EP0759244A1

EP0759244A1 - Telephone break-in security unit

Info

Publication number: EP0759244A1
Application number: EP95918539A
Authority: EP
Inventors: Christian Henri Krause
Original assignee: Krause Christian Henri
Current assignee: KRAUSE, CHRISTIAN HENRI
Priority date: 1994-05-02
Filing date: 1995-04-26
Publication date: 1997-02-26
Also published as: AU2444895A; WO1995030297A3; DK11995A; WO1995030297A2

Abstract

A telephone break-in security unit (20) comprises a connection (15) for an incoming telephone line and a connection (16) for an outgoing telephone line, a current threshold comparator (6) arranged for sensing the current in the telephone line and thus for detecting when the receiver of a telephone apparatus connected to the security unit is lifted, a code memory (8) arranged so that any code may be stored and transmitted, a code modulator (18) arranged to apply a coded signal to the incoming connection, and a microprocessor (1). A chief security unit comprises a code demodulator (19) arranged to receive coded signals and a microprocessor programmed in such a manner that it reacts to a signal from the voltage threshold comparator by examining whether or not the use of the telephone line thus detected is authorised by the emission of the approved code. In case of unauthorised use, the line may be blocked by a short-circuit relay (5).

Description

TELEPHONE BREAK-IN SECURITY UNIT

The present invention relates to a security unit for a telephone system which makes it possible to distinguish between authorised and unauthorised use of a telephone line. According to a further embodiment, the invention relates to a security unit which automatically implements measures to prevent unauthorised use of a telephone line.

Today there is an increasing focus on the dangers presented by unauthorised use of telephone communication lines, first and foremost by unauthorised persons fraudulently connecting themselves to another person's telephone line with the intention of making telephone calls without having to pay therefor. Light, small and inexpensive telephones, a maybe increasing technological insight and the interest in ex¬ pensive telephone calls are factors which make many persons fear increasing problems in connection with the theft of telephone time. Telephone lines are available in out-of-doors junction boxes and telephone cabinets which are to be so designed that they are easily opened by any service engineer which also makes them easy to open for intruders. The telephone cords in the telephone cabinets are functionally exactly like the ones of normal telephone installations, i.e. a person may bring along his own telephone set and outside connect himself to a telephone cabinet parallel with another telephone subscriber's telephone line and use this line without any problems. Physical securing of cords and telepho¬ ne cabinets is not considered realistic and changes in the transmission procedures are hardly feasible as^' they would entail that all telephones and telephone exchange lines were to be changed.

FR patent application No. 2 655 224 discloses a device for protecting a subscriber's communication line against unaut¬ horised parallel use. This disclosure describes a unit which surveys the voltage on the telephone line whereby it is detectable whether an apparatus is coupled to that telephone line where the phone handle is lifted off. The unit is intended for insertion into the telephone line before the authorised apparatus and constructed to be able to measure the current which runs in the line. If a current is detected in the line it means that the apparatus used is the authori¬ sed one. If, on the other hand, the presence of an apparatus with the handle lifted is detected by the current measurement without simultaneous detection of a current, this means that the apparatus in question is coupled in between the exchange and the security unit, i.e. it is assumed that the apparatus in question is unathorised. The security unit reacts to this by drawing a loading current which reduces the voltage on the telephone line to a level which is too low for the exchange to detect number signals from a dialling apparatus. This known unit maintains a certain residual voltage in the line which is necessary to maintain the voltage supply and the functioning of the apparatus. However, the residual voltage is high enough for an apparatus with a sound signal device to function and communicate.

