US 3792493 A
A security system for detecting the opening or closing of the door of an enclosed area incudes an enclosure having a digital clock which drives a binary register, the binary register providing a binary decimal code a measure of time. The door includes a fin having a pair of magnets which enter a slot in the enclosure when the door is closed and are removed therefrom when the door is open. A detector and associated circuitry in the enclosure sense the presence of the magnets and provides a signal to a storage register each time the door is opened or closed, thereby causing the storage register to sample and hold the binary decimal code provided by the binary register at the time of an opening or closing. The storage register is coupled to an illuminated display by decoder/drivers, thereby causing the stored time to be displayed in arabic numerals. In addition, the outputs of the storage register are coupled to the writing styluses of an electromechanical recorder in the enclosure. A motor in the recorder, in response to the detection, moves a voltage sensitive paper under the styluses, thereby causing the stored time to be recorded in binary decimal code on the paper. Thus, each opening or closing of the door is recorded. The enclosure includes a transparent window through which several such writings on the paper are visible, and another transparent window through which the illuminated display is visible at a distance.
Description (OCR text may contain errors)
United States Patent 1 [111 3,792,493 Hughes 1 Feb. 12, 1974 DOOR ACTUATED TIME RECORDER of the door of an enclosed area incudes an enclosure  Inventor: David Hughes, Cos Cob Conn. having a digital clock which drives a binary register, the binary register providing a binary decimal code a Assigneel' Intelligence Services Products measure of time. The door includes a fin having a pair Division, y of magnets which enter a slot in the enclosure when  Filed: No 6 1972 the door is closed and are removed therefrom when the door is open. A detector and associated circuitry PP bio-1303904 in the enclosure sense the presence of the magnets and provides a signal to a storage register each time 52 us. Cl 346/20, 58/145 R, 346/42, the is Pened or thereby 9 the F"- 346/74 S, 346/93 age register to sampleand hold the binary decimal  Int. Cl G07c 1/10 C0de.pmvided i the bmary reg'ster. at h time of an  Field of Search 346/20 42 79, 80 93 74 CH opening or closing. The storage register 18 coupled to 346/74 E, 74 S 74 SB, 74 SC; 340/274 275; an rllummated display by decoder/drivers, thereby 58/152 R 145 R 39.5 causing the stored time to be displayed in arabic numerals. In addition, the outputs of the storage register  References Cited aLe coulpled to1 the wlriting lstylusesAof an electaomec amca recor er in t e enc osure. motor in t e re- UNlTED STATES PATENTS corder, in response to the detection, moves a voltage 2,05 l COO] ensitive paper under the tyluses thereby causing the stored time to be recorded in binary decimal code on 0 911971 j s g' 4 X the paper. Thus, each opening or closing of the door is 31686880 8/1972 Samejima..::::..............::::::.4. 58 395 recorded' The enclosure a transparent Primary Examiner loseph W. Hartary Attorney, Agent, or Firm-Alan A. Levine; Breitenfeld & Levine  ABSTRACT A security system for detecting the opening or closing dow'through which several such writings on the paper are visible, and another transparent window through which the illuminated display is visible at a distance.
12 Claims, 6 Drawing Figures /9 20 Z 26 Q 2/ 62/6/24 wWaE/g E/IVAZ/ 505465 0Ec'a05e/ W504; air/M 4m? cam/r52 Zia/57722 az/t/ses iff afl LJ 22 20 m/ma 05756705 406/: zeaaeazz 1 DOOR ACTUATED TIME RECORDER The subject invention relates to security systems, and in particular to a security system for permanently recording the time of each opening or closing of the door of an enclosure, and for providing an illuminated display of the time of the last recorded event. The invention finds particular utility in connection with the cargo d or o a ehic e Each year people in the business of transporting goods lose money because goods are stolen from warehouses and vehicles used to transport the goods. In an attempt to minimize loses, and to determine the place or places where the thefts are occuring, seals have been placed on vehicle doors after the vehicles have been loaded. Thus, if a vehicle is dispatched with a seal and arrives at its destination with the seal broken and goods missing, it is known that the theft occurred while the goods were in transit and in the custody of known personnel. However, vehicle drivers usually carry a supply of seals with them, and hence are free to open and reseal the vehicle doors at will. In addition, due to human falibility, the sealing process is seldom carried out properly. Moreover, even when the seal is properly applied the use of seals has not been very effective because the time during which a seal is broken and a theft occurs is not known with any degree of accuracy. For example, if a sealed truck is driven by more than one driver and the drivers forget to check the seal when the switching of drivers takes place, or if a driver stops overnight and neglects to check the seal when he resumes his trip, the time or place of the theft and the custodian of the truck at the time of the theft will not be known.
