Reverse Operation Warning Signal System

Abstract

A warning signal system effective upon reverse movement of a vehicle. A sensor is mounted for free arcuate movement upon and independently of a shaft which is coupled to the speedometer cable of the vehicle for rotation therewith. Magnetic coupler means associated with the shaft effects arcuate movement of the sensor between two stop positions in accordance with the direction of motion of the vehicle. Movement of the sensor corresponding to reverse movement of the vehicle effects energization of an electrical circuit which operates a warning device. The circuit includes means for varying the nature of the warning as well as means to return the sensor to a circuit-deenergizing position, and from which latter the sensor immediately returns under magnetic coupling if reverse movement of the vehicle continues, thereby reenergizing the warning signal circuit.

Parent Case Text

This patent application is a continuation in-part of Ser. No. 488,447, filed Sept. 20, 1965 entitled Reverse Operation Warning Signal System, now abandoned.

Description

This invention relates to a system for signaling, and more particularly to a warning signal system for indicating reverse movement of an automobile or other vehicle.

It is well known that reversal of the normal forward movement of operation of a vehicle often presents hazards to the operator and to others in the immediate vicinity. This is particularly so when an unexpected reversal or backing up operation of the vehicle occurs. A number of devices have been proposed in an effort to warn inattentive or unaware persons near the vehicle that it is operating in reverse direction. However, these devices have been too slow in operation, too complex in manufacture, or otherwiese inadequate in reliably effecting their warning function.

The present invention provides a system that is extremely sensitive to the reverse movement of a mechanism such as an automobile, to the extent that a signal is given even when such movement may be practically imperceptible to those nearby.

Accordingly, it is a primary object of the present invention to provide a sensor device that triggers the system as applied to a vehicle when the wheels of the vehicle have turned only a short distance in rearward rotation.

Another object of the present invention is to provide novel and effective means for automatically discontinuing the warning signal, such as a horn, after a brief and predetermined time interval following termination of any rearward motion of the vehicle or other reversible direction mechanisms.

Still another object of the present invention is to provide relatively small and lightweight elements in the system so that the moving parts thereof offer a minimum of resistance to operation so that all units of the system may be compact and thus readily added to any vehicle.

Other objects and advantages of the present invention will be apparent upon reference to the following detailed description which when read in connection with the accompanying drawings discloses preferred embodiments of the invention as associated with a motor vehicle.

Referring to the drawings:

FIG. 1 is a schematic diagram showing the several units of the system adapted for use with an automobile;

FIG. 2 is a side view of the clutch and sensor unit with an upper quadrant in longitudinal section;

FIGS. 3 and 4 are transverse sections taken at line 3- 3 and 4--4 respectively in FIG. 2, showing magnetic clutch details;

FIG. 5 is a transverse section taken at line 5- 5 of FIG. 2, showing sensor details;

FIG. 6 is a schematic diagram of the electrical circuit elements in accordance with one embodiment of the invention;

FIG. 7 is a cross-sectional view of a preferred embodiment of the present invention;

FIG. 8 is a cross-sectional view taken along line 8- 8 of FIG. 7;

FIG. 9 is a cross-sectional view taken along line 9- 9 of FIG. 7;

FIG. 10 is a schematic diagram of the electrical circuit elements of another embodiment of the invention;

FIG. 11 is a fragmentary plan view drawn to an enlarged scale illustrating a means for limiting the motion of a sensor unit; and

FIG. 12 is a schematic diagram of the electrical circuit elements of another form of the present invention.

Referring now to FIG. 1 of the drawings, there is illustrated a sensor unit assembly 11 mounted adjacent the vehicle transmission on the vehicle frame 13. The conventional vehicle speedometer 12 is connected to sensor assembly 11 on one end by a length of cable 14 (FIG. 2), the other end of sensor 11 being connected by a length of cable 15 to the transmission power takeoff commonly employed for the conventional speedometer. The distance between the sensor 11 and the transmission connection is preferably relatively short, on the order of 16 inches or less in order to minimize any cable twist effects with respect to sensor 11.

The sensor unit assembly 11 is also connected electrically at 18 as well as through ground 19 to an electronic control unit 20 which, in turn, is connected electrically at 21, 22 respectively to the automobile battery 23 and a warning horn 24, for purposes more fully described hereinafter.