However, such unit only operates as intended when coupled to the telephone line between the authorised apparatus(es) and the telephone exchange. In case of an installation with several local telephone outlets which are to be used, it is therefore necessary to identify where the telephone line may be interrupted and the unit inserted so as to ensure that it is located before all the telephone outlets which are to be used. Therefore, in such cases, the unit requires inter- vention into the fixed installation which may be complex and involves the risk of defects. If the subscriber should later need to install more telephone outlets, it may even be necessary to move the security unit or to branch the wiring even more than would otherwise have been necessary. Besides, the known device does not know how to distinguish between authorised and unauthorised connecting to the telephone line inasmuch as couplings after the device are concerned. EP patent application No. 0 126 496 discloses an electronic lock for a subscriber's telephone line which comprises a circuit to be coupled in to load the telephone line and a code receiver unit which may detect whether a predetermined code is emitted onto the line and which may act on the right code by disconnecting the load. The subscriber to use the telephone apparatus therefore needs to emit the code by means of the numerical generator of the apparatus which is a troublesome and unwanted procedure. The load prevents calls with dialling telephones. However, a telephone apparatus with a sound signal device may readily be used without the known device being able to prevent it.

According to the invention a security unit is provided comprising a coupling to an incoming telephone line and a coupling to an outgoing telephone line, a current threshold comparator designed to detect the current in the telephone line and determine whether the numerical value of the current I in the telephone line exceeds a determined threshold value I_τ and to emit a corresponding logical signal, a code memory so constructed that any code can be stored and transmitted, a code modulator arranged for dialling a code signal on the incoming connection under the control of a logical unit, and a microprocessor arranged for functioning by power supply from the incoming telephone line and in such a manner that it can receive signals from the current threshold comparator and the code memory and emit control signals to the all-or nothing relay and the code modulator, the microprocessor being programmed to react to a signal from the current threshold comparator indicating that the threshold value has been exceeded by emission of the code from the code memory through the code modulator, following which it becomes passive, in which state it does not react again until the comparator has indicated that the threshold value has not been exceeded for a given period of time. Hereby a very convenient security unit is obtained which operates automatically to emit a pre-programmed code which indicates that the relevant use is authorised without any inconvenience being experienced by the user of the telephone. The only thing experienced by the user is a short delay, e.g. of less than one second, before the line is open, and follo¬ wing opening of the line the telephone is experienced as operating as usual. Since the security unit comprises a particular number emitter, this number emitter may easily be arranged to transmit the code signal in a manner which will not be detected as a dialling number by the telephone exchange. Realization of the unit according to the invention is furthermore easy since it does not require any other energy supply that the one obtained through the telephone line. It is suitable for integration into a telephone apparatus or a telephone outlet.

According to a convenient embodiment the user's apparatus is disconnected while the code is emitted thereby avoiding any inconvenience to the user caused by code signals.

According to a convenient embodiment the unit is so arranged that, during the period when it maintains the user's telepho¬ ne apparatus disconnected, it draws a current of such magnitude that the telephone exchange will detect that the telephone line from the subscriber's apparatus is constantly connected and in the state "handle lifted off" . Hereby it is ensured that the exchange is not disturbed by the disconnec¬ ting procedure, including that its debitor counter will not register it as a new call.

According to a convenient embodiment a chief security unit is provided which, compared to the ordinary security unit, further comprises a current threshold comparater arranged to detect the voltage U across conducter pair in the telephone line and to determine whether the numerical value of the voltage drops below a determined threshold value U_τ and to emit a corresponding logical signal, and a code demodulator arranged to detect and decode a signal emitted from a code modulator in a security unit coupled to the same telephone line, said microprocessor being further arranged to receive the signals from the current threshold comparator and the code demodulator and to emit an output signal, said micropro¬ cessor being programmed to react to a signal from the current threshold comparator indicating that the threshold value is below the threshold by checking (a) whether the current threshold comparator indicates that exceeding has taken place, and (b) whether it receives through the code demodula¬ tor a code signal which corresponds to the code stored in the code memory, and in the instance that just one one of the qu¬ estions (a) and (b) is responded in the affirmative, it passes to a passive state without further reaction whereas, in the opposite case, it emits an output signal to indicate that an unauthorised situation is going on.

