US3284571A

US3284571A - Combined intercom and door opener system

Info

Publication number: US3284571A
Application number: US259040A
Authority: US
Inventors: Wittig Gerhart
Original assignee: HANDELSKONTOR HEIDER GmbH
Current assignee: HANDELSKONTOR HEIDER GmbH
Priority date: 1962-02-24
Filing date: 1963-02-18
Publication date: 1966-11-08
Anticipated expiration: 1983-11-08

Description

Nov. 8, 1966 G. WlTTlG COMBINED INTERCOM AND DOOR OPENER SYSTEM Filed Feb. 18, 1963 5 SheetS-Sheet l Nov. 8, 1966 G. WIT-nca COMBINED INTERCOM AND DOOR OPENER SYSTEM 5 Sheets-Sheet Filed Feb. 18, 1965 Vst ZA MPL /F/E/e Nov. 8, 1966 G. wlTTlG COMBINED INTERCOM AND DOOR OPENER SYSTEM 5 Sheets-Sheet 3 Filed Feb. 18, 1963 Fig. 3

BELL? United States Patent Otiice 3,284,571 Patented Nov. 8, 1966 3,284,571 COMBINED INTERCOM AND DOOR OPENER SYSTEM Gerhart Wittig, Mannheim, Germany, assgnor to Handelslrontor Heider GmbH., Westphalia, Germany Filed Feb. 18, 1963, Ser. No. 259,040 Claims priority, application Germany, Feb. 24, 1962, H 45,001; Dec. 1, 1962, H 47,569 7 Claims. (Cl. 179-2) The present invention concerns a combined intercom and door opener system of the general type conventionally `used in b-uildings, particularly in buildings containing one or more apartments and one nonmally closed door, such system comprising facilities for enabling a caller at the door t-o talk to a party in an apartment, and vice versa, and for enabling the party in the apartment to lopen the door by remote electrical control.

It is an object of the invention to provide for a system of the type set forth containing a comparatively small number of components so that the syste-m 'can be installed at low cost for time and material.

lt is another object of t-he invention to provide for a system as mentioned above which requires between the door station and the cooperating station in .any one of the apartments only a two-line connection so that the system according to the invention can be installed in a particularly inexpensive and simple manner if in the respective building already a two`line connection exists between the area of the door and the area of :the respective apartment or apartments.

With above objects in view the invention includes a combined intercom and door opener system with a door station and at least 4one remote lcooperating station, comprising, in combination, a door station having two first terminals and including first elect-roacoustical transducer means and electrical door opener means connected in parallel with each `other to said terminals; a central station having two second terminal-s respectively connected with said first terminals, and including two third terminals. amplifier .means connected between said second and third terminals lfor cooperating with said transducer means, a plurality of relay means for controlling the operation of said amplifier means and the operation of said door opener means, respectively; and supply means for applying electrical energy to said relay means, amplifier mea-ns and door opener means, respectively, at least one cooperating station having two fourth terminals and including second electro-acoustical transducer means and control means including switch means for controlling the operation of said second transducer means and the ope-ration of said relay means, respectively, said second tnansd-ucer means and said control means being connected with said 'fourth terminals; and ltwo-line connection means arranged between said two third terminals and said two Ifounth terminals, respectively, for transmitting control criteria from said switch means in said cooperating station to said relay means in said central station and for transmitting signals between said first and second electroaco-ustical transducer means.

The novel features which are considered as characteristic for t-he invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation together with additional objects and advantages thereof, will be best understood from the following description yof specific embodiments when read in connection with the accompanying drawings, in which:

FIG. -1 is a schematic circuit diagram illustrating one embodiment of the invention including a door station, a central station and one of cooperating stations in an apartment;

FIG. 2 is a circuit diagram illustrating the main amplifier arrangement in the central station according to FIG. l;

FIG. 3 is a yschematic circuit diagram of a second embodiment of the invention including a door station, a central station and a cooperating stati-on in one of the apartments; and

FIG. 4 is a partial circuit diagram illustrating details of the main amplifier arrangement in the central station of FIG. 3.

The system or installation according to FIG. 1 comprises a door station TST, a central station ZST and at least one cooperating station WST in any one of the apartments of the respective building. It is evident that any number of such cooperating stations may be connected in the manner shown with the two-line connection St which may be assumed to extend .through the entire building.