As shown more particularly in FIG. 2, the sensor unit 11 comprises a suitable housing 27 and an end cover 28 therefor which enclose and support the sensor assembly. The ends of short section 14 and long section 15 of speedometer cable are connected and keyed at 29, 30 into opposite ends of a short speedometer connector shaft 31 mounted for rotation in bearings 32, 33 formed in the housing 27 and its cover 28 respectively. The two sections of the cable 14, 15 and the shaft 31 thus revolve as a unit when the vehicle moves in either the forward or reverse direction, and it will be seen that there is no interference with the normal operation of the speedometer 12. The usual flexible housings 36, 37 of the cable sections are secured in conventional manner to the housing 27 and cover 28 as by caps 39, 40. Accordingly, sensor unit 11 may be readily interconnected with the speedometer cable without affecting normal operation thereof.

The sensor incorporates magnetic clutch elements mounted on shaft 31 (FIGS. 2, 3 and 4) between the bearings 32, 33 within the housing, and comprise two iron discs 43, 44 axially spaced by means such as a brass sleeve 45. The hub 46 of disc 43 is pinned at 47 to shaft 31 and accordingly rotates therewith. Hub 48 of disc 44 as well as sleeve 45 are freely mounted on shaft 31 so that there may be independent and relative rotational movement between disc 43 and disc 44 about the axis of shaft 31.

Disc 43 is preferably formed with a number of radial slots 51 (FIG. 3) extending from its hub to and through its periphery. Disc 44 comprises a sensor element and is provided with a plurality of permanent magnets 53, shown as three, secured to its face 54, as best seen in FIG. 4. The discs 43 and 44 are so spaced by the sleeve 45 that the faces 56 of the magnets 53 are maintained at a uniform closely spaced realtion to the confronting face 57 of the slotted iron disc 43, preferably on the order of one-sixteenth inch or less. Thus, the magnetic flux therebetween will provide a discs or clutching effect between the discs normally causing freely-mounted disc 44 to rotate with and in response to rotation of shaft-secured disc 43.

For illustrative purposes and referring to FIG. 5, it will be assumed that forward motion of the automobile imparts a counterclockwise rotation to the speedometer cable and its associated connector shaft 31, and, likewise, backing-up or reverse automobile motion rotates shaft 31 in a clockwise direction, the speedometer 12, of course, as is usual, being unaffected in the reverse direction despite normal rotation of the cable at such time. As seen in FIGS. 2 and 5 radially extending sensor contact arm 61 is secured to hub 48 of sensor element disc 44 and is limited in its counterclockwise rotation as indicated by dashed lines in FIG. 5 by a fixed stop 62 provided on the end of a return electromagnet 64 mounted within housing 27, so that further counterclockwise movement of the arm 61 and therefore disc 44 is prevented as long as the car is moving in a forward direction, the magnetic flux coupling between discs 43 and 44 permitting relative slippage therebetween.

A spring 66 is associated with arm 61 and serves a dual function: it grounds the arm 61 and its contact point 67 to the vehicle body 13, and while the vehicle is idle it retains arm 61 in a neutral position shown in full lines in FIG. 5 substantially midway between stop 62 and a stationary contact point 68. This point 68 is insulated from ground by means of a nonconducting block 69 that is secured to the housing 27, point 68 having the electrical lead 18 extending therefrom to electronic unit 20.

When the vehicle commences movement in a reverse direction, the sensor arm 61 is immediately moved clockwise (FIG. 5) from its above-mentioned neutral position by virtue of the clockwise movement of shaft 31 and disc 43 being imparted thereto through the magnetic coupling therewith of disc 44. Contact points 67 and 68 thus move into abutment after only a few degrees of rotation, and further clockwise movement of the arm 61 is stopped, as before.

The above-described movements of the sensor arm 61 serve important functions in the operation of the system when connected into an electric circuit 20 such as shown in FIG. 6, to which reference is now made in description of the circuit and its relation to the mechanical elements above described.