Hereby a security unit is obtained which may be connected anywhere along the telephone line and which may communicate with any number of ordinary security units and which may continuously monitor the entire telephone line and react as soon as an unauthorised state occurs.

According to a convenient embodiment the unit comprises a short circuit relay whereby the telephone line is short circuited within e.g. one fraction of a second in case there is any sign of unauthorised use. Hereby very efficient security against theft of telephone time is obtained with an apparatus which may be connected anywhere on a line, e.g. in a usual telephone outlet without the need for any sort of intervention into the fixed installation.

The system works through the exchange of a code or password which is programmed into the individual security units and which may be changed according to need thereby allowing the connections to be opened or closed as desired. The system is active in case of both outgoing and incoming calls; thus it is not possible either to answer an incoming call through an unauthorised apparatus.

According to a convenient embodiment, the unit is provided with a device for monitoring the voltage on the telephone line and to emit a particular signal in case the line voltage is disconnected due to external disturbances. Hereby the user is informed in case the apparatus has been disconnected or short-circuited for external reasons, e.g. in case an unauthorised person should have found out how to avoid the short-circuit in the chief security unit by interrupting this portion of the telephone line to make an unauthorised call.

According to a convenient embodiment the chief unit is constructed in the same way as the ordinary security unit, it being determined by simple switching whether it operates as a chief or as an ordinary security unit. Such switching may e.g. be realised by operating the functions of the chief security unit by means of a battery whereas, when no battery is inserted, the unit automatically switches to function as an ordinary security unit.

The invention will be described in further detail in the following detailed description with reference to the accompa¬ nying drawings wherein

DESCRIPTION OF THE DRAWINGS

Fig. 1 is a flow chart depicting the security unit according to the invention,

Fig. 2 is a more detailed diagrammatical view of the security unit according to the invention, and Fig. 3 is a diagrammatical view of a telephone line with several apparatuses connected thereto, said apparatuses being provided with security units according to the invention.

All figures are schematical and simplifed so as to show only details which are essential to the understanding of the invention while other details have been omitted. The various electronic components have been shown by drawn symbols, which symbols are not intended to limit the ways in which circuits with corresponding functions may be realised. It is consi¬ dered to be comprised by the state of the art to implement e.g. relays in purely electronic form without movable components and to realise components in integral circuits, etc., which components are shown separately in the drawings. In all figures in the drawings, identical or corresponding components have been designated with identical reference numerals or designations.

Reference is first made to fig. 1 which is a flow chart of a security unit. The security unit, which is designated as a whole with the reference numeral 20, comprises a connector 15 for an incoming telephone line and a connector 16 for an outgoing telephone line. Between these connectors which are realised in the form of telephone outlets, supply terminals or the like, an electric communication in the form of the line 25 is established, thereby allowing the security unit to be inserted into an ordinary telephone line. The telephone line, which in fig. 1 is shown in one-line-form, comprises a conductor pair extending from a telephone exchange or the like and to a telephone apparatus or the like installation so as to conduct electric current in the loop formed by the two conductors in the conductor pair and by the lines which are in the equipment of the exchange and the telephone apparatus, respectively, said equipment being constructed to generate and detect variations in the electric currents thereby allowing different signals to be transmitted in both direc^¬ tions in the telephone line.

In the operational states normally occuring, the values of current and voltage in a telephone line may be listed like in the following table:

State of line Volta e (U) Loop current (I)

Resting 43-60 V = 0-1 mA

Handle lifted off 5-15 V about 35 mA

Ringing 0-110 V

Shut down 0 V

In its resting state the line in the subscriber's apparatus is disconnected or substantially disconnected, and the exchange supplies the line with a constant voltage of a magnitude of 43-40 V, thereby making a certain electrical effect available to the telephone subscriber. Some state of the art telephone sets draw a weak current, e.g. for maintai¬ ning an electronic memory or the like. This current is not considered to substantially exceed 1 mA.