Each of the cooperating vapartment stations WST comprises an electro-acoustical transducer RL which may be a conventional type of aloud speaker which can also be used as a microphone, moreover a changeover switch I shown in its 4normal position, a singleat-hrow double-pole switch II the switch portions Ill and 112 -t'hereof being held preferably by spring means, not shown, in open position, and a capacitor C1 'arranged in the circuit of the transducer RL. The just listed components are connected with each other and with the terminals a and b", respectively, as shown in FIG. 1.

The central station ZST containing an amplifier may be located 4anywhere in the building, e.=g. in the basement, or in any event not too far `away from the door station. This central station ZST comprises an amplifier arrangement V including control means as indicated by IFIG. 2, a speech direction reversing relay RS (its Icontacts rs sh-own in FIG. 2), 'a door opener relay RT with relay contacts rt, and including a .series resistor RV, and finally an amplifier starting relay RE, its contacts re shown in FIG. 2. The last mentioned relay RE is provided with conventional means for delaying its deenergization after the current supply has been cut off. By way of example these delaying means may comprise a capacitor C2 connected in parallel with the relay coil RE in combination with a series-connected rectifier or diode G.

Since the .amplifier V is only provided for amplifying speech `currents or similiar signals transmitted between the electro-acoustical transducers of the system, this amplifier V is blocked against direct current inpnt by capacitors C3 and C4 arranged between the amplifier and the terminals a Iand b to which the above mentioned twoline connection St is connected. Similarly the other side of the amplifier V is blocked against direct current input by capacit-ors C5 and C6 arranged between the amplifier V and the terminals c and d.

The central station ZST comprises further supply means for furnishing direct current to the above mentioned relays and to the amplifier V, and for furnishing alternating current to the door opener as described further below.

The direct current supply may comprise conventional means like a transformer, rectifier, current smoothing device, etc., not shown and symbolically indicated by a battery B. The supply of alternating current is indicated at D.

The door station TST comprises an electro-acoustical transducer TL of the same type as the above described transducer RL, a conventional electrical or electro-mechanical door opener To and decoupling capacitor C7. The devices To and TL are connected in parallel with the terminals e and f, these terminals being connected with terminals c and d of the central station ZST by a twoline connection L as shown in FIG. 1.

FIG. 2 illustrates details of the amplifier arrangement V of FIG. l. It can be seen that energization of the relay RE will result in switching on the conventional amplifier Vst by closing the relay contact re. It can be seen further that the input and output lines of the amplifier Vst are connected in the manner shown by FIG. 2 with the terminals a and b, i.e. with the two-line connection St, as Well as with the terminals c and d i.e. with the twoline connection L, via relay contacts rs which are in the positions shown by FIG. 2 when the relay RS is not energized, but change to the respective second positions when this relay RS is energized. Thus, by energization of the relay RS the input connection of the amplifier is exchanged against the output connection, and vice versa.

The operation of the arrangement according to FIGS. 1 and 2 is as follows. After a caller at the door has alerted in conventional manner, e.=g. by ringing a bell, the party in the apartment where the cooperating station WST is installed, the alerted party would operate the change-over switch I so as to move it from its normal position to its second position whereby the transducer RL is connected with the terminals a" and b" and thereby with the two-line connection St. Hereafter the double- -pole switch II is actuated so that its arms III and II2 are moved to closed position. Hereby communication between the station WST and the door station TST is established. This is due to the fact that the switch arm -II2 in closed position shunts the capacitor C1 whereby a direct current circuit is established as follows: from battery B via rectifier G, -relay coil RE, terminal b, line b', terminal b", transducer RL, closed switch 112, switch I, terminal a, line a', terminal a and back to the battery B. Thus, the relay RE is energized and closes its contact re (FIG. 2) so as to switch on the amplifier Vst whereby the amplifier arrangement V is rendered operative.

Simultaneously with the energization of the relay RE also the delaying capacitor C2 is charged and the relay RS is energized. Consequently, the respective relay contacts rs shown in FIG. 2 are moved from the illustrated position to their respective second position whereby the connection coming from the cooperating station WST is connected with the inputs of the amplifier Vst while its outputs are connected with the terminals c and d and thereby with the transducer TL in the door opener station TST. Thus, it is now possible to talk from the station WST to the door station TST.

