US 3784839 A
Starting of a motor vehicle is controlled from electronic combination lock circuitry which may be placed in a turnover mode of operation after the engine has been started to thereafter permit a predetermined number of engine starts without the entering of the lock combination.
Description (OCR text may contain errors)
United States Patent [191 [111 Weber 1 1 Jan. 8, 1974  ANTI-THEFT APPARATUS INCLUDING 3,670,836 6/1972 Tonkowich et a1 307/10 AT TURNQVER MODE OF OPERATION 3,653,457 4/ 1972 Lopez 180/1 14  Inventor: John A. Weber, Franklin, Wis.
Primary Examiner-J. R. Scott  Asslgnee' g s? agi g Corporauon Assistant Examiner-M. Ginsburg e to] Attorney-Eugene W. Christen et al.  Filed: July 12, 1972  App]. No.: 271,141
 ABSTRACT  US. Cl. 307/10 AT, 180/114, 317/134 Starting of a motor vehicle is controlled f else. [5 l] Int. Cl. B60! 25/00 tronic combination lock circuitry which may b  Field of Search 307/10 AT; 317/134; placed in a turnover mode of operation after the 340/633; 180/1 gine has been started to thereafter permit a predetermined number of engine starts without the entering of  References cued. the lock combination.
UNlTED STATES PATENTS 3,718,202 2/1973 Brock 307/10 AT 2 Claims, 8 Drawing Figures x KEYBOARD m B] m an m iiigg fi m LOGIC TRANSMIT TURNOVER ENGINE LOGIC RUN 20 4 0 IGNITION MODE TURNOVER I SWITCH LOGIC y l l k n 1 RECEIVER LOCK CODE 44/ .1 AND MEMORY I DECODER comRoLi ic i I I I LATCH RELEASE i l STARTER FLQEgHNG I? i RELAY \zz L I PAIENIEIJJIIII 3 m4 l RUN I I LOGIC TO IGNITION SYSTEM START CONTROL J I CIRCUITRY I 30 l Z6 18 .26 I STARTER I SOLENOID 9' I as I I I I KEYBOARD KEYBOARD KEYBOARD w m IZI [51 El E1 FILTER E C DE AND [TE] LOGIC TRANSMIT TURNOVER ENGINE LOGIC RUN TURNOVER IGNITION MODE SWITCH LoOIc M l m a u I I 42 I REOENER LO;I CODE 44;
I AND MEMORY AND I 4 I DECODER CONTROL LOGIC I 1 I LATCH RELEASE I I I l LATCHING pgI 2 I RELAY I 9 STARTER I l RELAY L 1 ANTI IHEFT APPARATUS INCLUDING TURNOVER MODE OF OPERATION This invention relates to a system for inhibiting motor vehicle operation by unauthorized persons and more particularly to a system for preventing starting of a motor vehicle until the operator has entered a predetermined code into the system.
It is an object of the present invention to provide an improved anti-theft system for preventingunauthorized operation of a motor vehicle.
It is another object of the present invention to provide such a system wherein knowledge of a particular code is normally required to start the motor vehicle but wherein one who has knowledge of the code may permit others a limited number of starts of the vehicle engine without the necessity for disclosing the code.
It is another object of the present invention to provide electronic combination lock circuitry including a turnover mode of operation which permits another operator a limited number of engine starts as selected by the owner of the vehicle without the necessity for disclosing the code and wherein the turnover mode is terminated in response to removal of the ignition key.
It is a further object of the present invention to provide an anti-theft system including akeyboard unit for entering a particular combination of numbers and means for serially transmitting the data entered to a receiver unit located in a comparatively inaccessible location of the vehicle to thereby render difficult and time consuming any attempt to defeat the system.
It is another object of the present invention to provide electronic combination lock circuitry requiring the entering of a particular code in order to start the vehicle but which permits restarting of the vehicle without the necessity for entering the codeaslong as the ignition switch remains in the On position.