When the subscriber lifts the handle of his telephone set, circuits are connected which draw a current of constant or varying magnitude. The current magnitude may vary e.g. depending on signals for the transmission of speach, digital information, number calls, etc. The current magnitude is assumed to vary about approximately 35 mA, and the line voltage varies within the interval of 5-15 V.

In case of the "ringing" state, the exchange emits a ringing signal in the form of a voltage which varies between 0 and 110 V.

Finally, the "shut-down state" is mentioned, which is the state in which the power across the line is 0, e.g. because the line has been disconnected at the telephone exchange, the line has been interrupted, or the line has been short-cir¬ cuited. Irrespective of the causes, in this state of the line anybody who uses a connected apparatus will perceive the telephone line as completely dead.

According to the flow chart, the security unit 20 further comprises the following components:

- A microprocessor 1 with a power supply inlet 1, an analogous entry port 7, digital entry ports 8 and 13 through 22 and digital exit ports 6, 9, 12, and 23 through 27.

- A battery 17 which may supply power to the microproces¬ sor 1 at its inlet 1 through a rectifier.

A code modulator 18 connected to the line 25 and having a connection to the logical exit port 6 of the micro- processor so as to permit the microprocessor to generate code signals through signals on its exit port, said signals being dialled onto the line 25. Moreover the code modulator 18 serves the purpose of trans¬ mitting electric power through a particular outlet connection which, through a rectifier, is connected to the microprocessor at its inlet 1, thereby allowing the microprocessor to receive electric current from the telephone line therethrough.

- A code demodulator 19 connected to the line 25 and having a line to the logical entry port 7 of the micro¬ processor 1, through which the microprocessor may read code signals off the telephone line and detect whether there is current on the line.

A current threshold comparator 4 which is connected to the conductor pair 25 and which generates the numerical value of the voltage U on the conductor pair and compares it to a determined threshold value voltage U_τ. The current threshold comparator 4 is connected to the battery 17 and operates actively only when supplied with power from the battery. The current threshold comparator 4 emits an affirmative signal to the microprocessor 1 on its logical entry port 8 in case the numerical value of the line voltage has been found to be below the threshold value, presupposing, however, that battery voltage is present. In all other instances no affirmative signal is emitted.

A short circuit relay 5 is connected to short-circuit the conductors in the line 25 under the control of a signal from the microprocessor 1 on the logical exit port 9 thereof. Short-circuited position is designated the activated position, and interrupted position as deactivated position.

A current threshold comparator 6 as inserted on the line 25 generates the numerical value of the loop current I in the line 25 and compares the latter to a threshold current value I_τ. In case the result of such comparison is found to be positive an affirmative signal is emitted to the microprocessor at the logical entry port 13 thereof.

A disconnecting relay 7 inserted in the line 25 to allow it to interrupt the communication with that portion of the line 25 which leads to the connector 16 to the outgoing line. The disconnecting relay is controlled by a signal from the logical exit port 12 of the microprocessor 1. "Disconnected state" is designa¬ ted the activated state whereas the "shut down state" is designated the deactivated state. An alarm signalling device 11 controlled by the microprocessor 1 through its logical exit port 25 and so devised as to emit a particular alarm signal, e.g. a visual or an acoustic signal.

A break-in indicator 12 controlled by the microproces¬ sor 1 through its logical exit port 24 and comprising e.g. a visual indicator which may be activated to inform the user that a telephone line break-in has been detected.

An interruption indicator 13 which is controlled by the microprocessor 1 through its logical exit port 23 and constructed to provide a particular indication, e.g. a visual indication, in case a telephone line break-in has been detected.

A reset panel 14 which is connected to the logical entry port 22 of the microprocessor 1 and so arranged that upon activation of e.g. a spring loaded push button, a user may cancel the indications and acknow¬ ledge receipt thereof, respectively.

Finally the security unit comprises a code memory 8 which may be read by the microprocessor 1 at the logical entry ports 14-21. A password or code number chosen by the user is stored in the code memory.