If now, while the change-over switch I is still in its second position, the direction of talk should be reversed so that speech can be transmitted from the transducer TL after amplification to the transducer RL, then the party at the station WST has only to release the doublepole switch II whereby the previously established circuit is immediately interrupted and the relay RS is deenergized. As can be seen from FIG. 2 the relay switches rs return now to their original normal position as illustrated so that now the input and output connections of the amplifier Vst are exchanged and speech can be transmitted from the transducer TL to the transducer RL. The relay RE, however, has not been deenergized when the circuit was interrupted by the return of the switch II to its normal position because, as mentioned above, the relay RE is provided with delay means e.g. with the capacitor C2 in cooperation with the rectifier G. However, it should be understood that the desired delay can be achieved also by other conventional means eig. by means 0f a clockwork controlling a current supply. The diode G is provided in order to prevent that the discharge of the capacitor C2 influences also the relay RS or RT, or both.

It can be seen that as long as the change-over switch I remains in its second position conversation between the stations can be carried out by alternatingly actuating the double-pole switch II as the change in the direction of the signal transmission changes. However, the door opener relay RT has not 4been energized up to now because this relay requires a current of greater strength than that of the current sufficient for energizing the relays RE and RS. For this purpose the relay coil of the relay RT may have a corresponding higher resistance 0r a series-resistor RV may be combined with that coil.

It should be noted that every time the double-pole switch II is depressed the relay RE is again energized and that every time the -double-pole switch is released the delay means mentioned above maintain for a predetermined period of time the energization of the relay RE. It is evident that the delay means can be chosen so that the predetermined delay time is long enough to meet prevailing conditions.

If the conversation -between the stations is completed the change-'over switch I is returned to its normal position as illustrated in FIG. 1 whereby the transducer RL is disconnected. If it is now desired to open the door the party at the station WST has only to actuate once more the double-pole switch II whereby the transducer RL and thereby its resistance is shunted by the closing of the switch arm III. This has the effect that now a current of ygreater strength is permitted to ow through the thus established circuit and that thereby now the -relay RT is energized in spite of its having a greater resistance, or in spite of the presence of the resistor RV, whereby the frelay contact rt is closed and alternating current from the supply D is applied to the door opener To. It is immaterial that at the same time also the relays RE and RS are energized because even if now the amplifier arrangement V is thus rendered operative this has no effect because the transducer RL has been disconnected. Incidentally, after the end of the predetermined delay time the relay RE is deenergized anyway so that thereafter the amplifier is not operating.

The above described embodiment may be modified in various ways without departing from the gist of the invention. For instance, the control criteria transmitted from Ithe station WST for establishing communication and for opening the door may not be :constituted by varying the strength of a direct current flowing through the system. In a similar manner alternating currents lhaving different characteristics may be used e.g. currents differing in amplitude as well as currents differing in phase.

The embodiment illustrated by FIGS. 3 and 4 differs from the above described embodiment in various ways. The station WST' is quite similar to the station WST of FIG. 1 except that a 'resistor R1 is connected between the switch arm II2 and the capacitor C1, and a second resistor R2 is connected in parallel with the transducer RL.

In the central station ZST an auxiliary amplifier VRE is provided for amplifying even a weak current sufficiently for energizing the associated rel-ay RE. Thus, the relay RE in combination with the auxiliary amplifier will respond to a current of lesser strength than that to which the relay RS will respond while, as in the `above described example, the relay RT will require a current of greater strength than that which suffices to energize the relay RS. The capacitors C3, C5 and C6 are omitted. The amplifier arrangement V is constructed in detail fas shown by FIG. 4, exactly like the amplifier )arrangement V according to FIG. 2. Also the door .station TST is identical with the same station as illustrated in FIG. 1.

The arrangement according to` FIGS. 3 :and 4 operates as follows. After the party at the station WST' has been alerted e.g. by the ringing olf a bell, the party may move the change-over switch I from the normal position illustrated by FIG. 3 to its second position whereby the transducer RL is connected with the terminals a and bi" and thereby through lines a and b with t-he terminals a and b. However, at the sa'me time the resistor R2 is connected across the lines a' and b' so that a ow of current is caused as follows: from the positive terminal of the direct current supply via the relay RE, relay contacts ft2 (described further below), amplifier VRE (which in this example is continuously oper-ative), terminal b, line b", terminal b, resistor R2, change-over switch I, terminal a, line a', terminal a `and back to the negative terminal of the supply.