In accordance with the present invention the ground path of the starter solenoid circuit on the motor-vehicle is controlled by a latching relay which is latched, to close the solenoid circuit, by combination lock circuitry responsive to operator actuationof a plurality of pushbuttons in a predetermined sequence. The lock circuitry includes a keyboard unit through which the combination code number may be entered. The digits of the number are encoded and serially transmitted to a receiver and decoder located in a relativelyinaccessible part of the vehicle. If the correct code number is transmitted the latching relay is latched to permit starting of the vehicle. If the code number is not-entered correctly the latching relay is released or if after the correct code is entered the ignition key is turned off the latching relay is released. If while theengine is running the operator actuates one of the numbered pushbuttons the system is placed in a turnover mode which permits the vehicle to be restarted for a numberof times corresponding to the pushbutton depressed. If at any time while the vehicle is in the turnover mode the ignition key is removed, the latching relay is released to return the system to the normal mode of operation.
Other objects and advantages of the present invention will be apparent from the following detailed description which should be taken in conjunction with the drawings in which:
FIG. I is a schematic diagram of the system of the present invention;
FIG. 2 is a block diagram of the combination lock circuitry of the present invention; and
FIGS. 3 through 8 are more detailed logic diagrams of the lock circuitry of FIG. 2.
Referring now to the drawings and initially to FIG. 1, the anti-theft system of the present invention is generally designated 10 and comprises logic control circuitry 12 and a latching relay 14. The control circuitry 12 is connected with a source of direct current potential such as the vehicle battery 16 through a conductor 18. The conventional vehicle ignition switch generally designated 20 has one side connected with the battery 16 and is movable from a normally open position as shown to engage a Run contact connecting the ignition system with the battery 16 and is further movable to engage a Start contact connecting the starter solenoid 22 of the vehicle to the battery 16 while maintaining engagement with the Run contact. As usual, the ignition switch 20 is biased to return from the Start position to the Run position. The position of the ignition switch 20 is sensed by the control circuitry 12 through a conductor 24. The latching relay 14 includes a latch winding 26 and a release winding 28 which controls a relay contact 30 to respectively close and open the starter solenoid circuit.
Referring now to FIG. 2, the anti-theft system 10 is shown in block diagram form and includes a master unit located within the vehicle compartment, preferably on the dashboard and a remote unit located in a relatively inaccessible area such as withinthe starter motor enclosure. The master unit comprises a keyboard unit 32 having a plurality of operator actuable pushbuttons bearing the numerals 1 through 5 and the legend RESET. The particular combination code number is entered by the operator through the keyboard unit 32. The six pushbuttons on the keyboard unit 32 are connected with keyboard filter logic 34 which is in turn connected with a keyboard encode and transmit block 36 which encodes the digits entered through the keyboard unit 32 and serially transmits such data through a conductor 38 along with a clock signal through a conductor 40 to the remote unit generally designated 41. The remote unit 41 includes a receive and decode block 42 where the data is received and decoded and fed to a lock code memory and control logic block 44 which provides either the latch or a release signal to the latching relay 14. The starter solenoid 22 is energizable from the ignition switch 20 and controls the usual engine starter motor (not shown) of the vehicle. The master unit further includes mode logic generally designated 46 which senses the position of the ignition switch 20 as well as whether the ignition key is inserted in or has been removed from the usual ignition lock on the vehicle. A turnover logic block generally designated 48 receives an input from a condition responsive switch 50 which responds, for example, to engine manifold vacuum so as to be indicative of whether the engine is running or not. The logic block 48 also receives inputs from the keyboard encode and transmit block 36 and provides an output designated TURN- OVER to the mode logic 46. The logic 48 when placed in the TURNOVER mode by the vehicle owner permits the starter relay 22 to be energized from the ignition switch 20 through the mode logic 46 for a particular number of starts without the necessity for entering the particular combination code number through the keyboard unit 32. The mode logic 46 provides a reset output to the keyboard encode and transmit unit 36 whenever the number of starts in the turnover mode have been completed or whenever the ignition switch 20 is removed from the ignition lock.
Referring now to FIG. 3, the overall timing for the system of F IG. 2 is provided from a basic clock oscillator 52 which produces, for example, a 20 KHZ square wave output signal to the clock input of a counter 54. A reset input, designated RST, to the counter 54 is provided from an initialization circuit generally designated 56 which produces a reset pulse upon connection of the logic control circuitry with the battery 16. The circuit 56 comprises a resistor 58 and capacitor 60 with an inverter 62 connected to the junction between the resistor 58 and capacitor 60 so that a pulse with a positive going leading edge is generated upon connection with the battery 16. RST will then go low upon charging of the capacitor 60 above the threshold of the inverter 62. The counter 54 is a divide-by-l28 counter producing seven successively lower frequency outputs designated TF1 through TF7. TP2 through TP7 g low upon charging of the capacitor 60 above the threshold of the inverter 62, and are inverted by the inverters generally designated 64 to produce the outputs TP2* through TP7*. TF3 and TP2* provide inputs to' a NOR gate 66 which produces an output designated CPB which is inverted by an inverter 67 to produce an output designated CPB. TP3* and TP2* provide inputs to a NOR gate 68 which produces an output designated CPA which is inverted by inverter 69 to produce the output designated CLOCK*.