When the vehicle is moving forward, the fixed contact 68 (FIGS. 5 and 6) is not grounded by arm 61 which is held against stop 63, and a transistor 81 is held in cutoff by a fixed reverse bias provided by a resistor 82 connected to the positive electrode of battery 23. Therefore an associated relay 83 does not close, but remains in the FIG. 6 condition. However, when the vehicle commences rearward movement, the fixed contact 68 is immediately grounded by contact with point 67, providing a triggering pulse to apply a forward bias to the base of transistor 81. Relay 83, which is in the emitter circuit thereof, now closes. A first section 84 of relay 83 now removes from ground a timing capacitor 86, and connects this capacitor to the base of transistor 81, whereby the capacitor now charges to positive potential (or reverse bias) sufficient to cut off transistor 81, after which relay 83 falls out. The time of charging capacitor 86 determines the length of time the signal or alarm is on, assuming reverse motion of the vehicle is terminated to remove contact 67, as set forth hereinafter.

A second section 88 of relay 83 also closes upon the above mentioned triggering pulse provided by the sensor 11. This applies power to transistors 91, 92 through connection 90, comprising a low frequency multivibrator. Simultaneously power is supplied to the return solenoid 64 which pulls the arm 61 to stop 62, thus breaking contact between the points 67 and 68. Solenoid 64 thus comprises a moving means for sensor element 44 and the switch arm 61 thereof. When this contact is thus opened, capacitor 86 begins to charge and the timing cycle starts. Capacitor 86 thus provides a time delay during which horn 24 continues to sound while arm 61 moves toward stop 62, and upon deenergization of the circuit and therefore electromagnet 64, arm 61 immediately rotates through the very short arc to reengage contact 68, providing reverse (clockwise) shaft movemenet continues, whereby there is no discernible interruption of the horn signal.

When power is applied to transistors 91, 92 a further relay 95 successively closes and opens at the cycling rate of the above-mentioned low-frequency vibrator. Therefore the relay contacts 96, 97 open and close in like manner, whereby power is applied to the emitters of transistors 101, 102 of a higher frequency power multivibrator circuit. In the collector circuits of transistors 101, 102 is the usual output transformer 105 which matches the impedance of these transistors to the external load of the warning horn 24.

While the audible signal emitted from the horn is preferably rather loud to insure that the warning will be heard despite noisy surroundings such as a busy street, factory, etc., in certain instances it is desirable or mandatory that the noise level be somewhat lower, as in hospital zones, for example. Means may be provided for accomplishing this, such as a suitable time delay switch arrangement as shown in the speaker line from transistor 101 in FIG. 6. In the exemplary form, switch 108 is under control of the vehicle operator normally connects directly with the horn transformer 105. In a "Quiet" zone, however, the operator actuates switch 108 against the force of dashpot 106 to connect resistance 107 in the speaker line, thereby diminishing the volume of the warning signal. Dashpot 106 will restore switch 108 to its original position after a predetermined time, thereby restoring normal full volume. The provision of dashpot 106 or equivalent means relieves the vehicle operator of any necessity to manually restore switch 108.

It will be understood that the disclosed volume cutdown means is exemplary only and that other and different forms and various circuit locations may be employed if desired.

One difficulty with the device illustrated in FIGS. 1 through 6 is that the iron and steel discs tend to become polarized so that they do not continue to produce the necessary magnetic torque and as a result the device may suffer large errors for a short period. Accordingly, a preferred form of the invention not subject to this difficulty is illustrated in FIGS. 7, 8, 9 and 11.

Referring now to FIG. 7, there is illustrated a sensor unit assembly indicated generally at 110 encased within end bell units 112 and 114. End bell units 112 and 114 may overlap as illustrated in FIG.7 or may be in abutting end relation and define an enclosing housing. A circular magnet 116 is illustrated to be secured to a through shaft 118 by a key 120. Alternatively, entirely satisfactory results have been obtained when the circular magnet is held on to shaft 118 by means of a press fit between the magnet and shaft 118. The latter turns with the speedometer cable as in the previous embodiment. Ball bearing 122 and 123 are illustrated to serve as end supports for shaft 118 by insertion thereof in machined shoulders in the end bells 112 and 114 respectively. Another ball bearing 124 is mounted on shaft 118 with a plastic disc or sensor 126 free to rotate thereon. A series of four snap rings 125 are received in machined grooves in shaft 118 so as to position with extreme accuracy ball bearing 122, circular magnet 116 and plastic disc or sensor 126.

