Munition control system

Abstract

1. A munition safing, arming and firing system comprising means for transting a code consisting of pulses of differing frequencies, means for receiving said code, means for segregating the received pulses according to frequency, a stepping relay having a plurality of contact decks each of which corresponds to one of the frequencies contained in said code and one additional contact deck, each of said contact decks having an associated wiper arm, said wiper arms being ganged for conjoint movement, means connected to said segregating means for applying ground potential to only one of the wiper arms of said plurality of contact decks, in response to a pulse of the corresponding frequency means interconnecting the contacts of said plurality of contact decks in correspondence with a preselected code, switch means for advancing said wiper arms one contact position in response to each pulse received by said receiving means, code check means connected to said interconnecting means for free-running said stepping switch back to its initial position after a predetermined time delay in response to a pulse which deviates from said preselected code, time delay means connected to said code check means for free-running said stepping relay back to its initial position after a predetermined time independently of the receipt of an incorrect pulse, arming circuit means connected to a predetermined contact of said additional contact deck, firing circuit means connected to a contact succeeding said predetermined contact and to said code check means so that upon the receipt of a correct series of pulses followed by an incorrect pulse the firing circuit will be activated through said code check means.

Government Interests

The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to me of any royalty thereon.

Description

This invention relates to improved means for arming, firing and safing (that is, disabling) a munition by means of a radiofrequency signal radiated to a munition control circuit from a master station located at a distance from the munition.

In the arming, firing and safing of a munition at a distance by means of a transmitted radiofrequency signal, it is necessary to code the transmitted radiofrequency signal in such a way that the enemy cannot duplicate the signal and thereby cause premature operation of the munition. Also, it is necessary to prevent the enemy from sending a few simple codes to cause reject (error code) relays to operate continuously. Continuous operation of these relays would reduce the life of the battery power supply and make the unit inoperative. Accordingly it is the broad object of this invention to provide improved means for arming, firing and safing a munition at a distance so that the munition can neither be prematurely detonated, nor can its power supply be run down by an enemy.

Another object of this invention is to provide an improved munition control circuit operated at a distance by a transmitted radiofrequency signal which, in addition to having the features of the above object, also incorporates fail-safe features to prevent accidental firing or arming of the munition in the event of failure of some component in the electrical control circuit of the munition.

A further object of this invention is to provide improved means for safing the munition at a distance by means of a properly chosen radiofrequency safing signal which disconnects anti-disturbance devices and other active components in order to make the munition safe to handle.

A still further object of this invention is to provide a munition control circuit which incorporates means for radiating a signal to the master station after the munition has been made safe, thereby making it possible to determine whether the transmitted safing signal has achieved its purpose.

Yet another object of this invention is to provide a munition control circuit having all of the above features which in addition is relatively simple, compact and inexpensive.

In a typical embodiment of the invention, the above objects are accomplished by means of a specially designed electrical control circuit for the munition which permits arming, firing and safing only in response to specially chosen coded signals which amplitude modulate a radiofrequency carrier of predetermined frequency, transmitted from a master station. A coded signal consists of a plurality of pulses, each of which comprises high or low frequency energy, and separated by a time interval of no signal. These pulses of high and low frequency energy modulate the carrier in a predetermined sequence, such as low, high, low, low, high, low, high, etc., for example. The munition control circuit is designed to arm, fire or safe the munition only in response to the receipt of the proper predetermined sequences of pulses. It is to be noted that by requiring a proper sequence of say 25 pulses, over 30 million combinations are possible, only a few of these being used to operate the munition. By employing a stepping relay with a plurality of decks in combination with associated relays and other circuit components, the desired code sequences are recognized from these 30 million combinations and caused to arm, fire or detonate the munition in an amazingly simple manner.

The specific nature of the invention, as well as other objects, uses, and advantages thereof, will clearly appear from the following description and from the accompanying drawing, in which:

The drawing is a schematic and block diagram of a munition control system in accordance with the invention. In describing and explaining the operation of this invention in the clearest manner so that its inventive features may be clearly revealed, no attempt will be made to describe in detail the structural features of the circuit components of the interconnections therebetween. The circuit components employed are of well known types and readily providable by those skilled in the art, and the interconnections therebetween are clearly shown in the drawing, the invention residing chiefly in the combination of circuit components and the cooperation therebetween. Instead, the circuit components and their functions in the circuit will be summarized briefly, and this will be followed by a detailed description of the various operations and functions provided by the invention which is believed to be entirely sufficient to enable one skilled in the art to make and use the invention.