Referring now to FIG. 4, the individual pushbuttons 1 through and RESET of the keyboard unit 32 control switches designated K1 through K6 respectively, each of which had one side grounded and the other side connected to V+ through pull-up resistors generally designated 70. The high side of the switches K1 through K6 respectively, are designated Key 1* through Key 6* and are connected with the keyboard filter logic 34. Referring now to FIG. 5, the filter logic 34 comprises NAND gates 72 and 74 and a NOR gate 76 which produces an output designated Key* which is normally high but goes low in response to actuation of any of the switches K1 through K6 and returns high.
upon release of the switch. Key* is connected with the D input on the first stage of a three stage counter comprising D type flip-flops 78 80 and 82. The Q outputs of the flip-flops 78 and 80 designated KI* and K2* are connected with the D inputs of the flip-flops 80 and 82 respectively. The flip-flops '78, 80 and 82 are set from the initialization circuit 56 so that Kl*, K2* and K3* are high. The flip-flops '78 through 80 and 82 are clocked from the TP7* output of the counter 54.
The keyboard encode and transmit unit 36 comprises binary encoding circuitry generally designated 84 the output of which is strobed into a register generally designated 86 under the control of strobe circuitry generally designated 88. The binary encoding circuitry 84 includes NAND gates 90, 92 and 94 which are connected with the pushbutton switches K1 through K6 as indicated. The output of each of:the gates 90 through '94 is inverted by inverters 96, 98 and 100 and provide one input to NOR gates 102, 104 and 106. The other input to the gates 102 through 106 is provided from the strobe circuitry 88 which comprises NOR gates 108, 110 and 112 connected with the three stage counter and with the timing circuitry of FIG. 3 as indicated. The
outputs of the NOR gates 108 through 112 provide inputs to a NAND gate 114 the output of which is designated RSTRW which provides the other input to the NOR gates 162 through 106. The output of the gates 102 through 106 are connected with the set inputs of the first three stages of the register 86 while the output of the gate 114 is inverted by an inverter 116 and applied to the set input of the last two stages of the regis-, ter 86. The register 86 comprises D type flip-flops 118 through 126 which are clocked from the output, designated REG/C, of a'NAND gate 128 having its inputs connected to TF7 and CPB. The flip-flops 1 18 through 126 are reset to an initial condition from RST wherein their Q outputs REG1 through REGS are low. The output of the flip-flop 126 provides one input to a NAN D gate 130, the other input of which is designated Run* and is high to open the gate whenever the engine is not running. The output of the gate 130 is designated Data".
Briefly, the operation of the circuitry thus far described is as follows: Depression of one of the switches K1 through K6 causes thebinary equivalent of the corresponding digit to be entered in the flip-flops 118, 120
and 122 of the register 86 when RSTRB* goes low. At the time the flip-flops 124 and 126 of the register 86 are set high from RSTRB. Consequently, as the data is shifted out of the register 86 by the clock pulses REG/C the data is always preceded by two logic 1 s foridentification purposes.
Referring now to FIG. 6, the receive and decode logic block 42 comprises a data register generally designated 134 which includes D type flip-flops 136 through 144. The flip-flops 136 through 144 are set so that their Q outputs are high by a reset signal designated RSTA. RSTA is derived from the Q output of a flip-flop 146 which is initially set high from an initialization circuit 147 which is identical to the circuit 56. The circuit 147 produces a pulse designated RSTD when the logic circuitry is first connected with the vehicle battery. Thereafter, the flip-flop 146 is toggled and the register 134 is set by the rising edge of CLOCK* whenever the 2 output of the flip-flop 144, designated RA1, is high. The data is shifted into the register 134 by a NAND gate 148 having its output connected to the clock input of each of the flip-flops 136 through 144. One input to the gate 148 is from a NAND gate 150 which has inputs connected to the 6 outputs of the flipflops 142 and 144, designated RA2 and RAl respectively, which go low when the register 134 is set. The other input to the gate 148 is from CLOCK* which is inverted by an inverter 152 the output of which is designated CLOCK- The gate 148 is open whenever the register 134 is set so that the data is shifted into the re gister on the following edge of CLOCK until the first two bits, which are logic 1s," are entered in the flip-flops 142 and 144, whereupon RA] and RA2 go high and the gate 148 is closed-F The register 134 stores the data for a one CLOCK pulse interval and thereafter theregister 134 is set from the flip-flop 146; During this one CLOCK pulse interval the data stored in the register 134 is decoded.