The circular magnet 116 is so magnetized that the outer faces of the magnet are respectively north and south. A typical magnet for use with this invention is one which is seven-eighths of an inch in diameter with center hole three-eighths of an inch in diameter with a height of approximately one-half inch. The circular magnet 116 is of the ceramic type and provides extremely good stability. The snap rings 125 are positioned within grooves (not shown) which may be accurately machined into shaft 118. Thus it is convenient to mount bearing 122 in end cap 112 between a pair of snap rings 125. The circular magnet 116 is then accurately positioned and retained in position with a snap ring or key 120, after which the plastic disc is placed in position and secured by a snap ring 125.

Therefore, the magnetic lines of force from circular magnet 116 pass in FIG. 7 from its left end surface through the fixed and movable parts of ball bearing 122, through shaft 118, through the fixed and movable parts of ball bearing 124 and return to the right end surface of circular magnet 116. Shaft 118 is made from a mild steel so that it is capable of providing the necessary return magnetic path for the magnetic circuit.

Plastic disc 126 constitutes a sensor element and carries a permanent magnet seen best in FIGS. 7 and 8 at 128. The permanent magnet 128 is in close proximity to a glass encapsulated reed switch 130 which is accurately positioned with respect to permanent magnet 128 in an insulation block 132 by means of bolts 136. Electrical leads may be brought to the reed switch 130 such as is illustrated at 134 and taken conventionally through one of the end caps. Since reed switch 130 is encapsulated in glass there is never any danger of contamination of the contact points by dirt or the like during usage of the vehicle.

As in the previous embodiment, the plastic disc sensor 126 is mounted for relatively free arcuate movement on the shaft and is restrained in movement within narrow limits. In this instance, there is shown a stop arm 140 which consists of a bolt extending through the plastic disc 126 and retained thereon by nut 145. The stop arm 140 may move between mechanical stops 142 and 144 of a stop assembly 146.

As in the previous embodiment illustrated in FIGS. 1--6, an electromechanical means such as solenoid 148 is provided to move the sensor element comprising plastic disc 126 and its assembly away from the mechanical stop 144 on stop assembly 146 against which the stop arm 140 moves whenever rearward movement of the vehicle occurs. Solenoid 148 is illustrated to have a projecting actuator arm 150 and a projecting actuator head 152 which is engageable with stop arm 140 on the sensor unit to move it away from mechanical stop 144 shown in FIGS. 9 and 11.

FIG. 7 illustrates that the through shaft 118 has been internally threaded so as to receive internal bushings 154 at each end thereof into which is received an adapter 156 in series with the speedometer cable. These bushings 154 and adapters 156 can, of course, take any suitable form to accommodate the speedometer cables of any desired vehicle.

Referring now to FIG. 10, there is illustrated a preferred control unit indicated generally at 157 which is capable of producing sequential and differing warning signals when energized. In control unit 157, the sensor unit is indicated generally and schematically at 258. A battery 160 supplies B+ voltage to the control circuit. When a glass encased reed switch 130 is used, it has been found that in order to prevent overdriving of the transistor such as transistor 166, a resistor network 162 and 164 is placed in series with a battery 160 and connection is made to the base of transistor 166 intermediate the two resistors so that voltage on the base of transistor 166 does not go all the way to ground potential thereby avoiding overdriving the transistor.

As in the previous embodiment, a relay coil 168 is in the emitter circuit of transistor 166 which serves to actuate contact arms 170 and 171 of the relay. Capacitor 172 functions as the timing capacitor so that once current flows through relay coil 168 so as to actuate contacts 170 and 171, they will remain in their energized position until the capacitor 172 becomes fully charged.

Relay coil 174 and relay switch coil 168 are in series with each other. Potentiometer 180 which is in parallel with relay switch coil 174 thereby controls the length of time relay switch 240 remains closed. Therefore, by adjusting potentiometer 180 it is possible to vary the duration of fast and slow pulses within an overall time determined by timing capcaitor 172.

