US3831065A

US3831065A - Electronic push button combination lock

Info

Publication number: US3831065A
Application number: US00348753A
Authority: US
Inventors: R Martin; P Quinn
Original assignee: Integrated Conversion Tech
Current assignee: JP INDUSTRIES Inc A MI CORP; WESLOCK Corp A CORP OF; Integrated Conversion Tech; INTEGRATED CONVERSION TECHNOLOGY US
Priority date: 1973-04-06
Filing date: 1973-04-06
Publication date: 1974-08-20
Anticipated expiration: 1991-08-20
Also published as: JPS604352B2; DE2416442A1; JPS501000A; FR2224629A1; GB1404748A; FR2224629B1; CA1021432A

Abstract

The electronic lock includes a plurality of push buttons which are sequentially actuated to provide a series of output binary signals. These signals are successively compared with a series of stored coded signals in a plurality of circuits which are caused to be successively actuated by a ring counter, a correct comparison providing an advance signal for the ring counter so that the next stored coded binary signal can be compared with the next applied output binary signal from the push button console. If the input code corresponds correctly with the codes in the coded circuits, the ring counter will complete its complete count, the last stage of the counter providing an unlocking signal to open the lock.

Description

United States Patent [191 Martin et a1.

ELECTRONIC PUSH BUTTON COMBINATION LOCK Inventors: Ricky Martin, Santa Ana; Paul Quinn, Anaheim, both of Calif.

Assignee: Integrated Conversion Technology,

Anaheim, Calif.

Filed: Apr. 6, 1973 Appl. No.: 348,753

References Cited UNITED STATES PATENTS l/l972 Gilbert.... 4/1972 UTTON [111 3,831,065 Aug. 20, 1974 3,676,849 7/1972 Malandro an 340/149 R 3,754,164 8/1973 Zorzy ..3l7/134 Primary Examiner-J. D. Miller Assistant Examiner-Harry E. Moose, Jr. Attorney, Agent, or Firm-Pastoriza & Kelly 5 7] ABSTRACT The electronic lock includes a plurality of push buttons which are sequentially actuated to provide a series of output binary signals. These signals are successively compared with a series of stored coded signals in a plurality of circuits which are caused to be successively actuated by a ring counter, a correct comparison providing an advance signal for the ring counter so that the next stored coded binary signal can be compared with the next applied output binary signal from the push button console. If the input code corresponds correctly with the codes in the coded circuits, the ring counter will complete its complete count, the last stage of the counter providing an unlocking signal to open the lock.

7 Claims, 3 Drawing Figures DECIMAL To BINARY CONVERTER OUTPUT CODE RESET ADVANCE PAIENIEU 3,831,065

SHEET 1 [If 2 PUSH BUTTONS m iivfii/ 0+ III I I I I I I DECIMAL To BINARY CONVERTER TIMER OUTPUT I RESET CODE '2 v '8 AIVBIVCIVDIV 22 RESET COMPARATOR RING COUNTER ADVANCE/ E AIIBICD 2| I 2345 GIVEN CODE I3 SELECTOR CKT I CKT 2 CKT 3 LOCK J CKT 4 FIG].

7 CKT I 4 CKT 2 5 CKT 3 2 CKT 4 FIG.2

PATENTEDAUBZOIHH SIEEIZBF 2 PUSH BUTTONS I a @C 6 8 9 23 )HHH) 2/ TTTITTTTTT TIMER- DECIMAL To BINARY CONVERTER. RESET A" B' C' D" 22 I RESET A COMPARATOR ADVANCE RING COUNTER A I B C D I 2 3 4 5 SELECTOR 4 I9 CKTI CKT2 CKT3 CKT4 I I ABCD ABCD ABCD ABCD 4 LOCK II I IIII II IIII STATIONN 25 ,C. ,J DATA CODE INs RTMuLTIPLE BIT SHIFT I A REGISTER CLOCKZ 27 SERIAL TO PARALLE CONVERTER.

ADDRESS MULTIPLE BIT SHIFT T STA SERIAL To PARALLEL CONVERTER;

' sIGNAL ADDRESS COMPARATOR 3| IIIIIII IIII III I 39' I O O 40 35 ADDRESS LOCK GATE I E: IIIIIIIIIIIIIIII IIIIIIIIIIIIIIII 36 COMBINATION CLOCKZ IIIIIIIIIIIIIIII OOOIOOOIOOOOOOOO OIIIOIOOOIOIOOIO IIOO-ADOREss 7452-COMBINATION 37 FIGS ELECTRONIC PUSH BUTTON COMBINATION LOCK BACKGROUND OF THE INVENTION Every year hotels and motels incur an enormous loss in keys. Most of this loss is accounted for by inadvertent carrying away of the key by a guest.