Accordingly, it is an object of the present invention to provide a security system which automatically records on a permanent record, the time of each opening and closing of the door of an enclosure, such as a truck, thereby providing information with regard to when a theft has occurred, and, in the case of a vehicle, identifying the custodian of the vehicle at the time of the theft.
It is another object of the present invention to provide a security system which automatically displays a sealing number with each closure of the door, the sealing number being related to the time of the closure.
It is still another object of the present invention to provide an electromechanical security system which detects and records the opening and closing the doors of an enclosed area for a predetermined amount of time after its primary source of electrical power has been removed.
Additional objects and features of this invention will become apparent by reference to the following description in conjunction with the accompanying drawings, in which:
FIG. 1 is a perspective view of a mobile security system according to the invention;
FIG. 2 is a schematic block diagram of the security system;
FIG. 3 is a schematic block diagram of a part of the security system which provides the visual indication and some or its connections to that part of the security system which provides a written record;
FIG. 4 is a schematic block diagram of a part of the security system which provides the written record of the times when the door being monitored is opened and closed;
FIG. 5 is a set of waveforms which are useful for describing the sequenceof events occurring within the security system each time the door being monitored is opened or closed; and
FIG. 6 is a fragmentary cross-sectional view of the security system, showing part of an electromechanical re corder and a window for displaying the written record of the openings and closings of the door.
A security system, according to the invention, which provides a written record of the time during which the door of an enclosure were opened or closed, and an illuminated indication of the time when the door was last opened or closed, is shown in FIG. 1. In the following description, the invention will sometimes be referred to as associated with a truck having doors being monitored. However, it is to be understood that the invention can be used with any enclosure having an access door which it is desired to monitor.
The system chosen to illustrate this invention is housed within an enclosure 13 (FIG. 1) having a slot 14. The door 12 to be monitored is furnished with a fin 11, in which two magnets 10 are embedded, the fin moving into slot 14 when the door is closed and moving out of slot 14 when the door is opened. Within the enclosure 13 there is located an electromechanical system which responds to the presence or absence of magnets 10 in the slot 14 and provides, in binary decimal code, a written record of the times when the door 12 was opened or closed and, in arabic numerals, an illuminated display of the time when the door was last opened or closed. The written record sets forth the time in weeks, days, hours, and minutes, and is visible through the transparent window 15. The illuminated display sets forth the time of the last opening or closing in days, hours, and minutes, and is visible through the transparent window 16.
In general, the electromechanical system, a block diagram of which is shown in FIG. 2, includes a crystal oscillator 17, which because of its stability functions as a digital clock. In the system, the crystal oscillator 17 is connected to a divider 18 which provides several outputs. One of the outputs is a one pulse per minute signal and the other outputs are signals having higher repetition rates. The signals from the divider 18 are separately connected to the terminals of a switch 19 whose wiper arm may be used to select one of the signals for driving a binary counter 20. In response to the one pulse per minute signal, the binary counter 20 provides, in binary decimal code, a measure of time in minutes, hours, days, and weeks. As more fully described below, the higher frequency outputs of the divider 18 and the switch 19 may be used to accelerate the count in binary counter 20 when it is desired to correlate the time reading in the counter with a particular time reference such as eastern daylight saving time.
