, the activation of any one of the detection systems would sound bell B, while energized light L would indicate to the motel operator which room to investigate.
Although the above embodiment describes the invention in reference to the prevention of theft of TV sets, it is also applicable to the protection of other electrical appliances, such as stereo sets, office machines and the like, where removal of the appliance requires unplugging or severing of the power cord thereof.
In another embodiment of the invention, V 1 was connected directly to the anode of T 1 and resistor R 4 and R 5 were eliminated from the circuit, as was thyristor T 2 and capacitor C 2 and C 3 . Thus, when T 1 is converted to the on position, a circuit is completed from V 1 through T 1 through closed switch S and through light L, thereby energizing light L. The audible alarm system was activated as in the previous example.
1. Field of the Invention
This invention is in the field of theft prevention apparatus that is automatically responsive to attempts to remove protected electrically operated articles.
2. Description of the Prior Art
Various methods have been employed to prevent the theft of electrically operated applicances but have so far been relatively unsuccessful. One such method which has been employed, for example, to prevent the theft of television sets from motels where such thefts are prevalent, consists in affixing a self-contained alarm system to the inner portion of the television chassis. Although variations of this method have resulted in quite complex circuit concepts, the device mechanism usually comprised a bell, a battery, and a self-activated switch. When a television set was moved, the switch, usually a mercury relay, closed, causing current to flow from the battery to the bell sounding the alarm.
This method could be easily by-passed by removing the alarm from the set or by cutting the wire to the bell, thereby preventing completion of the circuit.
Another method for preventing such thefts included a concealed electrical signal generator that transmitted one or more RF signals throughout the room where the appliance to be guarded was positioned. The appliance had a special reflector concealed within its structure which continuously reflected the signal back to the source. When the appliance was removed from the room, the signal was no longer reflected back to the generator and a complex alarm system was activated.
This method, besides being very expensive, has not been too successful since the reflector could be interrupted in other ways besides removal of the set and spurious alarms were sometimes sounded, to the great distress of the occupant of the room and to the embarrassment of the innkeeper when such appliance was a television set in a hotel or motel room, as was quite often the case.
My invention overcomes all the above mentioned obstacles, is inexpensive, and cannot be by-passed or caused to become non-operative by any known means.
It is an object of this invention to provide a theft detection system for electrically connected appliances wherein an alarm is activated when the electrical power supply to said appliances is interrupted, either by unplugging or severing of the power input cord or even by a power failure.
It is a further object of this invention to describe a theft prevention system for electrically connected appliances wherein the power input cord to said appliances also serves to supply a voltage and/or to a programmable unijunction transistor in order to keep the transistor in an off state of conduction until the signal is interrupted by disconnecting the power cord.
Another object of this invention is to describe a theft detection system where the principal components comprise a gated transistor and one or more silicon controlled rectifiers whereby the power input source to the electrically connected appliances to be protected also serves to energize the detection system and whereby an alarm is activated when the power input to the appliance is removed.
A further object of this invention is to describe a theft detection system for electrically connected devices comprising a visual light source and an audible alarm, wherein both audible and visual alarms are activated by disconnecting the power input supply cord to said elements to be protected.
SUMMARY OF THE INVENTION
Theft prevention apparatus, in accordance with this invention, comprises an electrical circuit which contains both a detection system and an alarm system.
The detection system includes a programmable unijunction transistor which is in an off position as long as a suitable DC voltage is applied to the gate thereof. Means for supplying said DC voltage includes the power input cord of an electrical appliance protected by the theft prevention apparatus. Thus, as long as the appliance is connected to a power source, such as by having the cord plugged into the usual 110 volt AC wall outlet, said DC voltage is applied to the gate of the transistor. Upon electrical disconnection of the cord, the gate voltage drops toward zero and the transistor is converted to the on position, thereby permitting discharge of a capacitor through the transistor to the gate of a normally off thyristor which, in turn, fires and stays in the on position. The thyristor completes the circuit to a suitable alarm system which becomes activated thereby.