The foregoing problem is solved if combination locks are provided for doors. Conventional combination lock's, however, require that the guest memorize his combination in order that only he can gain access to his room. It would be possible, of course, to set the combination lock to a particular combination of numbers with which the guest was already familiar; for example, the date of his birthday or his automobile license number or some other code wherein the problem of the guest having to memorize a number is avoided. This solution with conventional combination locks is unsatisfactory because of the enormous time involved in resetting the combination lock each time a new guest arrives. Moreover, it would present problems to the manager of the hotel or motel in keeping track of the various combinations for each room.

Nevertheless, the use of a combination type of lock would be preferable to the key type if a convenient type of combination lock could be devised wherein the code could readily be changed without having to take the lock apart at each individual room.

BRIEF DESCRIPTION OF THE PRESENT INVENTION With the foregoing in mind,.the present invention contemplates an electronic push button combination lock wherein the particular combination for opening the lock can very easily be set into the lock and wherein it is very simple for a guest to thereby gain access to his room by simply pushing in proper sequence a series of the push buttons.

Basically, the invention contemplates a means for generating an output code such as a series of push buttons operable by a person seeking to open the lock. The lock in turn includes means for storing a given code. Comparator means are provided which receive the output code and given code and compare the same, the comparator means providing a given signal only when the output code corresponds to the given code. Finally, means are provided responsive to the given signal to provide an unlocking signal which opens the lock.

The stored given code is established by a plurality of circuits each capable of storing a given number of bits of information such as a binary code. With such circuits, it is a very simple matter to electronically change the particular stored code in each of the circuits so that the entire combination can readily be changed at will.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the electronic push button combination lock of this invention will be had by referring to the accompanying drawings in which:

FIG. 1 is a block circuit diagram of the electronic combination lock; and,

FIG. 2 is a table showing an example of a particular combination set into the electronic lock of FIG. 1; and,

FIG. 3 is a system to change the combination.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1 the electronic lock includes a plurality of push buttons 10 which when depressed will close a suitable switch to connect power to a decimal to binary converter circuit 11. The various push buttons are numbered with the

digits

0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. The buttons themselves may be formed in a rectangular array such as found on an adding machine.

The decimal to binary converter 11 serves to generate an output binary signal in response to depression of a selected button equivalent to the number associated with the selected button. This output binary signal also referred to as an output code or part of an output code may constitute, by way of example, four bits of information designated by the letters A, B, C and D. As shown, this code is received in a first input of a comparator 12. The comparator 12 includes a second input for receiving a stored given code also in the form of a binary signal from a selector 13. This given code may also constitute four bits of information designated by the letters A, B, C, and D.

A given number of coded

circuits

14, 15, 16 and 17, by way of example, each provide a coded binary signal upon actuation for reception through the selector 13 to the second input of the comparator circuit. Actuation of the coded circuits 14 through 17 is effected by corresponding stages in a ring counter 18.

In the particular example chosen for illustrative purposes, the ring counter 18 has five stages; that is, the number of stages is one greater than the given number of coded circuits. The first and subsequent stages of the ring counter are connected to the coded circuits respectively as shown by the leads from the first four stages of the ring counter the arrangement being such that energization of any one stage of the ring counter will actuate its connected coded circuit.

The fifth stage of the ring counter when energized provides an unlocking signal through a relay coil 19 which may connect to a lock 20 to throw a bolt or striker and open the lock.

The comparator 12 is connected to the ring counter 18 by an advance lead connection 21 which provides a signal to advance the ring counter to a next stage whenever a correct comparision is made in the comparator 12. Also there is provided a reset signal lead 22 for providing a reset signal from the comparator to the ring counter 18 whenever the comparison made by the comparator 12 is incorrect.

The circuit is completed by the provision of a timer 23 connected to receive the unlocking signal from the fifth stage of the ring counter 18 which functions to reset the ring counter after a given period of time as measured from the moment that the unlocking signal is generated.

Each of the coded

circuits

14, 15, 16, and 17 include four hits of information defining a binary code the four circuits together making up an overall given code stored in the lock.