The binary outputs of counter 20 are connected to the inputs of a storage register 21. When the magnets 10 are moved into or out of the enclosure 13, a detector 22 applies a signal to a record logic circuit 23. The record logic circuit 23, in turn, activates a digital recorder 24 and sends a signal to the storage register 21, thereby causing the storage register to sample and hold the binary decimal code provided by binary counter 20. The coded time information in the storage register 21 is then coupled to the digital recorder 24 where it is re- 'duced to a writing. That part of the binary decimal code which corresponds to time information in terms of minutes, hours, and days is also coupled to a decoder/driver unit which drives the incandescent tubes of a visual display unit 26.
Referring to FIG. 3, the binary counter 20 includes nine counter units 28 (not all shown), a typical unit being the Motorola 7490 circuit, which are interconnected according to ways which are well known in the art so as to provide from a periodic pulse signal seven sets of outputs, each set of outputs 29 providing in parallel form bits which define a number in binary code. When the one pulse per minute signalfrom the switch 19 is connected to the binary counter 20, the seven output sets provide minutes, tens of minutes, hours, tens of hours, days, weeks, and tens of weeks numbers in binary decimal code.
Storage register 21 includes seven storage units 30 (not all shown), the Motorola 7475 circuit being typical, each of which is connected to one of the sets of outputs 29 and to the record logic circuit 23 via line 31. When, as more fully described below, the logic circuit 23 applies a digital command signal to the register 21, the storage units 30 sample and hold the time information provided by the seven sets of outputs.
Each of the storage units 30 corresponding to the minutes, tens of minutes, hours, tens-of hours, and day numbers is coupled by a decoder/driver 32 (not all shown), such as the Motorola 7447 circuit, in the decoder/driver unit 25 to an incandescent display device 33 such as the RCA DR 2010 Numitron. Thus, five Numitron devices 33 in the visual display unit 26 provide, in arabic numerals, the time when the digital command signal is applied to the storage register 21, this time being related, as more fully described below, to the opening orclosing of the door 12. It should be noted that because the storage register 21 holds the sampled binary decimal code, the Numitron devices provide a display of the sampled time until a subsequent opening or closing of the door 12 occurs. Thus, the visual display unit 26 always indicates the last opening or closing of the door. 1
As shown schematically, by lines 35-39 connected to brackets, the binary outputs of the storage units are connected to the digital recorder 24, and are used therein, as more fully described below, to control writing styluses.
In FIG. 4, there is shown a pair of magnetic reed switches 41, the switches being grounded at one end and connected at the other end to a line 43. Adjacent to the switches 41 there is located a pair of magnets 42. The magnets 42 and switches 41 make up the contents of the detector 22 (shown in FIG. 2) and, physically, are located in the walls of the slot 14 in enclosure 13 (see FIG. I). The magnets 42 are disposed in the Walls in a parallel opposing arrangement (FIG. 4) but at sufficient distance apart so their fields do not neutralize each other. Each magnet 42 is spaced with its associated switch 41 so as to produce a closing. Neutralization of the fields of magnets 42 will occur only when two magnets of sufficient strength and appropriate polarity are introduced into the slot as is the case when fin l1 enters. Thus the door is registered OPEN when either switch 41 is'closed but will only register CLOSED when both switches 41 are open.
Line 43 is connected within the record logic unit 23, via resistor 44 to a grounded capacitor 45, to one end of a resistor 46 whose otherend is connected via line 47 to a power supply 48, and to the input of an inverter 49. When the switches 41 are closed, the input to the inverter 49 is determined by the power supply 48 and the voltage divider, i.e., the resistors 44 and 46, and when the switches 41 are open, the input to the inverter 49 is determinedby the power supply, the resistor 46 and the capacitor 45. Thus, when the door 12 is open, the switches 41 are closed and the voltage at the input to the inverter is low relative to the voltage present at that input when the door 12 is closed, the switches 41 are opened and the capacitor 45 is charged.