In this now established circuit the resistance of the resistor R2 is so high that t-he current flow in this circuit is not suflcient to energize the relays RS and RT, w-hile the relay RE is energized by the comparatively weak current owing through the circuit because of the presence of the auxiliary amplifier VRE. The energization of relay RE `closes the contact re (FIG. 4) whereby the amplifier Vst is switched on. Since the relay RS requiring a stronger c-urrent has not been energized the amplifier is now connected via the relay contacts rs1-r5.1 in the position illustrated by FIG. 4 between the transducer TL and the transducer RL so that now a ca'ller at the door station TST can talk to the party at the station WST.

It can be seen that the advantage of this arrangement resides in the fact that after :actuation of the changeover switch I alone the relay RE is energized so that the conversation from the door station to the station in the apartment can be carried out for any desired length of time as long as the switch I remains in its second position. Thus, the relay RE does not have to be equipped with delay means.

If now the party at the station WST wishes to talk to the party at the door station TST the double-pole switch II has to be actuated while the change-over switch I still remains in its second position. Hereby the resistor R1 is connected in parallel with the resistor R2 due to the closing of the switch portion lI2. Now the combined resistance is so low that the relay RS is energized by the current flowing as follows: from the positive terminal of the direct current sup-ply B through the relay coil RS, terminal b, line b', terminal b, lresistors R1 and R2, switch I, terminal a, line a', terminal a, and back to the negative terminal of the source B. However, it has to be understood that the thus established circ-uit still is not able to carry enough current for energizin-g the relay RT. The energization of the relay RS causes the relay contact rs1-m4 to move to their respective second position whereby the input and output connections of the amplifier Vst are exchanged against each other so that signals can be transmitted from the transducer RL through the Iamplifier to the transducer TL. If thereafter the direction of the transmission of signals should be reversed again the party at the station WST' has -only to release the double-pole switch II so that the switch portion H2 returns to its normal position which causes the relay RS t-o be deenergized.

When the conversation between the stations is terminated the change-over switch I is returned to its normal position illustrated in FIG. 3 whereby the transducer RL is switched off; at the same time the current ow through the resistor R2 is interrupted so that Ithe relay RE is deenergized and the amplifier Vst is also switched oif by the opening of the relay contact re.

If now, with the change-over switch I in normal position, the double-pole switch II is :again actuated a direct connection between the lines a and b is established through the switch portion H1. Consequently, any resistance existing in the station WST' is removed from the circuit and a rather strong current can now ow from the positive terminal of the source B through the relay RT, resistor RV, terminal b, line b', terminal b, switch Ill, switch I, terminal a", Iline a', terminal a and back to the negative terminal of the source B. This current flow is strong enough to energize the relay RT. Consequently the relay contact rtl is changed from the normal position illustrated in FIG. 3 to its second position whereby the circuit between the alternating current source D and the door opener To is completed via the two-line connection L. At the same time the change of position of the relay contact rtl separates the amplifier arrangement V from the alternating current supply whereby possible damage is avoided. In addition, the second relay contact ft2 of the relay RT interrupts the circ-uit for the relay RE. It so happens that the relay RS is energized simultaneously with the relay RT 'but this is without any consequence because the amplifier arrangement is switched off.

In certain cases it may be desirable to utilize the alternating current supply D in the central station ZST also for operating a bell or buzzer connection between the door station and the station in the particular apartment or apartments. This can be done with the additional advantage that one of the lines of the two-line connection Sl is also utilized in the bell or buzzer system. This possibility is illustrated in FIG. 3. For this pu-rpose one line of the alternating current supply D is connected permanently by a connection g via terminal a with the line a. A bell or buzzer K is connected at one side with the line a while it is connected at the other side by a line i1, a bell button P1 and line h via terminal f and d with the other line of the alternating current supply D. By pressing the bell button P1 the bell or buzzer K is supplied with alternating current as can be seen readily. If there are more apartments in the system additional lines i2 and bell buttons P2 may be added as required. Thus, one line for a bell or buzzer system and an extra transformer is saved in this modified installation.

It should not be overlooked that the arrangement according to the invention entails the further advantage that, provided that the additional bell or buzzer installation is not present as shown, it is immaterial whether the line a is connected with terminal a and line b is connected with terminal b, or whether line a is connected with terminal b and line b is connected with terminal a". Similarly it is immaterial whether the terminal c is connected With terminal e, terminal d with terminal f, or whether terminal c is connected with terminal f and te-rminal d with terminal e. This feature greatly facilitates the installation of the various stations described above in a building, particularly if already a two-line connection exists which only has to be tapped for utilizing it for the connections between the stations.

It will be understood that each of the elements described above or two or more together, may also find a useful application in other types of a combined intercom and door opener system differing from the types described above.