Following this discussion of the main components of the security unit, the functioning thereof will be subject to a brief description.

The security unit is so constructed that it may operate with or without the battery. In case the unit is not provided with a battery, the microprocessor 1 obtains its power supply from the telephone line thorugh the particular line on the code modulator. As shown in fig. 1 a rectifier is inserted into the line and a second rectifier is inserted into the line from the battery to keep the two power supplies apart. The current threshold comparator is receives its supplies directly from the battery by a line connected before the rectifier in the battery connector which means that the current threshold comparator is not supplied with power unless a battery is mounted in the unit. A security unit without battery substantially acts as a code transmitter only whereas a security unit with battery acts as a code receiver and code approver and may activate alarms and short-circuit the telephone line according to certain criteria. On one telephone line several telephone apparatuses may be mounted which are each connected thereto through each their security unit. One of the security units is provided with battery and thus acts as chief security unit in the system.

ORDINARY SECURITY UNIT

A security unit without battery serves as ordinary security unit. When the telephone handle is lifted off the correspon¬ ding apparatus, the current threshold comparator 6 ensures that current is drawn in the line, and an impulse is trans¬ mitted to the microprocessor 1 on its entry port 13. The microprocessor checks whether it has received a positive signal on the logical entry port 8. In the negative, it switches to a state where it draws current from the line through the code modulator 18 of a current magnitude of above 15 mA, following which it activates the relay 7 that dis¬ connects the line to the local telephone set and it reads the code stored in the code memory 8 and transmits the code number on the telephone line through the code modulator 18. The disconnection of the local telephone apparatus by the disconnecting relay 7 serves the purpose of preventing that the transmission of the code number disturbs the user of the local apparatus. Following completion of this transmisssion, the disconnecting relay is again switched to deactivated state, i.e. the state in which the telephone line is shut down and the local apparatus connected to the line. The microprocessor 1 ceases to draw current through the code modulator and switches to a substantially passive state. Except from a very brief clicking sound, the user of the local telephone set will be persuaded that connection is not obtained until completion of the transmission. Since, however, the transmission may be carried out in less than one second, such additional waiting is considered acceptable. Essentially, the ordinary security unit does not fulfil any other functions, i.e. it does not monitor break-ins on the telephone line or interruptions and it does not transmit any alarm messages.

SECURITY UNIT WITH BATTERY

A security unit with battery serves as chief security unit. The current threshold comparator 4 is constantly active and in case the voltage on the telephone line drops below the threshold value, it will transmit a positive signal to the microprocessor 1 on the entry port 8. This means that the handle is lifted off a telephone set which is in communica¬ tion with the telephone line. In case the microprocessor simultaneously receives a signal on the entry port 13 from the current threshold comparator 6, it means that current is drawn in the local telephone set and the microprocessor 1 does not take further action.

However, if the microprocessor does not receive a positive signal on the entry port 13 it means that the handle is lifted off another telephone set connected to the telephone line. In this case the microprocessor awaits a code signal. Any such code signal will be received through the code demodulator 19 connected to the microprocessor entry port 7 so as to allow deciphering of an incoming code signal and comparison by the microprocessor to the code stored in its code memory 8.

For a very short period of time, e.g. less than 0.1 sec, the microprocessor awaits the transmission of code signals. If there is any sign of code pulsations, it awaits the code message and deciphers it.

If the codes match it means that the telephone apparatus in question is authorised, and then the microprocessor will not take any further action. If the microprocessor cannot match with its own code or if, following the expiry of said short waiting period, the microprocessor ascertains that no code at all was transmitted on the line, it means that the relevant use of the telephone line is unauthorised and the micropro¬ cessor will then activate the short-circuit relay 5 through its exit port 9 to short-circuit the telephone line. The telephone line is then dead and useless. At the same time the microprocessor makes an indication on the break-in indicator 12 and optionally it sets off an alarm on the alarm unit 11. In case no code signal is received, the line is shut down so fast that the apparatus of the unauthorised user will not have the time to starts its dialling tone.