The output of the inverter 49 and the output of a flipflop 52 are separately connected, via lines 50 and 53, respectively, to the input terminals of an EXCLUSIVE OR gate 51. The output of the EXCLUSIVE OR gate 51 is connected via line 54 to the trigger terminal of a flip-flop 55 and the output terminal of the flip-flop 55 is connected via line 56 to an input terminal of an AND gate 57. The power supply 48 is connected to a current source 59 and the current source'59 is connected via line 60 to a grounded storage capacitor 61. In addition, line 60 is connected to one end of a resistor 62. The other end of resistor 62 is connected to one end of a grounded resistor 63 and, via line 64, to the other input of the AND gate 57. In effect, resistors 62 and 63 comprise a voltage divider which is in parallel with the storage capacitor 61, the output of the voltage divider being connected to the AND gate 57. The output of the AND gate 57 is connected to the gate terminal of a silicon controlled rectifier (SCR) 68 via a resistor 65, the gate terminal also being connected toground by a resistor 66. In effect, the output of the AND gate 57 is connected to the gate terminal of an SCR 68 by a voltage divider. The cathode of the SCR 68 is grounded and its anode is connected to one of the terminals of a motor 67. The other terminal of the motor 67 is connected to the power supply 48. The circuitry described in this paragraph functions as follows.
Referring to FIGS. 4 and 5, if it is assumed that at a time prior to t the door 12 is open and the voltage on the output line 53 of the flip-flop 52 is high (see voltage waveform A in FIG. 5), the high output voltage on line 50 from inverter 49 (see voltage waveform B) plus the high on line 53 causes the voltage on the output line 54 of the EXCLUSIVE OR gate 51 to be low (see voltage waveform C). The output line 56 of the flip-flop 55 (see voltage waveform D) has been set low. If the capacitor 61 is fully charged, the voltage on line 64 will be high (see voltage waveform E). However, since the voltage on line 56 is low, the voltage at the output of the AND gate is low and will keep the SCR 68 in a nonconductive state. When, at a time t,, the door 12 is closed the output voltage of the inverter 49 drops to a low state (see voltage waveform B). The low input on line 50 and the high input on line 53 cause the output of the EXCLUSIVE OR gate 51 to rise to a high voltage state (see voltage waveform C). In turn, the high input to the flip-flop 55 causes the output of the flip-flop to go to a high voltage state (see voltage waveform D), thereby causing, via the AND gate 57, the SCR 658 to conduct. Conduction by the SCR 68 turns the motor 67 The motor 67 is mechanically coupled, via a gear train 71 (see FIGS. 4 and 6) to a sprocket drive 76, and to a shaft 72 on which there are mounted three cams 73, 74, 75. Cam 73 is used to control the state of a single pole double throw switch 77 Whose movable contact is grounded. One terminal of the switch 77 is connected to the anode of the SCR 68 and the other terminal is connected to one end of a resistor 80. The other end of resistor 80 is connected to a grounded capacitor 81, to one end of a resistor 82 whose other end is connected via line 47 to the power supply 48, and to the input of an inverter 83. In turn, the output of the inverter 83 is connected to the flip-flop 52 and as previously mentioned, via wire 31, to the storage register 21. When the motor 67 is at a standstill, the movable contact of the switch 77 is connected to the resistor 80. This switch condition is indicated by the lower level line in waveform F in FIG. 5. However, when the motor begins to rotate at for example time t the cam 73 throws the movable contact of the switch 77 into contact with the anode of the SCR 68. The thrown switch condition is indicated by the higher level line in waveform F. When the grounded movable contact of the switch 77 is disconnected from the resistor 80, the voltage input to the inverter 83 rises and-as a result, a voltage drop is provided over line 31 which causes the storage units 30 in the storage register 21 to sample and hold, as previously described, and causes the flip-flop 52 to go from its initially assumed high voltage state to a low voltage state (see waveform A). Thus, the inputs to the EXCLUSIVE OR gate 51 again have the same state (low) and the voltage state on its output line 54 drops without affecting the flip-flop 55. As will become appafent, this action prepares the circuit for another cycle of operation. It should be noted that when the cam 73 throws the switch 77 the motor is connected at one end to ground thereby causing the total voltage from the power supply 48 to be applied to the motor and reducing the voltage across the SCR to zero.