In the drawing, a master station 10 has a radiofrequency oscillator 11 of predetermined frequency to which is connected an A-M modulator 13 adapted to modulate the oscillator 11 with a coded signal fed to the input of the modulator 13. The oscillator 11 feeds an antenna 8 which transmits the coded radiofrequency energy to an antenna 14 connected to an A-M receiver 12. The master station 10 is located at a first location while the remaining circuitry shown in the drawing makes up the munition control circuit which is placed with the munition at a second location at a distance from the first location. The master station 10 also includes a receiver 17 connected to the antenna 8 and an indicator 24 connected to the output of the receiver 17 to indicate the receipt of a safing signal transmitted from the munition control circuit.

The coded radiofrequency signal from the master station transmitter 10 consists of a radiofrequency signal amplitude modulated by a sequence of pulses, the pulses comprising energy of at least two different frequencies. In the preferred form of the invention now to be described, the coded pulses from the master station consist of a sequence of pulses of high and low frequency energy as illustrated, feeding the A-M modulator 13 of the master station 10 in the drawing. The terms "high" and "low" are intended to designate pulses of energy having widely different frequencies so as to be easily distinguishable.

The A-M receiver 12 is tuned to the predetermined carrier frequency transmitted from the master station 10, and is adapted to detect the coded signal modulating the carrier and consisting of high and low frequency pulses of energy. These detected pulses of energy are fed to the low pass filter 16 and the high pass filter 18. The low pass filter 16 passes only the low frequency pulses of energy, while the high pass filter 18 passes only the high frequency pulses of energy. A low tone relay 25 has two normally open contacts 23 and 27 and a coil 21 connected to the output of the low pass filter 16. Similarly, a high tone relay 35 has two normally open contacts 33 and 37 and a coil 31 connected to the output of the high pass filter 18. In response to each pulse of low frequency energy, therefore, the low tone relay 25 is energized closing its contacts 23 and 27 for the duration of the pulse, while in response to each pulse of high frequency energy the high tone relay 35 is energized closing its contacts 33 and 37 for the duration of the pulse.

The heart of the munition control circuit consists of a stepping relay 55 having a coil 51, a normally closed interruption contact 52, a "home" contact 53 which is closed except when the relay 55 is in its home position, and three independent decks 56, 57 and 58, each having 25 contacts and a movable arm 66, 67 and 68, respectively. The movable arms 66, 67 and 68 of decks 56, 57 and 58, respectively, step synchronously from contact to contact in response to the application of pulses to the coil 51, the stepping relay advancing one step after each pulse of energy is received. The interconnections between the contacts of the decks 56 and 57 determine the coded sequence of high and low pulses of energy which will arm, fire or safe the munition, whatever the case may be, while the contacts of the deck 58 interconnect various other components.

Also provided in the embodiment of the drawing are a code reversing relay 45, a code check relay 85, three time delay relays 75, 105 and 135, a firing relay 115, a safing relay 125, and a relay 95. The code reversing relay 45 has a coil 41, two single-pole double-throw contacts 46 and 47, a normally closed contact 48 and a normally open contact 49. The code reversing relay 45 is used to reverse the code provided by the contacts of decks 56 and 57 of the stepping relay 55 in connection with the safing operation of the circuit. The code check relay 85 has a coil 81, three single-pole double-throw contacts 86, 87 and 88, and a normally open contact 89. The code check relay comes into operation when a wrong sequence of pulses is applied to the stepping relay 55. The time delay relay 75 provides a delay of 3 minutes, has a coil 71 and a normally closed contact 72, and is utilized in connection with the operation of the code check relay 85. The time delay relay 105 provides a delay of 1 minute, has a coil 101 and a contact 106, and operates in cooperation with the code check relay 85 and the relay 95 which has a coil 91 and a normally closed contact 96. The time delay relay 135 provides a delay of 3 seconds, has a coil 131 and a normally open contact 136, and operates with the code reversing relay 45 in connection with the safing operation provided by the munition control circuit. The firing relay 115 has a coil 111 and two normally open contacts 116 and 117, and operates with the code reversing relay 45 in connection with the safing operation provided by the munition control circuit. The safing relay 125 has a coil 121, a normally open contact 126 and a single-pole double-throw contact 127, and operates in connection with the safing operation of the munition.