The microprocessor 1 comprises an integral time circuit and it uses it in such a manner that, following a certain waiting period, e.g. 10 sees, it deactivates the short-circuit relay to open the telephone line whereupon it checks the signal from the current threshold comparator 4. If the current threshold comparator still emits a positive signal it means that the unauthorised use still takes palce and it will then reactivate the short-circuit relay 5 and the line is dead again. If there is no positive signal from the current threshold comparator it means that the relevant use of the telephone line has ceased and the microprocessor 1 sub¬ sequently switches to its normal resting state. An unauthorised user which tries to obtain communication with the telephone line will experience that the line is complete¬ ly dead and therefore it will be difficult for him to decide whether he has obtained any connection whatsoever to an open line. The periodical check-ups of the line by activation of the short-circuit relay are carried out so fast that they will be perceived as sporadic noise signals only.

According to a convenient embodiment the microprocessor is further constructed for continuously monitoring whether there is voltage on the line. This is effected through the port 7 which is in communication with the code demodulator 19. The code demodulator 19 is so constructed that, while there is voltage on the line, it continuously emits an off-set voltage in addition to the code signals, which voltage may be detected by the microprocessor on its entry port 7. The absence of offset-voltage on the entry port 7 means that the telephone line has been interrupted or short-circuited. The microprocessor being, of course, able to distinguish when when the lacking voltage is due to short-circuit through the short-circuit relay 5, it is programmed to react to the absence of voltage in all other instances by indicating line- interruption on the interruption indicator 13 optionally in combination with an alarm on the alarm unit 11.

The indications on the break-in indicator 12 and the inter¬ ruption indicator 13, respectively, remain, as they are cancelled manually by activation of the reset panel 14.

Reference is now made to fig. 2 for a more detailed descrip¬ tion of the components which are comprised by the security unit. To the left in fig. 2 the two conductors in the telephone line 25 are shown between the connector 15 for the incoming line and the connector 16 for the outgoing line. The short-circuit relay 5 is so disposed that it short-circuits the two conductors in the pair. The line unit 3 comprises the two functional blocks, the code modulator 18 and the code demodulator 19. The signal from the line is transmitted through a rectifier bridge connected to the modulator part and the demodulator part. The demodulator part consists substantially of the voltage divider constituted of the resistances R₂ and R₉ and a Zener diode Z₃ which serves to protect the microprocessor 1 against hypervoltages. The voltage divider converts the signals received to a level which allows them to be processed by the microprocessor.

In essence the modulator part comprises the transistor P_1# the resistance R₁₂ and the Zener diode Z₄. The modulator signal from the microprocessor unit is introduced on the basis of the transistor whereas current may be drawn from the joining point between the one pole on the Zener diode and the emitter of the transistor. From this point current can be drawn to supply power to those portions of the microprocessor which are to operate when the unit operates without battery, and a controlled current may be drawn which translates itself as a load on the telephone line so that the telephone exhange will also perceive the line as loaded while the connection is disconnected in the disconnecting relay 7.

The current threshold comparator 4 comprises a rectifier bridge, a transistor which derives its emitter voltage from the battery, and a resistance network adapted to effect that the transistor saturates when the voltage on the telephone line drops below a level of 28 V.

The current threshold comparator 6 comprises a rectifier bridge, the transistor P₃, the Zener diode Z-, two diodes D and D₂ and various resistances. Among these, resistances R₇ and R₈ are inserted serially into the conductors of the telephone line, and the circuit is substantially devised to react when the voltage above these resistances exceed a certain magnitude. The offset of the emitter is offset at the Zener diode Z_x thereby ensuring that the circuit reacts at a certain current value. The circuit is tuned so as not to react to small currents, e.g. the currents drawn by certain electronic telephone apparatuses in their resting states to maintain electronic memories and the like. Conveniently the circuit is constructed to react when the current magnitude exceeds 15 mA whereby it is ensured that the circuit reacts upon connection of any normal telephone set.