At a point during the time that the cam 73 keeps the movable contact of the switch 77 in contact with the motor, for example at time t cam 75 throws the movable contact of a switch 86 into contact with line 60. The thrown state of the switch is indicated by the higher level line of waveform G in FIG. 5. The armature of the switch 86 is connected to a metallic roller 87 (FIGS. 4 and 6) which is insulated from the enclosure arid over which a voltage sensitive paper 104 passes. As more fully described below, it is the discharge from the capacitor 61 through the roller 87 and the voltage sensitive paper which causes a writing to take place.
At a point during the time that the motor 67 is grounded and the capacitor 61 is connected to the rol ler 87, for example at time the cam 74 closes a switch 88 (FIG. 4). The closed state of switch 88 is indicated in FIG. 5 by the higher level line in waveform H. The movable contact of the switch 88 is connected via a resistor 89 to thepower supply 48, and a terminal of the switch 88 is connected via line 90 to the flip-flop 55. When the cam 74 closes the switch 88, a high voltage is applied to the flip-flop 55 and the flip-flop 55 is reset in preparation for another cycle. During the time that the high voltage is being applied to the flip-flop 55, at for example time 2 the cam 75 disconnects the storage capacitor 61 from the high voltage roller 87. Thereafter, the cam 74 opens the switch 88, at timer and cam 73 throws the switch 77 at time t It should be noted that at this point in the sequence all of the switches 77, 86, and 88 are in their initial positions.
Moreover, it should be noted that when switch 77 returned to its initial position, the ground was removed from the anode of the SCR 68 and the motor 67. Since the reset signal applied to the flip-flop 55 causes the output voltage on line 56 to go low, the output from the AND gate on line 69 (see waveform E) is low and keeps the SCR 68 in a non-conductive state.
As previously mentioned, the outputs of the storage register 21 are connected to the digital recorder 24. In FIG. 4 all of the outputs represented by lines 35-39 in FIG. 3 are represented by the single line and bracket 95. Although only one circuit is shown in FIG. 4, each of the actual outputs is connected, for example, by a resistor 96 to the base of a switching transistor 97 having a grounded emitter. In turn, the collector of transistor 97 is connected by a resistor 99 to one of the styluses 100. In addition to the outputs from the storage registers, the output of inverter 49 is connected via line 50 and a resistor 101 to the base of a transistor 102 having a grounded emitter, its collector being coupled to one of the styluses by a resistor 103. The styluses 100 are arranged in parallel and abut against the part of the voltage sensitive paper 104 (FIG. 6) which is in contact with the roller 87. Thus, when the storage capacitor 61 is connected to the roller 87 by switch 86, its discharge (see wave-form E at time t takes place only through those styluses 100 which are rendered conductive by the inputs to the transistors coupled to the styluses, i.e., the output of the storage register 21. The discharge process causes marks to be made on the voltage sensitive paper 104 which are related to the binary digital code held in the storage register 21, and a mark, due to the output of inverter 49, which indicates whether the recording corresponds to the time of an opening or closing of the door.
In the sequence of events described, it was assumed that the door 12 was moved from an open to a closed position. If at time t, the door 12 is opened, the output of the inverter 49 is caused to rise (see voltage waveform B) to a high voltage state. Since the output of the flip-flop 52 is, at that time, at a low voltage state, the outputs of the EXCLUSIVE OR gate 54 rises causing the reset flip-flop 55 output to rise. If the voltage across capacitor 61 has had a chance to build up to a level which is sufficient for writing purposes, the SCR 68 is again fired and another writing takes place. If the voltage across the capacitor 61 is too low for writing, the firing of the SCR 68 is delayed until the current source 59 has charged the capacitor 61. Typically the voltage required across the capacitor 61 to cause the SCR 68 to be fired is approximately 300 volts, and is a level which is dictated by the requirements of the voltage sensitive paper 104 used.