FIG. 2 is a table illustrative of a specific combination binary code set into the lock. Thus, for circuit 1 the binary code is 01 l l and when circuit 1 designated 14 in FIG. 1 is actuated by a signal from the first stage of the ring counter 18, the binary code 0111 will be passed through the selector 13 to appear on the second input A, B, C, and D of the comparator 12. The second coded circuit indicated as circuit 2 has a binary code 0100 and similarly this particular binary code is applied to the comparator when the circuit 2 is actuated by energization of the second stage of the ring counter.

In a similar manner the

circuits

3 and 4 include binary codes 0101 and 0010 respectively. The decimal equivalent of these binary signals respectively is the number 7452 and thus'for the example chosen, the number 7452 constitutes the combination of the electronic lock.

OPERATION OF FIG. 1

In operation, the ring counter is always reset to its first stage. Assuming that the coded

circuits

14, 15, 16, and 17 each include the four bit binary code information as described in conjunction with FIG. 2, a person may readily unlock the lock by depressing in sequence the various push buttons in FIG. 1 corresponding to the code 7452. These particular push buttons are circled in FIG. 1.

For example, when the guest entering the room presses the first push button 7, the decimal to binary converter 11 will provide the binary equivalent to the decimal number 7 which binary equivalent is 01 1 I, this binary signal appearing on the output code lead A, B, C, and D in the first input of the comparator 12. Since the first stage of the ring counter is energized to actuate the first coded circuit 14, the input leads A, B, C, and D to the second input of the comparator 12 will have the binary code 01 l l. The comparator circuit will now compare these two binary numbers and since they are identical, an advance signal will be sent from the comparator to the ring counter to advance the ring counter from its first stage to its second stage.

When the ring counter is advanced to its second stage, the coded circuit is energized to provide the code 0100 to the second input of the comparator 12.

The guest next depresses the push button 4, the binary equivalent thereof applied to the first input of the comparator 12 being 0100. Since this code again corresponds to that applied to the second input of the comparator, an advance signal is again applied to the ring counter to advance the ring counter to the third stage.

The third stage code 0101 is then compared with the output code applied by the push button 5 and again since the output code corresponds to the given code, the ring counter is again advanced to energize the last coded circuit 17.

Assuming the guest then depresses the last number 2 of the code, the output binary signal 0010 at the first input of the comparator will be compared with the given code from the coded circuit 17 and since these binary numbers correspond, an advance signal will be applied to the ring counter 18 to energize its last or fifth stage.

When the fifth stage of the counter 18 is energized, the solenoid coil 19 receives an unlocking signal to open the lock 20. Simultaneously, the counter 23 is energized to start a given time period running after which the ring counter 18 is automatically reset to its first stage.

Should the guest depress the wrong button any place in the sequence, an incorrect comparison will be made in the comparator 12 causing a reset signal to be generated on the lead 22 to reset the ring counter 18 to its first stage.

It should be understood that the comparator 12 only operates when one of the push buttons 10 is in a depressed state so that the advance and reset signals will only pass to the ring counter while a selected button 10 is held closed.

Since the coded circuits 14 through 17 are made up of simple one-off memory circuits; for example, simple flip flops, the particular code can easily be changed. Thus a particular combination of numbers which is very easy for a guest to remember can readily be inserted in the electronic combination lock.

Referring now to FIG. 3 a suitable address and combination code inserting circuit for selecting a particular room combination lock and inserting a desired code is shown. I

In FIG. 3, the electronic lock portion is identical to that described in FIG. 1 except that the

circuits

14, 15, 16 and 17 of FIG. 1 are incorporated in a large selector block designated 24. The other components of the electronic lock portion in FIG. 3 are designated by the same numerals as the corresponding components shown in FIG. 1.

Essentially the code insert system of FIG. 3 enables the successive coded binary signals in the given number of coded circuits designated CKT. l, CKT. 2, CKT. 3 and CKT. 4 in the selector block 24 to be changed to new codes from a remote location. Towards this end, the system includes a code insert multiple bit shift register 25 for effecting a serial to parallel conversion of successive bits of code information. The circuits 1 through 4 in the selector 24 are connected to the parallel conversion output of the shift register 25 as shown by the letters A, B, C, D for each of the circuits. A data signal line means includes a data line 26 carrying a control data wave form and a line 27 providing a series of uniformly spaced clock pulses. The data wave form essentially gates certain ones of the clock pulses which are fed into the serial input of the shift register 25 in the form of successive bits of code information in accordance with the new codes to be supplied to the circuits in the selector 24.