The outputs of the flip-flops 136 through 144 in the register 134 are connected with decode logic generally designated 158 which comprises NAND gates 160 through 168 connected with the outputs of the flipflops 136 through 144 in the register 134 as indicated. The decode logic 158 is hard wired with the register 134 to decode the particular combination code number which in the example shown is 53124. The outputs of I the gates 160 through 166 are connected with the D input of flip-flops 170 through 176 respectively while the output of the gate 168 is connected to the D input of flip-flop 178 through a NOR gate 179. The flip-flops 170 through 178 are clocked through NOR gates 180 through 188 which are open in sequence to insure that the five numbers of the code are inserted ir the keyboard unit 32 in the proper sequence. The Q outputs of the flip-flops 170 through 176 and the Q output of the flip-flop 178 are designated MEMl through MEM 5 respectively. The 6 output of the flip-flop 178 is designated MEM 5*. The sequential control of the gates 180 through 188 is controlled from a BCD counter 190 which is advanced on the rising edge of CLOCK through a NAND gate 192 and an inverter 194. The other inputs to the gate 192 besides CLOCK are RAl and RA2 so that the gate 192 is opened for the CLOCK pulse interval following the entering of the binary data in the register 134. In other words, after CLOCK goes low to enter the most signficant bit in flip-flop 136, the gate 148 is closed and the gate 192 is opened. On the following rising edge of CLOCK, CL* goes low and CL goes high and the counter 190 is incremented and on the following falling edge of CLOCK, i.e. CLOCK* goes high, the flip-flop 146 is toggled causing RSTA to go high and reset the register 134. When the register 134 is reset the gate 148 is opened and the gate 192 is closed. The outputs of the counter 190 designated CTRl through CTR3 are inverted by inverters 196, 198 and 200 to provide the outputs designated CTR1* through CTR3* respectively. CTRl through CTR3 and CTR1* through CTR3* are connected with NAND gates 202 through 210 as indicated. Theoutputs of the gates 202 through 210 are designated CHl* through CH5* respectively. The counter 190 is reset from RSTC* through a NAND gate 216, the output of which is designated RSTB, or from a NAND gate 214. The input to the gate 214 is the CTRl, CTR2*, CTR3 outputs of counter 190 and the MEM 5* output of the flipflop 178. The RSTC* input to gate 216, as will be shown hereinafter, goes low and RSTB goes high when RSTD goes high to initially reset the counter 190. When RSTB goes high the flip-flops 170 through 176 are set to drive MEMl through MEM4 low and the flipflop 178 is reset to drive MEMS low and MEM5* high. With the counter 190 reset CHl* is low and CH2* through CI-I5* are high. Accordingly, when CL* goes low, just prior to incrementing the counter 190, the flip-flop 170 will be toggled to cause MEMl to go high if the pushbutton switch K5 had been depressed since in that event RA3, RA4* and RAS would be high. As the counter 190 is incremented CH2* goes low and the next binary coded number entered through the keyboard unit 32 is decoded by the gate 162. If the number 3 is entered MEMZistoggled high on the falling edge of CL*. Similarly, MEM3 and MEM4 go high if the pushbutton switches K1 and K2 are successfully depressed. The four outputs of the flip-flops 170 through 176 are inputs to a NAND gate 212, the output of which is connected with the NOR gate 179 so that if the first four numbers of the code are entered and in the proper sequence and the pushbutton switch K4 is thereafter depressed, the outputs of the flip-flop 178 designated MEMS is toggled high end MEM5* is toggled low.