Although the capacitor 61 and the current source 59 could be replaced with a b 300 volt power supply, the arrangement described is preferred when the security system is to be used with a truck or the like which is already equipped with a 6 or 12 volt power supply.
Preferably, the power supply 48 is located within the enclosure 13 and is a battery which is continuously charged by the power supply in the truck. Thus, if for any reason the external power is lost, the system will continue to operate. Although it is not shown, the illuminated display may be used to indicate the loss of power. The latter may be achieved by connecting the power supply of the truck to the circuits associated with the display, and to the power supply 48, the power supply 48 only being connectedto the circuits associated with the recorder. Thus, if the power supply on the truck is disconnected or ceases to operate, the illuminated display will be extinguished but the recorder will continue to operate for a period of time determined by the capacity of the battery within the enclosure 13.
Referring to FIGS. 2 and 4, it may be noted that line 31 is connected to a switch 106 which is grounded. When switch 106 is in the closed position, the storage register 21 continuously samples the binary counter and the display unit 26 operates continuously. In this mode of operation, the switch 19 may be used to apply the higher frequency signals from the divider 18 to the binary counter 20 until the time shown by the display unit 26 corresponds to a desired reference such as eastern daylight time or the like. Once the display has been caused to correspond to a particular reference, the switch 106 may be moved to its open position and the system will operate as previously described.
Since the operation of the system requires that the switches 77, 86, and 88 operate sequentially, and that the most recently recorded times be visible without having to open the enclosure 13, a mechanical arrangement of parts for the recorder such as is shown in FIG. 6 is preferred. Referring to FIG. 6, it may be seen that the motor 67 is mounted on the base 110 of the enclosure 13 between aback wall 111 and a partition 112 (partly shown) which is parallel to the back wall. Partition 112 and back wall 111 rotatably support the rod 72' on which the cams 73-75 (not all shown) are mounted. Partition 112 and back wall 111 also support between them the sprocket drive 76, the high voltage roller 87, via insulated bearings (not shown), a rod 105 for supporting the supply roll of voltage sensitive paper 104, and a rod 114 which supports an insulated block 115 carrying the styluses 100. In addition, the partition 112 and back wall 111 support a pair of rods on which the switches 77, 72, and 86 (not all shown) are mounted, the switches having followers which, respectively, ride the cams 73- 75. It should be noted that the voltage sensitive paper 104 extends from the rod 105, passes between the styluses 100 and the high voltage roller 87, under the transparent window 15, on to the sprocket drive 76, and then to a take-up roller (not shown). Thus, when the motor 67 is turned on, the gear train 71 rotates the cams 77-75 and the sprocket drive 76, thereby sequentially operating the switches 72, 77, 86 and advancing the resulting writings on the voltage sensitive paper 104 under the transparent window 15. The size of the window is preferably large enough so that several most recently recorded openings and closings on the paper are visible at any one time.
To insure that the security system itself is not tampered with, it is preferred that the enclosure 13 and the transparent windows 15 and 16 (see FIG. 1) be made from metal and unbreakable or bullet-proof glass, respectively. Further, since the top 27 of the enclosure must be opened periodically to reload the system with voltage sensitive paper, it is preferred that a hidden combination lock (not shown) be provided to keep the top secure and that the combination of the lock be known only by trusted personnel.
In summary, a secure enclosure 13 houses an illuminated display and an electromechanical recorder which respond to the opening or closing of a door 12. Each response causes the time of the event to be incandescently displayed in arabic numerals and permanently recorded in binary decimal code on voltage sensitive paper. The system operates from a low voltage source such asis found on vehicles and as a result, the system may be used to monitor the cargo doors of vehicles, the information obtained from the system being useful to determine when thefts occur.
Although the embodiment of the invention described herein provides a coded record on voltage sensitive paper, other embodi-ments of the invention could, in
place thereof, utilize equivalent imprinters which are heat, pressure or light sensitive to provide coded or uncoded e.g., arabic, written records. Moreover, equivalent displays which are luminescent, florescent, etc. may be substituted by those skilled in the art for the incandescent display described, and movers other than the DC. motor, e.g., stepping motors, solenoids, may be used to drive the recorder.