While the invention has been illustrated and described as embodied in a combined intercom and door opener system with a door station and at least one remote cooperating station, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit ofthe present invention.

Without further analysis the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed and desired to be secured by Letters Patent is:

l. A combined intercom and door opener system with a door station and at least one remote cooperating station, comp-rising, in combination, a door station having two first terminals and including first electro-acoustical transducer means and electrical door opener means connected in parallel with each other to said terminals; a central station having two second terminals respectively connected with said first terminals, and including two third terminals, amplifier means connected between said Second and third terminals for cooperating with said transducer means, a plurality of relay means for controlling7 the operation of said amplifier means and the operation of said door opener means, respectively; and supply -means for applying electrical energy to said relay means, amplifier means and door opener means, respectively; at least one cooperating station having two fourth terminals and including second electro-acoustical transducer means and control means including switch means for controlling the operation of said second transducer means and the operation of said relay means, respectively, said second transducer means and said control means being connected with said fourth terminals; and two-line connection means arranged between said two third terminals and said two fourth terminals, respectively, for transmitting control criteria from said switch means in said cooperating station to said relay means in said central station and for transmitting signals between said first and second electroacoustical transducer means.

2. A combined intercom and door opener system according to claim 1, wherein said plurality of relay means includes first relay means for connecting, when energized, said amplifier means with said supply means, second relay means for interchanging, when energized, the connections between said second and third terminals, respectively, and the input and output, respectively, of said amplifier means, and third relay means for connecting, when energized, said door opener means with said supply means, said third relay means requiring for its energization a current having a characterstic different from that of currents required by said first and second -relay means, and wherein said control means is constructed so as to determine, depending upon actuation of said switch means, said characteristics of said currents energizing said various relay means, respectively.

3. A combined intercom and door opener system according to claim 1, wherein said plurality of relay means includes first relay means for connecting, when energized, said amplifier means with said supply means, second relay means for interchanging, when energized, the connections between said second and third terminals, respectively, and the input and output, respectively, of said a-mplifier means, and third relay means for connecting, when energized, said door opener means with said supply means, said third relay means requiring for its energization a current of greater strength than that of currents required by said first and second relay means, and wherein said control means is constructed so as to determine, depending upon actuation of said switch means, the strength of said currents energizing said various relay means, respectively.

4. A combined intercom and door opener system according to claim 1, wherein said plurality of relay means includes first relay means for connecting, when energized, said amplifier means with said supply means, second relay -means for interchanging, when energized, the connections between said second and third terminals, respectively, and the input and output, respectively, of said amplifier means, and third relay means for connecting, when energized, said door opener means with said supply means, said first relay means including a relay and current amplification means for raising the strength of current applied thereto to that required for energizing said relay, said third relay means requiring for its energization a current of greater strength than that of the current required by said second relay means, and said first relay means requiring a current of lesser strength than that required by said second relay means, and wherein said control means include resistor means, a change-over switch and a singlethrow double-pole switch connected with each other and with said second transducer means in such a manner that when said change-over switch is in its normal position and said double-pole switch is closed a current of suflicient strength is caused to flow through the thus established circuit to energize said third relay means, and that when said change-over switch is moved to its second position while said double-pole switch is in its normal open position only that amount of current is caused to flow through the thus established circuit which suffices only to energize said first relay means and said first and second transducer means are operatively connected, and that when said change-over switch is in its second position and additionally said double-pole switch is moved to closed position a current is caused to fiow through the thus established circuit which suffices to energize said second relay means and said first and second transducer means are operatively connected.

5. A combined intercom and door opener system according to claim 1, wherein said plurality of relay means includes first relay means for connecting, when energized, said amplifier means with said supply means, second relay means for interchanging, when energized, the connections between said second and third terminals, respectively, and the input and output, respectively, of said amplifier means, and third relay means for connecting, when energized, said door opener means with said suppy means, said third relay means requiring for its energization a current having a characteristic different from that of currents required by said first and second relay means, said first relay means including means `for delaying through a predetermined time period the deenergization thereof after the energy supply thereto has been cut off, and wherein said control means is constructed so as to determine, depending upon actuation of said switch means, said characteristics of said currents energizing said various relay means, respectively.