Referring now to FIG. 7, MEMS is connected to the D input of a flip-flop 218 which is initially reset from the RSTD output of the initialization circuit 147. The flip-flop 218 is clocked from the output of a BCD counter 220, designated SLOCLK, which is driven from and is at a frequency of, for example, 20 Hz. as compared with the CLOCK frequency of, for example, 2KHz. When the flip-flop 218 is clocked while MEMS is high its Q output designated LATCH goes high. The Q output of the flip-flop 218 is connected with the base of a transistor 222 through a resistor 224. The emitter electrode of the transistor 222 is grounded while its collector is connected to V+ through the latch winding 26 of the latching relay 14. Accordingly, if the correct combination is entered through the keyboard unit 32 the latch winding 26 is energized and its contact 30 is moved from the position shown to provide a ground path for the starter 22 solenoid.
If the reset button K6 of FIG. 4 is depressed the binary equivalent of the numeral 6 is transmitted to the register 134 causing the inputs to a NAND gate 226 to go high and the output thereof designated COD6* to go low. The output of the NAND gate 226 is connected to the D input of a D type flip-flop 228 which is toggled from CL. Accordingly, on the rising edge of CL (FIG. 6) following depression of the reset pushbutton switch K6, the 6 output of the flip-flop 228 designated RST goes high. RST is connected with the D input of a D type flip-flop 230 which is clocked from SLOCLK and has its 0 output designated RELEASE connected to the base of a transistor 232 through a resistor 234. The emitter of the transistor 232 is grounded while the collector is connected to V+ through the release winding 28 of the latching relay 14. Accordingly, the release winding 28 may be energized to open the ground path of the starter solenoid 22 by actuating the reset pushbutton switch K6. The O outputs of the flip-flops 218 and 230 designated LATCH* and RELEASE* respectively are inputs to a NAND gate 236 the output of which is connected with a NOR gate 238. The other input to the NOR gate 238 is RSTD from the initialization circuit 147. The output of the gate 238 is the previously mentioned RSTC* which provides the other input to gate 216 of FIG. 6. Thus, whenever the latching relay 14 is either latched or released RSTC* goes low to reset the counter 190 and. the flip-flops 178 and set the flip-flops to 176 and 228. If the code number is incorrectly entered MEM5* will remain high and after the fifth pushbutton switch has been actuated CTRl, CTR2* and CTR3 will be high so the output of the gate 214 will go low and RSTB will go high to reset the counter and the memory flip-flops and release the latching relay 14.
Referring now to FIG. 8, the turnover logic 48 is disclosed in greater detail. The turnover logic 48 permits the owner of the vehicle or anyone else having knowledge of the combination to turn over limited use of the vehicle to one not having knowledge of the combination without the necessity of disclosing the combination. Use is limited by the operator prescribing the number of vehicle starts to be permitted. The logic 48 comprises a three stage counter 242 which is set through a NAND gate 244 which performs an OR function. One input to the gate 244 is K OUT* which is derived from a switch 246 which closes when the ignition key is placed in the ignition lock and opens when the ignition key is removed from the ignition lock. One side of the switch 246 is grounded while the other side is connected with an inverter 248 which is tied to -V+ through a pull-up resistor 250. Another input to the gate 244 is from the reset pushbutton switch K6 and the accompanying RSTRB* through a NOR gate 252 and an inverter 254. The other input to the NAND gate 244 is from the initialization circuit 56 through a NOR gate 256. The counter 242 comprises flip-flops 258, 260 and 262. The flip-flops 258, 260 and 262 are initially set from RST so that their Q outputs are high and their 6 outputs are low. The O outputs of the flip-flops 258 through 262 provide inputs to a NOR gate 264, the output of which is designated TURNOVER*, which is inverted by an inverter 266 to provide the output designated TURNOVER. Thus, initially the output of the gate 264 is high and the output of the inverter 266 is low indicating that the vehicle is not in the turnover mode. If at any time the ignition key is removed from the ignition lock or the reset pushbutton switch K6 is actuated the counter 242 is set so that the output of the inverter 266 goes low. The output of the inverter 266 provides one input to a NOR gate 268. The other input to the NOR gate 268 is tied to ground through a pull down resistor 270 and is also connected with the Run position of the ignition switch 20. The output of the a gate 268 provides one input to a NOR gate 272 the other input of which is K OUT. The output of the gate 272 is designated KRST* and provides one input to the gates 92 and 94 of the encoding circuitry 84 (FIG. Thus, if KRST* goes low the binary equivalent of the reset pushbutton switch K6 is transmitted to the receive and decode unit 42 to release the