In the situation depicted in FIG. 3, there are provided a plurality of additional electronic locks at different stations all identical to the electronic lock shown in FIG. 3. Such stations are indicated in the lower right hand portion of FIG. 3 by the block labeled STA l and by the block STA 2 which might, for example, constitute different rooms in a motel each provided with individual electronic locks. The complete electronic lock system shown in detail in FIG. 3 is designated STA- TION N.

In order to insert a new code in any one particular electronic lock associated with a particular room, an address system is provided so that only the particular lock selected can receive the new code information. This address system is provided for each of the electronic locks.

Thus, for STATION N the address system includes an address multiple shift register 28 for effecting a serial to parallel conversion of successive bits of address information. This address information-may be provided from the same data line 26 as by branch lead 29 in conjunction with a series of clock pulses provided on a line 30, certain of these pulses being gated by the address wave form on the data line 29 to provide the successive bits of address information into the register 28. The parallel conversion outputs of the register 28 connect to a first input of an address comparator 31. The second input to the address comparator 31 is provided with an address code identifying the particular electronic lock. This address code is indicated at 32 and in the example chosen includes a four digitnumber l 100. Thjis number is provided in binary form to the second input of the comparator for comparision with the parallel conversion output of the register 28 passing to the first input of the comparator. If the input address code to the comparator corresponds with the particular address code identifying the electronic lock from the second inputs 32, an open gate signal will be provided by the comparator on line 33 to a gate 34 positioned between the clockproviding the series of code pulses in the serial input to the code insert multiple bit shift register 25 thus opening this register to receive the data designating the new codes.

Referring to the bottom portion of FIG. 3, there 'is shown a clock 1 designated 35 for providing the series of pulses utilized in the address code, a clock 36 providing the seriesof code pulses for inserting the new codes and a data generating circuit-37 for providing an address wave form cooperating with the clock 1 and a combination code data wave form for cooperation with the clock 2.

The

components

35, 36 and 37 may be at a remote location such as the clerks office in a hotel or motel. All of the information necessary to address a particular electronic lock associated with a particular room and to insert a desired code can be carried on the three leads designated 38, 39, and 40. (There would, of course, be provided a common return or ground lead not shown).

OPERATION OF FIG. 3

As stated heretofore, it will be understood that there is provided an electronic lock together with a code insert and address system for each particular stationor room in the hotel or motel. Input leads such as 30, 27, and 26 passing to the electronic lock and associated insert and address equipment in STATION N are tapped off from the

main lines

38, 39, and 40 from the remote station. The designated

stations

1 and 2 shown in the blocks of FIG. 3 constitute other electronic locks with the necessary input leads being taken in parallel from the

leads

38, 39, and 40.

With respect to the lock at STATION N as shown in FIG. 3, assume that it is desired to insert a new code to provide new binary code signals from the

circuits

1, 2, 3, and 4 in the selector 24 to thus provide a new combination to the room. This operation can be effected from a remote location by the clerk. Assuming that the room number or identification address for STATION N is '1100 as indicated at 32, the clerk will first address this particular station by sending out clock pulses from clock 1 designated 35 in the lower left portion of FIG. 3. In the example where there are provided four coded circuits in the combination lock, the code insert multiple bit shift register is a 16 bit shift register and the clock 1 will provide 16 uniformly spaced pulses in time. Simultaneously, the data wave from designating the address identification 1100 will be transmitted from the data block 37. This wave form essentially gates certain of the pulses from the clock 1 to provide a series of address pulses designating the binary address code in a serial form. The two pieces of information will pass on the

leads

38 and 40 to the

leads

30 and 26 respectively.

The data on the lead 26 will be passed by the branch lead 29 into the serial input of the address multiple bit shift register along with the clock pulses on line 30.

The bits of address information corresponding to the identifying address number 1 are shown above the address data wave form from the data block 37 in the lower portion of FIG. 3. Thus, the identifying numbers 1 100 at 32 fed into the second input of the comparator 31 in binary form will be: 0001 0001 0000 0000. Since this same binary code is received in the first input of the comparator from the address multiple bit shift register 28 as a consequence of the data fed into the serial input thereof, the address comparator 31 will generate an open gate signal to the gate 34 and thus ready the code insert multiple bit shift register 25 to receive a new code for changing the combination of the electronic lock.