Accordingly, it is to be understood that the description herein of a preferred embodiment, according to the invention, is set forth as an example thereof and is not to be construed or interpreted as a limitation on the claims which follow and define the invention.
What is claimed is:
l. A security system for an enclosed area having a door, comprising:
a. means for providing a periodic pulse signal;
b. means responsive to the pulse signal for providing a continuous indication of time;
c. a detector for sensing each opening or closing of the door;
(1. means responsive to the detector for sampling and storing the indication of time corresponding to the time when the last sensed opening or closing of the door occurred; and
e. means coupled to the sampling and storing means (d) and to the detector, for providing awritten record of the time when each of the sensed openings or closings of the door occurred,
said means for providing a written record including signal sensitive paper, means for linearly moving in succession a different section of the signal sensitive paper past a predetermined location each time an opening or closing of the door is sensed, and means for applying a signal to the section of paper moving past the predetermined location; thereby writing the time of said opening or closing of the door.
2. A security system as defined in claim 1 wherein the means for providing a continuous indication of time include means for correlating the time indication provided with a real time reference.
3. A security system as defined in claim 1 wherein the detector includes magnetic means which are actuated by the opening or closing of the door, and means for sensing the presence therein of the actuated magnetic means.
4. A security system as defined in claim 1 further including:
a digital display device; and means coupled to the sampling and storage means (d) for driving the display device, whereby the display device provides a visible indication of when the door was last opened or closed.
5. A security system as defined in claim 4 wherein the means for providing a continuous indication of time includes means for correlating the visible indication prodoor, comprising:
vided by the display device with a real time reference.
a. means forproviding a periodic pulse signal;
b. means responsive to the pulse signal for providing a continuous indication of time;
c. a detector for sensing each opening or closing of the door;
d. means responsive to the detector for sampling and storing the indication of time corresponding to the time when the last sensed opening or closing of the door occurred; and
e. means coupled to the sampling and storing means (d) and to the detector, for providing a written record of the time when each of the sensed openings or closings of the door occurred, said means for providing a written recordincluding voltage-sensitive paper,
means for moving sections of the voltage-sensitive paper past a predetermined location each time an opening or closing of the door is sensed, and
means for providing a voltage across the thickness of the section of paper moving past the predetermined location, thereby writing the time of said opening or closing of the door.
7. A security system as defined in claim 6 further including means for applying a voltage to the paper indicating whether the movement of the door is an opening or a closing.
8. A security system as defined in claim 6 wherein the means for moving the voltage-sensitive paper includes:
exceeds a predetermined value and an opening or closing of the door is sensed, and wherein said means for applying a voltage across the thickness of the section of paper moving past the predetermined location is responsive to movement of said motor.
9. A security system as defined in claim 8, wherein the continuous indication of time is in binary decimal code; and wherein said means responsive to the movement of the motor include:
a switch connected to the voltage source;
a cam driven by the motor for closing the switch momentarily each time the door is opened or closed;
a roller electrically connected to the switch;
a plurality of writing styluses arranged in parallel adjacent the roller, the space between the writing styluses and rollers defining the predetermined location; and
means coupled to the sampling and holding means (d) for selectively grounding said writing styluses according to the binary decimal code corresponding to the time the door is opened or closed, thereby providing a writing on the paper which indicates in binary decimal code the times when the door is opened or closed.
10. A security system as defined in claim 9 wherein said means for selectively grounding said writing styluses include a plurality of solid state switches, each of said solid state switches being connected to only one of the writing styluses.
11. A security system as defined in claim 8 wherein the voltage source includes a power supply, a current source connected to the power supply, a capacitor connected to the output of the current source, whereby the capacitor may be charged to a voltage which exceeds the output voltage of the power supply and is sufficient for writing on the voltage sensitive paper.
12. A security system as defined in claim 11 wherein the security system includes means for delaying the writing of a time corresponding to the opening or closing of the door until the capacitor has been charged to a writing level.