Herein the code memory 8 comprises a manually activatable dip-switch, i.e. a series of switches which may each manually be set between to positions, all the positions of the switches being digitally readable. It lies within the scope of the invention to realise the code memory in other ways in the form of fixed wirings which may conveniently be arranged in a plug-in connector or in the form of a purely electronic memory wherein the code is programmed electronically.

The microprocessor 1 comprises a control unit which may e.g. be realised in a suitable microcomputer or microcontroller and various peripheral components. The transistor P_B may be activated directly by a signal from the current comparator through the condensator C₃ whereby it will substantially open the power supply from the code modulator and to the micropro¬ cessor at its entry port 1. Following power supply in this manner, the microcomputer will be able to maintain the transistor P_s open by providing a signal on the exit port 27 which extends to the basis of the transistor P_s. Hereby the transistor may also ensure that a current load is maintained on the telephone line. The transistor P₅ ensures that power will not run in the opposite direction due to the rectifier effect thereof corresponding to the one of the rectifiers shown in fig. 1.

Power from the battery 17 is conducted through the transistor P₄ which, on the one hand, has a rectifying effect correspon- ding to the second rectifier of those shown in fig. 1 and on the other enables the microcomputer to keep the connection open following power supply due to the impulse from the current threshold comparator by providing the exit port 26 with a signal.

The entry port 7 of the microcomputer is an analogous entrance which enables the microcomputer to measure voltage levels and to determine whether the signals detected are code signals, dialling impulses or constant voltages.

The alarm unit 11 comprises a loudspeaker while the break-in indicator 12 and the interruption indicator comprises suitable control lights. The reset panel 14 comprises a push button. Further details, including types and values for components, will not be described in further detail as they are considered to be within the knowledge of the skilled person.

Reference is now made to fig. 3 which is a one-line diagram¬ matical illustration of a telephone line 24 connected to an exchange 23 and to four different telephone sets which are each designated with the reference numeral 22. The telephone line comprises a number of parallel coupled telephone outlets 21 and each of the telephone sets are plugged into a telepho¬ ne outlet 21 through each their security unit 20 according to the invention. One of the security units is provided with a battery 17 whereby it reacts as chief security unit which is capable of short-circuiting the telephone line and to set off alarms and indications.

The chief security unit 17,20 continuously monitors the telephone line and, as soon as the line is loaded by lifting of telephone handles or by connection of an unauthorised telephone apparatus, it reacts by checking whether the approved code is emitted and in the negative, by short- circuiting the telephone line. The function is active for outgoing as well as incoming telephone calls, i.e. an unauthorised apparatus can neither dial nor respond to calls. Only telephone sets connected through a security unit with the same code number as the one stored in the chief unit are allowed to avail themselves of the telephone line.

Although above the invention has been described above with reference to one particular embodiment, it is not excluded that the inventive idea delimited by the appended patent claims may be executed in other ways and in other contexts than those expressly taught by the description

REFERENCE NUMERALS

1. Microprocessor

2. Operation panel 3. Line unit

4. Voltage threshold comparator

5. Short-circuit relay

6. Current threshold comparator

7. Disconnecting relay 8. Code memory

9. 10.