Further shown in the drawing is a firing circuit 150 adapted to fire a detonator 170 when a suitable pulse is applied to the firing circuit 150. In addition, an anti-disturbance device 160 is provided to fire the detonator 170 in the event that the munition is disturbed, such as by handling in an attempt to disarm the munition by the enemy. Anti-disturbance devices are usually in the form of a switch which closes momentarily to fire the detonator 170 if the munition is subjected to a disturbance. A transmitter 180 is connected to the antenna 14 and is adapted to radiate a predetermined signal to the master station 10 when the munition has been made safe.

The operation of the munition control system shown in the drawing will now be described in detail. The master station 10 transmits a radiofrequency signal of predetermined frequency, amplitude modulated by a coded signal preferably consisting of high and low frequency pulses of energy, as described previously. These pulses are detected by the A-M receiver 12 tuned to the predetermined carrier frequency and then fed to the low pass and high pass filters 16 and 18. A pulse of low frequency energy closes the contacts 23 and 27 of the low tone relay 25 for the duration of the pulse, while a pulse of high frequency energy closes the contacts 33 and 37 of the high tone relay 35 for the duration of the pulse. The coil 51 of the stepping relay 55 has a first end connected to a grounded power source represented by the battery 240, and a second end connected to the commonly connected moving arm elements 23a and 33a of the contacts 23 and 33 of the relays 25 and 35, respectively. The commonly connected fixed elements 23b and 33b of the contacts 23 and 33, respectively, are connected to circuit ground through the unenergized contact 86 of the code check relay 85. Thus, when either of the relays 25 and 35 are energized by its respective low or high frequency pulse of energy, the coil 51 is energized for the duration of the pulse, thereby causing the stepping relay 55 to advance one step when the coil 51 is no longer energized.

The code reversing relay 45 is initially unenergized so that the movable arms 66 and 67 of the stepping relay decks 56 and 57, respectively, are effectively connected to the movable arm elements 27a and 37a of the contacts 27 and 37 of the relays 25 and 35, respectively, the fixed elements 27b and 37b being connected to circuit ground. It can thus be seen, therefore, that with the code reversing relay 45 unenergized, a pulse of low frequency energy which energizes the relay 25 places a ground on the movable arm 66 of the deck 56, while a pulse of high frequency energy which energizes the relay 35 places a ground on the movable arm 67 of the deck 57.

The contacts 1-25 of each of the decks 56 and 57 of the stepping relay 55 shown in the drawing are set up, for illustrative purposes, for the following coded sequence of pulses:

(1) low, (2) low, (3) low, (4) low, (5) high, (6) high, (7)-(11) low, (12)-(16) high, (17) low, (18) high, (19) low, (20) high, (21)-(25) low. The terms low and high refer to pulses of low frequency energy and pulses of high frequency energy, respectively.

To now illustrate how the stepping relay 55 acts to provide operation only in response to the receipt of the proper sequence of high and low pulses of energy as given above, it will first be assumed that with the stepping switch in its home position as shown in the drawing, the following wrong code sequence is received: (1) low (correct), (2) low (correct), (3) high (wrong), (4) high (wrong), and (5) to (25) (correct). The receipt of the first pulse (1), which is low, thus places a ground on the movable arm 66 and the contact 1 of the deck 56. After the pulse is completed, the de-energization of the coil 51, which became energized when the contact 23 closed along with contact 27, causes each of the arms 66, 67 and 68 of the stepping relay 55 to advance one step to contact 2 of their respective decks 56, 57 and 58. The receipt of the second pulse (2), which is also low, places a ground on the movable arm 66 and the contact 2 of the deck 56, and after the pulse is completed, causes the arms 66, 67 and 68 to advance to contact 3 of their respective decks 56, 57 and 58.

The third pulse (3), which is high and wrong, places a ground on the movable arm 67 and the contact 3 of the deck 57. The pulse is wrong because it causes a ground to be placed on the lead 59. The grounding of the lead 59 is how the stepping relay 55 recognizes that the code sequence is wrong, as will now be explained. It can be seen that for each step of the stepping relay 55, either the contact of the deck 56 is connected to the lead 59 and the other one is left unconnected, or vice versa. For the proper code sequence the pulses will so energize the relays 25 or 35 that as the stepping relay 55 advances, the lead 59 is never grounded by the movable arm 66 or 67; or stated another way, for the proper code sequence, the arm 66 or 67 which is grounded at each step is the one whose respective contact is left unconnected. This proper code sequence permits the stepping relay 55 to continue to advance in order to provide the desired operation. But when the lead 59 is grounded because of a wrong pulse, as is the case for the third pulse (3) of the assumed wrong code as described above, the coil 81 of the code check relay 85 becomes energized, since one of its ends is connected to a grounded power source represented by the battery 210 and its other end is connected to the lead 59.