6. A combined intercom and door opener system according to claim 1, wherein said plurality of relay -means includes first relay means for connecting, when energized, said amplifier means with said supply means, second relay means for interchanging, when energized, the connections between said second and third terminals, respectively, and the input and output, respectively, of said amplifier means, and third relay means for connecting, when energized, said door opener means with said supply means, said third relay means requiring for its energization a current of greater strength than that of currents required by said first and second relay means, said first relay means including mean for delaying through a predetermined time period the deenergization thereof after the energy supply thereto has been cut off, and wherein said control means is constructed so as to determine, depending upon actuation of said switch means the strength of said currents energizing said various relay means, respectively.

7. A combined intercom and door opener system according to claim 6, wherein said plurality of relay means includes first -relay means for connecting, when energized, said `amplifier means with said supply means, second relay means for interchanging, when energized, the connections between said second and third terminals, respectively, and the input and output, respectively, of said amplifier means, and third relay means for connecting, when energized, said door opener means with said supply means, said third relay -means requiring for its energization a current of greater strength than that of currents required by said first and second relay means, said first relay means including means for delaying through a predetermined time period the deenergization thereof after the energy supply thereto has been cut ofi, and wherein said control means include a change-over switch and a single-throw double-pole switch connected with each other and with said second transducer means in such a manner that when said change-over switch is in its normal position and said double-pole switch is moved to closed position 9 a current of suficient strength is caused to ilow through the thus established circuit to energize said third relay means, and that when said change-over switch is moved to its second position and additionally said double-pole switch is moved to closed position only that amount of current is caused to flow through the thus established circuit which suces to energize `only said first and second relay means and said rst and second transducer means are operatively connected, and that when thereafter said change-over switch is in its second position and 10 said double-pole switch is returned to its normal open position said second relay means is deenergized While References Cited by the Examiner UNITED STATES PATENTS 1,857,107 5/1932 Cadieux 179-2 2,372,435 3/1945 LaForest 317--137 X 2,387,443 10/ 1945 Hasenberg 179-1.4

DAVID G. REDINBAUGH, Primary Examiner.

J. T. STRATMAN, Assistant Examiner.

Claims

1. A COMBINED INTERCOM AND DOOR OPENER SYSTEM WITH A DOOR STATION AND AT LEAST ONE REMOTE COOPERATING STATION, COMPRISING, IN COMBINATION, A DOOR STATION HAVING TWO FIRST TERMINALS AND INCLUDING FIRST ELECTRO-ACOUSTICAL TRANSDUCER MEANS AND ELECTRICAL DOOR OPENER MEANS CONNECTED IN PARALLEL WITH EACH OTHER TO SAID TERMINALS; A CENTRAL STATION HAVING TWO SECOND TERMINALS RESPECTIVELY CONNECTED WITH SAID FIRST TERMINALS, AND INCLUDING TWO THIRD TERMINALS, AMPLIFIER MEANS CONNECTED BETWEEN SAID SECOND AND THIRD TERMINALS FOR COOPERATING WITH SAID TRANSDUCER MEANS, A PLURALITY OF RELAY MEANS FOR CONTROLLING THE OPERATION OF SAID AMPLIFIER MEANS AND THE OPERATION OF SAID DOOR OPENER MEANS, RESPECTIVELY; AND SUPPLY MEANS FOR APPLYING ELECTRICAL ENERGY TO SAID RELAY MEANS, AMPLIFIER MEANS AND DOOR OPENER MEANS, RESPECTIVELY; AT LEAST ONE COOPERATING STATION HAVING TWO FOURTH TERMINALS AND INCLUDING SECOND ELECTRO-ACOUSTICAL TRANSDUCER MEANS AND CONTROL MEANS INCLUDING SWITCH MEANS FOR CONTROLLING THE OPERATION OF SAID SECOND TRANSDUCER MEANS AND THE OPERATION OF SAID RELAY MEANS, RESPECTIVELY, SAID SECOND TRANSDUCER MEANS AND SAID CONTROL MEANS BEING CONNECTED WITH SAID FOURTH TERMINALS; AND TWO-LINE CONNECTION MEANS ARRANGED BETWEEN SAID TWO THIRD TERMINALS AND SAID TWO FOURTH TERMINALS, RESPECTIVELY, FOR TRANSMITTING CONTROL CRITERIA FROM SAID SWITCH MEANS IN SAID COOPERATING STATION TO SAID RELAY MEANS IN SAID CENTRAL STATION AND FOR TRANSMITTING SIGNALS BETWEEN SAID FIRST AND SECOND ELECTROACOUSTICAL TRANSDUCER MEANS.