A multivibrator circuit is indicated generally at 200 having transistors 202 and 204 with capacitors 206, 208 and 210 determining the capacitor feedback in the multivibrator circuit to establish the flipflop time thereof. The relay switch 240 illustrated in deenergized position provides a short across capacitor 206 when relay switch 240, is actuated, thereby operating the signal device or horn in a first manner with a slow pulse sequence. As reverse bias builds up in the timing capacitor, the emitter current of transsitor 166 begins to decay through relay coil 168 and coil 174. When it reaches the correct value as set by potentiometer 180, switch 240 opens and the short is removed across capacitor 206 thus giving rise to signal device operation in a second manner with a fast pulse sequence. Therefore, it is possible to obtain a plurality of warning signals in sequence from the electrical circuit following its energization. This is accomplished effectively by varying the capacitive feedback in the multivibrator circuit. The means for varying the capacitive feedback is the reed switch whose actuation is controlled by the position of the sensor.

Relay switch 220 controls its associated contact arms 176 and 178. Contact 178 is optional and is used in place of the electronic oscillator circuit consisting of transistors 222, 224 and output transformer 226 with the magnitude of signal being determined by the position of poteniometer 228. If the electronic oscillator is deleted, namely transistors 222, 224 and related circuitry, the additional contact 178 may be used to control the horn relay and thereby use a conventional 12-volt horn rather than the speaker type horn for certain applications.

In operation, with the vehicle moving forward transistor 166 is held in a cutoff position and the magnetic coupling provided by circular magnet 116 maintains the plastic disc sensor 126 against stop 142. It is an important feature of the present invention that the stop arm 140 of the sensor 126 will be held against the forward stop position developed by the circular magnet. The force is adequate to assure nontriggering even under heavy road shock.

When the vehicle commences rearward movement, circular magnet 116 rotates in the opposite direction with shaft 118 and the magnetic coupling between the circular magnet 116 and ball bearings 122, 124 and shaft 118 is sufficient to move plastic sensor disc 126 in the opposite direction toward reverse stop 144 thereby bringing circuit-actuating permanenet magnet 128 sufficiently close to reed switch 130 to close its contacts, thereby providing a triggering pulse to apply forward bias to the base of transistor 166. As current flows in the emitter circuit of transistor 166, relay coil 168 energizes sufficiently to close contacts 170 and 171. This in turn permits timing capacitor 172 to begin to charge until such time as it becomes sufficiently charged to cause transistor 166 to stop conducting and permit the normally open contacts 170 and 171 to reopen. When this occurs the warning signal, e.g. the blowing of the horn of the vehicle, ceases provided that the rearward motion has ceased. If the rearward motion has not ceased, the magnetic coupling between the circular magnet 116 and plastic disc 126 causes the plastic disc to rotate toward the reverse stop again thereby reactuating the reed switch 130 and causing the warning signal to continue. Also with energization of relay coil 168 and reed coil 174, the closing of contact 171 sends a pulse to the multivibrator circuit 100 causing it to flip-flop as current flows through relay coil 220, contacts 176 and 178 are closed. The closure of contact 176 causes the second multivibrator circuit comprising transistors 222 and 224 to function whereas closure of contact 178 causes the horn to relay to sound. A warning signal is obtained whenever contact 176 is closed by virtue of actuation of the multivibrator circuit comprising transistors 222 and 224. The contact 178 is optional as evidenced by the dashed lines in FIG. 10. Contact 171 also sends a pulse to solenoid 148 thereby returning plastic disc 126 to the forward stop 142.

An optional electrical circuit is illustrated in FIG. 12 wherein this circuit may be used with an external horn for continuous nonpulsed type of operation. The control unit is designated generally by the numeral 256 and the sensor unit generally by the numeral 258. Once again resistor network 262 and 264 is in series with battery 260 with an intermediate connection to the base of transistor 266 so as not to overdrive transistor 266. A relay coil 268 functions in a manner somewhat similar to relay coil 168 and the actuation of its relay contacts 270 and 271. Capacitor 272 serves as a time delay device during the charge of which the horn will sound by means of the lead directly to the horn relay as indicated by the legend in FIG. 12. This circuitry eliminates the multivibrator circuits and produces an external horn warning sound with continuous nonpulsed operation.