When the code check relay 85 is energized by the placing of a ground on lead 59 as the result of a wrong pulse, the following functions are performed: (a) the moving arm element 86a of contact 86 moves to the fixed element 86b to keep the lead 59 grounded through the normally closed contact 72 of the time relay 75, thereby keeping the relay 85 closed regardless of whether or not the lead 59 is grounded by a movable arm 66 or 67; because the stepping relay is of the type which advances only after the energization pulse to coil 51 is removed, ample time is available to permit the contact 86 of the relay 85 to operate; (b) the movement of the moving arm element 86a from the fixed element 86c prevents further incoming pulses of energy from energizing the coil 51 of the stepping relay by removing the ground from the common fixed elements 23b and 33b of relays 25 and 35, respectively; (c) the moving arm element 87a of the contact 87 disconnects from the fixed element 87c, which is connected to the movable arm 68 of the deck 58 and which performs the arming, firing and safing functions, as will hereinafter be described; (d) the movement of the grounded moving arm element 87a to the fixed element 87b places a ground on the normally closed interrupter contact 52 of the stepping relay 55, causing the stepping relay to "free-run" itself to its home position (no. 1), where the home contact 53 (which is in series with the interrupter contact 52 and remains closed except in the home position) opens to terminate this free-running action; (e) the movement of the grounded moving arm element 88a of the contact 88 to the fixed element 88b energizes the coil 71 of the 3-minute time delay relay 75, since one end of the coil 71 is connected to the grounded battery 210 and the other end is connected to the fixed element 88b; (f) the movement of the grounded moving contact element 88a away from the fixed element 88c breaks the ground circuit to the coil 41 code reversing relay 45 so that if the relay 45 had been energized, it would be unlatched by the operation of the contact 88; the operation of the code reversing relay 45 is fully described in connection with the description of the safing function of the circuit; (g) the closing of the normally open contact 89 closes the input lead 153 to the firing circuit 150 with which it is in series.

From the above description, it can be seen that as soon as a wrong pulse is received, the code check relay 85 energizes, disabling the arming, firing and safing functions and causing the stepping relay 55 to free-run to its home (no. 1) position. The continued energization of the code check relay 85 maintains the circuit inoperative until the three-minute time delay relay 75 operates, opening the normally closed contact 72, thereby deenergizing the coil 81. The provision of the time delay relay 75 is useful to prevent the enemy from continuously operating the above-described error-check system in the event they knew the predetermined carrier frequency of the master station 10 so as to reduce the life of the batteries or damage the mechanism by constant starting and resetting operations. A 3-minute time delay, such as provided by the relay 75, permits the enemy to cycle the equipment only 20 times an hour.

If a noise burst were to pass through the receiver 12 with the relay 55 in its home (no. 1) position, as might happen during an electrical storm, and were to operate the correct relay 25 or 35, the stepping relay 55 would advance to contact 2 and might remain there indefinitely without the sender at the master station 10 being aware of it. Or, the master station 10 might start to transmit a code signal and fail to send the remainder of the sequence due to some difficulty, causing the stepping switch to remain at some intermediate contact. To prevent such possibilities, the one-minute time delay relay 105 and the relay 95 are provided with their coils 101 and 91 connected in parallel through the normally closed contact 106 of the time delay relay 105, each coil thereby having one end connected to the grounded battery 220 and the other end connected to the contact 2 of the deck 58 of the stepping relay 55. Thus, if the stepping relay 55 remains at or passes the contact 2, the relays 105 and 95 will both be energized, the normally closed contact 96 of the relay 95 keeping the coils 101 and 91 energized, even when the stepping relay 55 advances past the contact 2. After one minute, the time delay relay 105 operates, causing the normally open contact 107 to apply a ground to the lead 59, thereby energizing the code check relay 85 and causing the stepping switch to free-run to the home position (no. 1), as previously described. Also, when the time delay relay 105 operates, the normally closed contact 106 opens, de-energizing the coil 91 of relay 95, which opens the contact 96, thereby also de-energizing the time delay coil 101. The time that the contact 107 is closed so as to ground the lead 59 should obviously be made long enough to permit the code check relay 85 to become energized. It is evident, therefore, that if the stepping relay 55 unintentionally leaves the home position, the action of the relays 95, 105 and 85 will again return the stepping relay to the home position after 1 minute, and the circuit will be ready to receive a code sequence 3 minutes later when the 3-minute time delay relay 75 operates to open the relay 85. This action is also used to prepare the circuit to receive the firing code sequence after it has been armed, as will hereinafter be described.