The various other stations would have different identifiying addressed numbers and thus could be individually address by sending a-proper address code in the manner described for the STATION N to ready their corresponding code insert multiple bit shift registers to receive a new combination.

Assume now that the combination of the electronic lock is to be changed to the number 7452 which is the number of the combination utilized in the description of the electronic lock of FIG. 1. The clerk at the remote station will now generate pulses from clock 2 designated 36 at the lower portion of FIG. 3 to provide sixteen uniforrnly spaced time pulses. At the same time, the data block 37 will be caused to generate a data wave form which gates the pulses from clock 2 in such a manner as to serially define in binary form the combination 7452. This combination in binary form is shown as:0l1l 01000101 0010. I

When this data is received on the line 26 to the serial input of the shift register 25 along with the pulses from clock 2 on line 27 which can now pass through the open gate 34, the shift register will insert the four binary numbers into the circuits designated 1, 2, 3, and 4 in the selector 24.

After the code has been inserted, the clerk may send a second series of 16 clock pulses from clock 1 to the address multiple bit shift register 28 which will provide a binary number to the comparator 31 which does not correspond to any particular station identifying address number. As a consequence, the address comparator will cease providing the open gate signal to close the gate 34 and thus prevent additional data sent along the line 40 from thereafter affecting the electronic lock in STATION N.

With the combination 7452 now set into the

circuits

1, 2, 3, and 4, the electronic lock may be operated precisely as described for the electronic lock of FIG. 1.

It will be understood, of course, that while only a five stage counter has been shown cooperating with four coded circuits, a ring counter with many more stages could be provided with additional coded circuits. Similarly, the multiple bit registers 25 and 28 of FIG. 3 could be made to handle more than sixteen bits of information so that a great many different combinations can be coded into the lock and a great many more rooms can be individually addressed.

It should also be understood that while the invention has been described with respect to an electronic lock for room doors, the circuit set forth can be used to operate any particular mechanism since the solenoid coil 19 can effect other functions when energized aside from specifically throwing a bolt or striker. The term lock" should therefore be considered in its figurative sense rather than as confined to a specific functioning of locking or unlocking a door.

The invention, accordingly, is not to be thought of as limited to the particular examples set forth merely for illustrative purposes.

What is claimed:

1. An electronic push button combination lock comprising, in combination:

a. a plurality of push buttons corresponding to a plurality of different numbers respectively;

b. a decimal to binary converter circuit connected to said push buttons for providing an output binary signal in response to depression of a selected button equivalent to the number associated with the selected button;

c. a comparator circuit having a first input receiving said output binary signal and a second input receiving a coded binary signal;

d. a given number of coded circuits each providing a coded binary signal upon actuation for reception in the second input of said comparator circuit;

e. a ring counter having a number of stages one greater than said given number of coded circuits, the first and subsequent stages being connected to said coded circuits respectively to successively actuate the same when the connected stage is energized and the last stage providing an unlocking signal; and, a lock responsive to said unlocking signal to open, said comparator generating an advance signal to the counter to advance it to the next successive stage whenever said output binary signal corresponds to said coded binary signal, and generating a reset signal to reset said ring counter whenever said output binary signal does not correspond to said coded binary signal whereby said lock will not open unless a sequence of push buttons are depressed to provide successive output binary signals which equal the successive coded binary signals supplied by the coded circuits in response to the successive actuation of the stages of the ring counter.

2. An electronic lock according to claim 1, including a timer connected to said ring counter and responsive to generation of said unlocking signal to reset said counter to its first stage after a given period of time.

3. An electronic lock according to claim 1, including a code insert system to enable the successive code binary signals in said given number of coded circuits to be changed to new codes from a remote location, said code insert system including: a code insert multiple bit shift register for effecting a serial to parallel conversion of successive bits of code information, said given number of coded circuits being connected to the parallel conversion output of said code insert multiple bit shift register; and a data signal line means providing a series of code pulses representing the new codes connected to the serial input of said code insert multiple bit shift register to provide said successive bits of code information from a remote location.

4. An electronic lock according to claim 3, in which said data signal line means providing said series of code pulses includes a first line carrying a series of uniformly spaced clock pulses and a second line carrying a control data wave from defining the new codes for gating certain ones of the clock pulses to form said successive bits of code information in accordance with the new codes.