While the foregoing description is believed to provide a full understanding of the invention as to its structure, environment and mode of operation, certain features thereof should be clearly understood as contributing significantly to the effectiveness and desirability of the signalling system.

IMMEDIACY OF OPERATION

Inasmuch as rotation of only a few degrees of arc is necessary to energize the warning circuit, it will be apparent that very little backup movement by a vehicle such as an automobile is required before the speedometer shaft connector turns sufficiently to swing the trigger arm so as to energize the electrical circuit. For example, in a working signalling system made in accordance with the present invention, installed in a standard automobile, rearward vehicle movement was found to be less than 1 foot before the warning horn sounded.

AVOIDANCE OF UNNECESSARY SIGNALLING

Once the reverse motion of the vehicle stops, there is no further need for a warning signal. The vehicle may proceed forward or remain stopped and the warning device is quiescent. In the system of this invention, this is accomplished by a number of important features. In the event that the vehicle starts to move backwards, the magnetic torque developed by the ceramic magnet 116 causes the plastic sensor disc element 126 to rotate in the direction of rotation until stopped by stop 146. This stop has a slot which permits plastic disc 126 to travel an angular motion of one-fourth of an inch. Passage of the small fixed magnet mounted by being embedded in the outer rim of plastic disc 126 over the small fixed reed switch 132 causes the reed switch to close. The momentary contact to ground of the base of transistor 166 through limiting resistor 162 causes transistor 166 to draw current through relay 168 and reed relay coil 174. When forward bias is thus applied to its base, transistor 166 conducts and current flows through relay 168 and reed coil 174. At the same time relay 166 has closed contact 170 thereby maintaining timing capacitor 172 at a discharge or ground potential. At this instant timing capacitor 172 starts to charge and allows transistor 166 to conduct for a limited period of time such as for 7 seconds. Contact 171 remains closed during this period supplying power to multivibrator 200. Also during this time contact 171 supplies power to solenoid 148 causing it to kick plastic disc 126 through control arm 150 away from the reverse stop position 144 to the forward stop position 142. The disc will remain in this position if the vehicle stops or proceeds in a forward direction. If this condition exists, permanent magnet 128 is not in position to actuate reed switch 130 to close the electrical circuit.

However, if the vehicle continues its rearward motion the magnetic torque developed by magnet 116 will tend to return the plastic disc in the direction of rotation (rearward) and again cause the reed switch 130 to be closed by passage of magnet 128 thereover and the cycle will be repeated. It is important to note that the kick imparted to the plastic assembly 126 through arm 150 must be strong enough to insure instant removal of plastic assembly 126 to the forward stop 142. This is accomplished by the magnetic torque developed by the magnet 116 and in addition is against the shaft motion which is rotating in the opposite or reverse direction.

In summary, the ability of this device to terminate the warning signal when the vehicle stops and give instant warning whenever rearward motion commences as well as to remain quiescent when forward motion occurs is dependent upon the factors listed above.

It is important to note that the 7-second sequence will be observed in all cases after initial triggering caused by any rearward motion even though that motion stops instantaneously. This is because the 7-second warning sequence will continue until capicitor 172 is fully charged and normal reverse bias causes transistor 116 to again be cut off so as to return the system to its normal nonsignalling state.

This functioning contrasts markedly with certain commercially available backup warning systems wherein the warning signal circuit remains energized despite the absence of rearward movement until such time as (1) a switch is normally opened by the vehicle operator, or (2) the vehicle engine and ignition are shut, or (3) the vehicle actually commences forward motion, or (4) the operator moves the transmission shift lever from a "reverse" to a "neutral" or "forward" position. With the present invention, none of the foregoing operations is required to terminate the warning signal, once the vehicle has actually ceased rearward motion.

While the invention has been described in association with a motor vehicle, as for example a passenger auto, delivery truck, forklift vehicle etc. it is apparent that the advantages of the signalling system may be equally readily realized in association with any machine or mechanism employing a reversible drive means and wherein rotation of the mechanism in a reverse or undesired direction may cause a hazard to be warned against. Similarly, while the specific warning means herein disclosed comprises the conventional or an auxiliary automobile horn, other or additional warning means of an audible, visual or tactual nature may be employed as may be desirable or necessary.