If it is now desired to arm the munition, 19 pulses of high and low frequency energy having the correct code sequence, as given previously, are transmitted to the munition control circuit from the master station 10, advancing the stepping relay 55 to contact 20, thereby causing the movable arm 68 of deck 58, which is connected to ground through the contact 87 of relay 85, to apply a ground to the munition arming circuit 190. The munition arming circuit 190 is adapted so that the application of this ground thereto activates the circuit 190, causing it to arm the munition, in accordance with well known arming techniques. The 19 correct pulses are sent in about 40 seconds so that about 20 seconds later, the action of the relays 95, 105 and 85 causes the stepping relay 55 to free-run back to its home (no. 1) position.

When the time delay relay 75 opens about three minutes later, the circuit is ready to receive the coded sequence which will fire the munition. The sequence, which the master station 10 now transmits in order to fire the munition, consists of a sequence of 21 correct pulses of high and low frequency energy as given above, and then one deliberate wrong pulse of high frequency energy as the twenty-second pulse of the sequence. The 21 correct pulses advance the stepping relay 55 to contact 22, causing the movable arm 68 of the deck 58 to ground one end of the coil 111 of the firing relay 115 whose other end is connected to the grounded battery 250 through the normally closed contact 48 of the relay 45. The firing relay 115 thus becomes energized and the contact 116 thereof latches the relay 115 in the energized position, while the contact 117 thereof applies the voltage of the grounded battery 200 to one of the elements of the contact 89 of the code check relay 85. When the code check relay 85 is energized by the incorrect twenty-second pulse, therefore, the closing of contact 89 applies the voltage of the battery 200 to the firing circuit 150, thereupon causing the firing circuit 150 to apply a pulse to activate the detonator 170 and detonate the munition. The total time taken for the stepping relay 55 to advance to the contact 23 is obviously made less than 1 minute, otherwise the action of the 1-minute time delay relay 105 and the relays 95 and 85 would return the stepping relay 55 to the home position before firing occurred. From the description of the firing operation, it will be understood that a fail-safe situation is achieved because both a correct code and at least one error code is necessary to activate the firing circuit 150.

In the event that it is desired to place the munition in a safe condition so that it can be handled without the anti-disturbance device 160 activating the detonator 170, the master station 10 transmits a sequence of 14 correct pulses of high and low frequency energy, thereby advancing the stepping relay 55 from its home (no. 1) position to the contact 15, causing the movable arm 68 of deck 58 to ground one end of the coil 131 of the 3-second time delay relay 135 whose other end is connected to the grounded battery 200. Three seconds after the stepping relay 55 advances to contact 15, therefore, the normally open contact 136 of the time delay relay 135 closes, grounding one end of the coil 41 of the code reversing relay 45 whose other end is connected to the grounded battery 220. The code reversing relay 45 thus becomes energized and latches itself in the energized position by means of the contact 49 which provides a ground through the contact 88 of the code check relay 85. Thus, if an error in transmission should occur, or an error be introduced by a noise burst, the energization of the code check relay 85 would unlatch the code reversing relay 45 so as to permit the circuit to return to its initial conditions. The 3-second time delay relay 135 is necessary, because in the arming and firing sequences described the stepping relay 55 must pass the contact 15 without operating the relay 135 or the code reversing relay 45. Since in normal operation the arming and firing sequences are transmitted at a rate of about one pulse every 0.2 second, the 3-second time delay relay 135 will not operate to energize the code reversing relay 45 unless the stepping relay 55 is deliberately stopped for at least 3 seconds at the contact 15.