BRIEF DESCRIPTION OF THE DRAWING
The Drawing shows an electrical circuit of theft prevention apparatus in accordance with this invention.
DESCRIPTION OF THE PREFEFFED EMBODIMENT
As shown in the drawing, theft prevention apparatus in accordance with this invention requires an input voltage, shown as "Input" in the drawing, to the detection system of the apparatus. This voltage may be AC or DC; if AC, however, it is rectified to DC before it is applied to the anode gate of programmable unijunction transistor T 1 . Means for supplying the input voltage must be so associated with the line cord of an electrical appliance to be protected that interruption of electrical power through the cord will interrupt the input voltage to the detection system. As a result, the voltage on the anode gate of transistor T 1 will decrease toward zero. When this occurs, a positive voltage at the anode of transistor T 1 permits T 1 to "fire," or switch to the on position, thereby permitting capacitor C 2 , which is charged to +V 1 volts, to discharge through T 1 and apply a current to the gate of thyristor T 2 . This current triggers thyristor T 2 and switches it to the on position, thereby completing a circuit from DC voltage supply V 1 , through switch S (normally closed) to a visual alarm system, such as an indicator light L. Activation of the alarm indicates that the electrical appliance is no longer plugged into an energized electrical outlet. Once thyristor T 2 is triggered, it remains in the on position and, consequently, the alarm stays on, until switch S is manually opened to shut the alarm.
In addition to visual indicator light L, or in the alternative, an audible alarm B (a bell or buzzer or the like) may be activated by the triggering of thyristor T 2 , which results in application of current to the gate of thyristor T 3 and the triggering of T 3 , thereby completing the circuit to, and activating, audible alarm B.
In one particular application, where it was desired to protect from theft TV sets in the rooms of a motel, a small step-down transformer was installed in the back of each TV set. The primary leads of the transformer were connected to the power input cord of the TV set, so that aslong as the cord was plugged into the usual 110 volt AC wall outlet, the transfomer was energized. The secondary voltage of the transformer was 12 volts and was the input to the detection system. The detection and alarm systems were installed in the office of the motel and a twisted pair of 20 guage plastic insulated wires was connected between the transformer secondary and the input of the detection system. For convenience sake, said twisted pair was taped to the TV set cord and extended into and through the junction box containing the wall outlet into which the cord was plugged.
The 12 volt AC input was rectified into DC by diode D 1 with resistor R 1 in series therewith in order to limit the current through diode D 1 . Resistor R 1 also prevented excess current in the transformer secondary in the event of a short circuit. In this particular application, diode D 1 was type IN 914 and resistor R 1 had a value of 100 ohms.
Capacitor C 1 was in series with diode D 1 in order to limit AC ripple to an acceptable level for the purpose of maintaining the voltage at the anode gate of transistor T 1 at a higher voltage than the firing level. Here, capacitor C 1 had a value of 2 microfarads.
Resistors R 2 and R 3 were in series with diode D 1 and acted as a voltage divider in order to provide the desired DC voltage on the anode gate of transistor T 1 in order to maintain T 1 in the off condition as long as input voltage was applied to the detection system. The values of R 2 and R 3 were 22 kilohms and 33 kilohms, respectively, which resulted in a voltage greater than 6 volts on said anode gate.
Transistor T 1 was a programmable unijunction transistor, Type 2N 6027, for example, Transistor T 1 was of the type that will remain in an off condition until the anode voltage exceeds the gate voltage by one diode forward voltage drop, about 0.2 to 0.6 volts.
The anode of T 1 was connected through current limiting resistor R 4 to the positive terminal of voltage supply V 1 which was, in this case, a 6 volt battery. Resistor R 4 was in series with capacitor C 2 and provided a charging path therefor. The value of resistor R 4 had to be small compared to the sum of the leakage resistances for T 1 and C 2 in order that C 2 remain charged as long as T 1 was in the off condition: R 4 had a value of 68 kilohms.