It is thought that the invention and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of parts without departing from thespirit and scope of the invention.

Claims

We claim:

1. A device for signalling rotational movement in a first direction of mechanism capable of rotating in first and second directions, comprising:

a sensing unit assembly,

a sensor element mounted in said assembly for limited rotational movement in first and second directions,

said sensing unit assembly having means for coupling said sensor element to said rotatable mechanism such that rotation of said mechanism in said first direction imparts rotational movement to said sensor element in said first direction,

an electrical circuit including a signal device for effecting a signal when said circuit is energized,

switch means carried by said sensing unit assembly and responsive to rotative movement of said sensor element in said first direction for energizing said circuit, and responsive to rotative movement of said sensor element in said second direction for deenergizing said circuit,

sensor element moving means carried by said sensing unit assembly and operable to engage and move said sensor element in said second direciton,

means in said electrical circuit for operating said sensor element moving means when said circuit is energized, and,

time delay means associated with said electrical circuit for maintaining energization thereof for a predetermined limited time following deenergization-initiating sensor element movement in said second direction.

2. The device of claim 1 wherein said electrical circuit includes means for operating said signal device in a first manner and means for operating said signal device in a second manner, said time delay means energizing said operating means successively when said circuit is energized.

3. The device of claim 2 wherein said several operating means include a multivibrator circuit portion having means to vary the capacitative feedback therein during functioning thereof to alter the nature of the warning signal effected by said electrical circuit.

4. The device of claim 1 wherein:

said coupling means includes a shaft rotatably mounted in said sensing unit assembly,

said shaft having a member fixedly mounted thereon and having said sensor element carried thereby for free arcuate movement relative to said shaft;

stop means limiting arcuate movement of said sensor element about said shaft in either direction, and,

means magnetically coupling said member and said sensor element to effect arcuate movement of said sensor element within limits of said stop means; and

whereby rotation of said mechanism in said first direction results in a corresponding rotation of said shaft and said member, and thereby through said magnetic coupling to effect limited arcuate movement of said sensor element to energize said electrical circuit.

5. The device of claim 1 further including in combination said mechanism with which said sensing unit assembly is cooperatively associated and comprising a rotatable element of a motor vehicle whose said first and second directions of rotation are respectively responsive to reverse or forward vehicle movement.

6. The device of claim 5, wherein said vehicle rotatable element comprises a speedometer cable to which said coupling means is connected.

7. A sensing unit for signalling backing up movement of a vehicle comprising:

a sensing unit housing;

a shaft mounted for rotation in said housing having means for connection to a vehicle part which rotates in opposite directions in synchronism with forward and reverse movements of said vehicle;

a first member fixed to said shaft for rotation therewith;

a second member mounted on said shaft for arcuate movement thereon;

means magnetically coupling said first and second members;

a circuit-actuating element fixed on said second member;

a forward direction stop and a reverse direction stop on said housing defining opposite ends of the arcuate path of said member for limiting arcuate movement thereof;

an electrical circuit including a warning signal device;

said electrical circuit being energized in response to motion of said circuit-actuating element towards said reverse stop; and

an actuator responsive to circuit energization for engaging and moving said second member toward said forward stop thereby to deenergize said circuit and free said sensor for repeat movement toward said reverse stop.

8. The device of claim 7, wherein said actuator comprises an electromagnet in said housing adjacent said stops, said electromagnet having an arm controlled thereby and mounted for movement to engage said second member for urging the same away from said reverse stop when said electromagnet is energized, said electromagnet being energized upon energization of said electrical circuit.

9. The sensing unit of claim 8, wherein said electrical circuit includes means for maintaining said circuit energized for a predetermined time following removal of said sensor element from said reverse stop.

10. The device of claim 8 wherein said electrical circuit includes means for operating said signal device in a first manner and means for operating said signal device in a second manner, and timing means in said circuit for energizing said operating means successively when said circuit is energized.

11. The sensing unit of claim 7, wherein said electrical circuit includes a multivibrator circuit portion for operating the warning signal device, and including means to vary the capacitative feedback therein so as to alter the nature of the warning signal emitted by said warning signal device.