When the 3-second time delay relay 135 operates, causing the code reversing relay 45 to also become energized, contacts 46 and 47 operate to reverse the code provided by the stepping relay 55; it can be seen that this is accomplished by the contact 46 switching the movable arm 66 of the deck 56 from the output of the low tone relay 25 to the output of the high tone relay 35, and the contact 47 switching the movable arm of the deck 57 from the output of the high tone relay 35 to the output of the low tone relay 25. Also, the opening of the normally closed contact 48 by energization of the code reversing relay 45 causes the end 111a of coil 111 of the relay 115 to be disconnected from the grounded battery 250, thereby disabling the firing relay 115.

In transmitting the safing code sequence, the stepping relay is held at the contact 15 for about 5 seconds to permit the 3-second time delay relay to operate, the remainder of the safing code is now transmitted consisting of 10 pulses of high and low frequency energy to advance the stepping relay 55 from the contact 15 to the last contact 25. Because the code provided by the stepping relay 55 has been reversed by the code reversing relay 45, these 10 pulses, corresponding to pulses (15) to (24) of the correct sequence of pulses previously given, must be wrong in order to advance the stepping relay to the last contact 25. At contact 25, the grounded movable arm 68 of deck 58 grounds one end of the coil 121 of the safing relay 125 whose other end is connected to the grounded battery 250, thereby energizing the safing relay 125, the contact 126 permanently latching the safing relay 125 in an energized condition. The contact 127 of the safing relay 125 disables the anti-disturbance device by disconnecting the power source represented by the grounded battery 200 from the input of the device 160. Also, the contact 127 connects the battery 200 to activate the transmitter 180, causing it to radiate a predetermined signal to the master station 10, where it is received by the receiver 17 and appears on the indicator 24 to inform the sender that the munition has been made safe. If this predetermined signal is not received, the sender must try a second time, or more, to safe the device.

It can be seen that the safing operation of the circuit is such that because the stepping relay 55 is required to stop at the contact 15 for a predetermined time (3 seconds) to disable the firing circuit and reverse the code provided by the relay 55, any chance of accidentally firing the munition during the safing operation is eliminated. For example, if the stepping relay 55 had stopped at the contact 15 and then the master station 10 continued to send a proper firing code sequence by error, or because of some difficulty every pulse after the contact 15 is reached is in error, the stepping relay would be returned to its "home" position by action of the code check relay 85.

From the above description of a munition control circuit in accordance with the invention, it will be evident to those skilled in the art that the coding principle of this invention provided by the stepping relay 55 and the associated components is very flexible and easily permits changing or adding more codes. With the addition of a third tone relay to the relays 25 and 35 and another deck provided on the relay 55, for example, the number of possible combinations can be greatly increased. In the present stepping relay using two decks 56 and 57 each having 25 contacts to supply the code over thirty-three million combinations are available, while by using another tone relay and another deck, over eight hundred billion combinations would be available.

It should be noted that the above-described munition control circuit can be made amazingly simple and compact in view of the many, many possible combinations and variations of codes that are made available. It should also be noted that, in the standby condition, the circuit consumes no power from the batteries, and if so desired the batteries could be combined into a single power supply by suitable choice of operating voltages for the various components.

It will be understood, therefore, that the illustrative embodiment described is only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims.

Claims

I claim as my invention:

1. A munition safing, arming and firing system comprising means for transmitting a code consisting of pulses of differing frequencies, means for receiving said code, means for segregating the received pulses according to frequency, a stepping relay having a plurality of contact decks each of which corresponds to one of the frequencies contained in said code and one additional contact deck, each of said contact decks having an associated wiper arm, said wiper arms being ganged for conjoint movement, means connected to said segregating means for applying ground potential to only one of the wiper arms of said plurality of contact decks in response to a pulse of the corresponding frequency, means interconnecting the contacts of said plurality of contact decks in correspondence with a preselected code, switch means for advancing said wiper arms one contact position in response to each pulse received by said receiving means, code check means connected to said interconnecting means for free-running said stepping switch back to its initial position after a predetermined time delay in response to a pulse which deviates from said preselected code, time delay means connected to said code check means for free-running said stepping relay back to its initial position after a predetermined time independently of the receipt of an incorrect pulse, arming circuit means connected to a predetermined contact of said additional contact deck, firing circuit means connected to a contact succeeding said predetermined contact and to said code check means so that upon the receipt of a correct series of pulses followed by an incorrect pulse the firing circuit will be activated through said code check means.

2. The device of claim 1 in which there is additionally provided code reversing means for causing said code check means to respond to a correct rather than an incorrect pulse and switch means responsive to the deliberate stopping of said wiper arms at a predetermined contact for a preselected length of time for activating said code reversing means.