Upon triggering of transistor T 1 , capacitor C 2 would discharge therethrough and apply a current to the gate of thyristor T 2 which, in turn, would trigger T 2 and switch it to the on position. This, in turn, completed an electrical circuit consisting of thyristor T 2 , switch S (normally closed) and indicator light L, and light L would be lit by electrical power from voltage source V 1 . Thyristor T 2 could be a silicon controlled rectifier or triac or similar switching device; here, T 2 was a silicon controlled rectifier, Type ID 200.
Resistor R 5 and capacitor C 3 , in parallel, provided a network for stabilizing thyristor T 2 , the network being in series between the cathode of T 1 and switch S, and also being in series between the gate of T 2 and switch S. Resistor R 5 provided stability to T 1 by providing a leakage path therefor; R 5 had a value of 100 ohms. Capacitor C 3 provided stability by preventing dV/dt (spurious noise) triggering; C 3 had a value of 0.1 microfarads.
The motel operator, upon seeing light L energized, would immediately known that the cord to a TV set had been unplugged or severed and could immediately investigate as to the cause. Of course, there was a separate detection system and indicator light for each room in the motel, with suitable identification matching each indicator light to its room.
A power failure in the motel would also activate all the detection systems and indicator lights, since voltage source V 1 was independent of the power lines. In such a case, all the switches S could be opened to turn off all the indicator lights until power was restored; the switches would then be closed. The fact that the detection systems would be activated by a power failure is a minor inconvenience when compared to the reliability of the system in preventing thefts of TV sets; furthermore power failures are generally quite rare.
Another advantage of theft prevention apparatus in accordance with this invention is that all the components comprising the detection system are quite small and can be mounted on a relatively small printed circuit board. Thus, a moderately sized console could contain enough detection systems and indicator lights for a 50 room or 100 room or even larger motel without requiring much space. In addition, a single voltage source, such as a battery, could serve as the V 1 supply for all systems.
As an adjunct to the visual alarm system, an audible alarm system was also installed in order to be sure to obtain the attention of the motel operator. An audible alarm system is shown in the drawing and it includes a diode D 2 connected to the detection circuit at a point between switch S and light L.
The triggering of thyristor T 2 results in application of a voltage at diode D 2 , which results in a current through resistor R 6 to the gate of thyristor T 3 , thereby triggering T 3 and converting it to the on position. This completes a circuit from a separate voltage supply V 2 through bell B and thyristor T 3 , thereby energizing bell B. Resistor R 6 was a current limiting resistor similar to that of R 1 in the detection system and had the same value as R 1 . The purpose of resistor R 1 and capacitor C 4 was to stabilize thyristor T 3 in the same manner as resistor R 5 and capacitor C 3 stabilized thyristor T 2 ; resistor R 5 had a value of 270 ohms and capacitor C 3 had a value of 0.1 microfarad. Thyristor T 3 could be any suitable switching device and, in this case, was a silicon controlled rectifier, Type ID 200. V 2 was a separate voltage source and was a 6 volt battery. If desired, a single battery can serve as both V 1 and V 2 .
Since only one audible alarm system was necessary to gain the motel operator's attention, the outputs of all the detection systems was connected, through suitable diodes, to point A of the audible alarm system, as shown in the drawing. Thus, the activation of any one of the detection systems would sound bell B, while energized light L would indicate to the motel operator which room to investigate.
Although the above embodiment describes the invention in reference to the prevention of theft of TV sets, it is also applicable to the protection of other electrical appliances, such as stereo sets, office machines and the like, where removal of the appliance requires unplugging or severing of the power cord thereof.
In another embodiment of the invention, V 1 was connected directly to the anode of T 1 and resistor R 4 and R 5 were eliminated from the circuit, as was thyristor T 2 and capacitor C 2 and C 3 . Thus, when T 1 is converted to the on position, a circuit is completed from V 1 through T 1 through closed switch S and through light L, thereby energizing light L. The audible alarm system was activated as in the previous example.