5. The subject matter of claim 3, wherein there are provided a plurality of additional electronic locks at different stations all identical to said electronic lock, said electronic lock and additional electronic locks each including individual address responsive means comprising: an address multiple bit shift register for effecting a serial to parallel conversion of successive bits of address information; an address comparator having a first input connected to the parallel conversion output of said address multiple bit shift register; an address code identifying the particular electronic lock connected to a second input of said address comparator; an address signal line means providing a series of address pulses representing the address code connected to the serial input of said address multiple bit shift register to provide said successive bits of address information; and a gate connected between said data signal line means and said code insert multiple bit shift register and controlled by an open gate signal from said address comparator to open and permit a new code to be inserted, said open gate signal from said address comparator only being generated when the address code supplied by said address signal line means corresponds with the address code identifying the particular electronic lock.

6. The subject matter of claim 5, in which said address signal line means providing said series of address pulses includes a first line carrying a series of uniformly spaced clock pulses and a second line carrying a control data wave form defining the address for gating certain ones of the clock pulses to form said successive bits of address information in accordance with the address code identifying-the particular electronic lock.

7. An electronic push button combination lock including in combination:

a. a plurality of push buttons corresponding to a plurality of different numbers respectively; 7

d. a given number of coded circuits each providing a coded binary signal upon actuation for reception in the second input of said comparator circuit; and

e. a ring counter having a number of stages one greater than said given number of coded circuits, the first and subsequent stages being connected to said coded circuits respectively to successively actuate the same when the connected stage is energized and the last stage providing an unlocking signal, said comparator generating an advance signal to the counter to advance it to a next successive stage whenever said output binary signal corre- 10 vide successive output binary signals which equal the successive coded binary signals supplied by the coded circuits in response to the successive actuation of the stages of the ring counter.

Claims

1. An electronic push button combination lock comprising, in combination: a. a plurality of push buttons corresponding to a plurality of different numbers respectively; b. a decimal to binary converter circuit connected to said push buttons for providing an output binary signal in response to depression of a selected button equivalent to the number associated with the selected button; c. a comparator circuit having a first input receiving said output binary signal and a second input receiving a coded binary signal; d. a given number of coded circuits each providing a coded binary signal upon actuation for reception in the second input of said comparator circuit; e. a ring counter having a number of stages one greater than said given number of coded circuits, the first and subsequent stages being connected to said coded circuits respectively to successively actuate the same when the connected stage is energized and the last stage providing an unlocking signal; and, f. a lock responsive to said unlocking signal to open, said comparator generating an advance signal to the counter to advance it to the next successive stage whenever said output binary signal corresponds to said coded binary signal, and generating a reset signal to reset said ring counter whenever said output binary signal does not correspond to said coded binary signal whereby said lock will not open unless a sequence of push buttons are depressed to provide successive output binary signals which equal the successive coded binary signals supplied by the coded circuits in response to the successive actuation of the stages of the ring counter.

6. The subject matter of claim 5, in which said address signal line means providing said series of address pulses includes a first line carrying a series of uniformly spaced clock pulses and a second line carrying a control data wave form defining the address for gating certain ones of the clock pulses to form said successive bits of address information in accordance with the address code identifying the particular electronic lock.

7. An electronic push button combination lock including in combination: a. a plurality of push buttons corresponding to a plurality of different numbers respectively; b. a decimal to binary converter circuit connected to said push buttons for providing an output binary signal in response to depression of a selected button equivalent to the number associated with the selected button; c. a comparator circuit having a first input receiving said output binary signal and a second input receiving a coded binary signal; d. a given number of coded circuits each providing a coded binary signal upon actuation for reception in the second input of said comparator circuit; and e. a ring counter having a number of stages one greater than said given number of coded circuits, the first and subsequent stages being connected to said coded circuits respectively to successively actuate the same when the connected stage is energized and the last stage providing an unlocking signal, said comparator generating an advance signal to the counter to advance it to a next successive stage whenever said output binary signal corresponds to said coded biNary signal, and generating a reset signal to reset said reing counter whenever said output binary signal does not correspond to said coded binary signal whereby said unlocking signal will only be provided if push buttons are depressed in a sequence to provide successive output binary signals which equal the successive coded binary signals supplied by the coded circuits in response to the successive actuation of the stages of the ring counter.