latching relay 14. KRST* will go low if the ignition key is removed from the ignition lock or, if the vehicle is not in the turnover mode and the ignition switch is turned off. The flipflops 258, 260 and 262 are programmed by encoding logic comprising NOR gates 274, 276 and 278. The NOR gates 274, 276 and 278 are each controlled from a NAND gate 280. One input to the gate 280 is TURN- OVER*. The other input to the gate 280 is from the engine manifold, vacuum switch 50 or other appropriate engine run conditioning sensor and also to V+ through a pull up resistor 282. Thus, the gates 274, 276 and 278 are closed if the engine is not running or the system is in the turnover mode. In other words, the counter 242 can be programmed for a particular number of free starts only if the engine is runningand the system is not already in the turnover mode. The second input to each of the gates 274, 276 and 278 is from RSTRB* which goes low in response to actuation of any of the pushbutton switches Kl through K6. The third input to the gate 274 is from RA* (FIG. 5) which goes low in response to actuation of either of the pushbutton switches K1,
K3 or K5. The third input to NOR gate 276 is desig- V nated BCLR and is derived from the output of a NAND gate 284 through an inverter 286. The inputs to the switches K1, K2 or K5 is actuated. The third input to the NOR gate 278 is derived from a NAND gate 288 through an inverter 290. The inputs to the NAND gate 288 are KEY4*, and KEYl (FIG. 5) through an inverter 292. Accordingly, the output of the inverter 290 goes low if either of the pushbutton switches K1, K2, K3 or K4 is actuated. Thus, the outputs of the flipflops 258, 260 and 262 will be reset to the binary equivalent of any pushbutton switch depressed while the vehicle is running and the vehicle is not in the turnover mode. For example, if the register 242 is set so that 8 TURNOVER* is high and if the engine is running and .the driver depresses the pushbutton switch K4, the 6 output of the flip-flop 262 will be reset high and the 6 outputs of the flip-flops 258 and 260 will remain low.
When the 6 output of the flip-flop 262 goes high TURNOVER* goes low closing the gate 280. TURN- OVER* will remain low until the counter 242 has been toggled four times. The counter 242 is toggled through a NAND gate 294 and inverter 296 each time the switch 50 opens while TURNOVER is high. Thus, each time the engine is started during the turnover mode the counter 242 is toggled so that after four starts, all inputs to the gate 264 are low and TURNOVER goes low. Accordingly, when the ignition switch 20 is turned off after the fourth start the output of the gate 268 goes high and KRST* goes low to generate the reset number 6 and interrupt the circuit to the starter solenoid 22.
Having thus described my invention what I claim is: 1. In a motor vehicle provided with starter motor control means for controlling starting of the vehicle engine, including an ignition switch operable by an ignition key, apparatus for preventing unauthorized starting of said vehicle comprising:
a keyboard unitincluding a plurality of operator actuable switch means, latch means for enabling said starter motor control means, latch control means responsive to actuation of said switch means in a predetermined sequence for energizing said latch means to enable said starting motor control means, said latch controlmeans responsive to actuation of said ignition switch to an off position for releasing said latch means to disable said starter motor control means, override means for overriding said ignition switch and thereby prevent releasing of said latch means, said override means including counter means, means responsive to actuation of one of said switch means while said vehicle engine is running to program said counter means to a predetermined number of ignition switch actuations, said counter means being advanced each time said engine is started after said counter means is programmed whereby the override condition is terminated after said engine has been started said predetermined number of times, and means responsive to removal of said ignition key for resetting saidcounter means. 2. In a motor vehicle provided with an electrical load device for controlling usage of the vehicle, apparatus for preventing unauthorized usage of said vehicle comprising:
input means including a plurality of operator actuable switch means, latch means for enabling said load device, latch control means responsive to actuation of said switch means in a predetermined sequence for energizing said latch means to enable said load device, an additional operator'actuable switch means, said latch control means responsive to actuation of said additional switch means for releasing said latch means to disable said load device, override means for overriding said additional operator actuable switch means and thereby preventing release of said latch means, said override means inadvanced each time said additional switch means is actuated after said counter means. is programmed whereby the override condition is terminated after said additional switch means has been actuated said predetermined number of actuations.