11. Alarm

12. Break-in indicator 13. Interruption indicator

14. Reset panel

15. Line connector

16. Apparatus connector

17. Battery 18. Code modulator

19. Code demodulator

20. Security unit

21. Telephone outlet

22. Telephone set 23. Telephone exchange

24. Telephone line

25. The line

Claims

C l a i m s

1. Security unit comprising

- a connection to an incoming telephone line and a connection for an outgoing telephone line, a current threshold comparator constructed to detect the current in the telephone line and determine whether the numerical value of the current I in the telephone line exceeds a determined threshold value I_τ and to emit a corresponding logical signal, a code memory so constructed that any code may be stored therein and transmitted, a code modulator devised to dial a code signal on the incoming connection under the control of a logical unit, and a microprocessor constructed to operate by power supply from the incoming telephone line and such that it may receive signals from the current threshold comparator and from the code memory and emit control signals to the coupling relay and to the code modulator, said microprocessor being so programmed that it reacts to a signal from the current threshold comparator indicating that the threshold value has been exceeded by emitting the code from the code memory through the code modula¬ tor whereupon it switches to a passive state, wherein it does not react again until the comparator has indicated that the threshold value has not been exceeded over a given period of time.

2. Security unit according to claim 1, c h a r a c t e ¬ r i z e d in that it comprises a disconnecting relay constructed to provide respectively interrupt the communica¬ tion between incoming and outgoing connections of the telephone line, said microprocessor being programmed to maintain the communication to the outgoing connection interrupted through activation of the disconnecting relay while the code is emitted.

3. Security unit according to claim 2, c h a r a c t e - r i z e d in that the microprocesor is constructed to draw, while the disconnecting relay is in its "interrupted" position, a current of such magnitude that the relevant telephone line will be perceived by the telephone exchange as if the subscriber's apparatus is connected and is in the "handle lifted" state.

4. Security unit according to claims 1-3, c h a r a c ¬ t e r i z e d in that the code memory comprises a number of manually operable switches, the positions of which may each be read, or a multiple plug in which the code may be stored by forming a pattern of constant short-circuits between respective plug poles, or an electronic memory where the code is programmed electronically.

5. Chief security unit equipped like the security unit according to claim 1, but further comprising a current threshold comparator constructed to detect the voltage U across the conductor pair in the telepho¬ ne line and determine whether the numerical value of the voltage drops below a determined threshold value U_τ and to emit a corresponding logical signal, and a code demodulator constructed to detect and decode a signal emitted from a code modulator in a security unit connected to the same telephone line, said microprocessor being further constructed to receive the signals from the current threshold comparator and the code demodulator and to emit an output signal, said microprocessor being programmed to react to a signal from the current threshold comparator indicating that the threshold value has not been reached, by checking:

(a) whether the current threshold comparator indicates that exceeding has taken place, and (b) whether it receives through the code demodulator a code signal which corresponds to the code stored in the code memory, and in case just one of the questions (a) or (b) is answered in the affirmative, switches to passive state without further reaction, while in the opposite case it emits an output signal to indicate that an unauthorised situation is going on.

6. Chief security unit according to claim 5, c h a ¬ r a c t e r i z e d in that the unit comprises a short- circuit relay controlled by said output signal and connected to short-circuit the conductor pair in the telephone line as a reaction to the output signal, and that the unit optionally comprises a visual or acoustic alarm indicator which is activated by the output signal.

7. Chief security unit according to claim 5, c h a ¬ r a c t e r i z e d in that the unit comprises a device for monitoring the discontinuation of line voltage, said micro¬ processor being programmed and equipped to emit a particular line-discontinuation signal in case the line voltage is discontinued without the line being short-circuited by the short-circuit relay, and that the unit optionally comprises a visual or acoustic alarm indicator which is activated by the line-discontinuation signal.

8. Chief security unit according to claim 5, c h a ¬ r a c t e r i z e d in that the unit is constructed to deactivate the short-circuit relay after the expiry of a determined period during which the short-circuit relay has been activated, and to control whether the unit voltage is subsequently still below the threshold value in which case the short-circuit relay is reactivated.

9. Chief unit according to any one of claims 5-8, c h a ¬ r a c t e r i z e d in that the unit is so constructed as to be readily switched to serve as an ordinary unit.

10. Chief unit according to any one of claims 5-8 so constructed that power may be supplied by a battery, said unit being so constructed that it will operate as chief unit when power is supplied thereto by battery, whereas it will automatically change function to act as an ordinary unit when the battery supply is not available.