Abstract:
A digital measuring instrument includes a housing containing a measuring line, a rotatable drum engaging the line whereby movement of the line causes rotation of the drum, a storage spool mounted in the housing for storing the line, a crank assembly for rotating the storage spool for winding the measuring line about the storage spool, and a rotatable disc that rotates simultaneously with the measuring drum. The instrument includes an optical sensor in communication with the rotatable disk for compiling data related to rotation of the disk, and electronic circuitry in communication with the optical sensor for calculating a length of the line drawn from the housing. The housing has a well for receiving the storage spool and a side cover with a slideable lock for covering the well.

Description:
FIELD OF THE INVENTION 
   The present invention is related to tape measures used for measuring distances and is specifically related to electronic tape measures. 
   BACKGROUND OF THE INVENTION 
   Tape measures have been used for many years to accurately measure distances between two points and the lengths of objects. Tape measures typically include a housing having an opening through which a measuring tape is drawn. The measuring tape has indicia printed thereon that are used to measure distances. When measuring a distance, the measuring tape is unwound from a storage spool as it is drawn through the opening of the housing. When the measuring task is complete, the measuring tape is rewound onto the storage spool. 
   In recent years, electronic tape measures have been developed which electronically track the length of a measuring tape unwound from a storage spool. Electronic tape measures typically include a reading element in communication with the storage spool that generates electric pulses corresponding to the length of the measuring tape drawn from the spool. The electric pulses are counted by operational circuitry, which converts the pulses into a digital readout provided on a visual display. 
   One electronic tape measure is disclosed in U.S. Pat. No. 5,142,793 to Crane which teaches a housing, a reel located within the housing and a measuring tape wound on the reel. The measuring tape is extendable through an opening in the housing as the reel is unwound. A first measuring element is associated with the reel for generating incremental measuring data and a second measuring element generates absolute measuring data as the measuring tape is extended. A processor responsive to both the incremental measuring data and the absolute measuring data generates an output that reflects the linear extension of the measuring tape from the housing. The device also includes a visual display for showing information related to the linear extension of the measuring tape. 
   U.S. Pat. No. 5,426,863 to Biggel discloses a combination tape measure and calculator. The tape measure includes a measuring tape having equally spaced holes along the length thereof. The holes are read by closely spaced optical readers arranged to generate incremental unit signals that are transmitted to a microprocessor mounted in a case. 
   U.S. Pat. No. 5,433,014 to Falk et al. discloses a digital tape measure including a transmitter for transmitting measurements to a remote location. The tape measure includes a housing, a measuring tape having a series of optical markings, an optical sensor for sensing the optical markings when the tape is extended or retracted, a logic circuit in communication with the optical sensor for providing a digital signal representative of the length of the tape drawn from the housing, and a transmitter connected to the logic circuit for transmitting the digital signal to the remote location. A receiving unit is provided at the remote location for receiving, storing and displaying the measurements. 
   U.S. Pat. No. 5,286,972 to Falk et al. discloses a photoelectric measuring device with a digital display. The measuring device measures distance by optically scanning the length of a line as it is unwound from a reel. The line is provided with a plurality of dye marks placed at spaced intervals. The dye marks are detected by an optical sensor in communication with a circuit that maintains a count indicative of the length of the line. The count is displayed on an LED display. 
   The above-described devices generally use measuring tapes having a substantial width and thickness that limits the length of tape, such as 100 meters or less, that can be conveniently stored on a single spool. Tape measures having distances of over one hundred meters are rare because longer tapes will substantially increase the overall size of the tape measure housing, thereby making the tape measure extremely bulky and burdensome to operate. 
   Another problem with conventional tape measures relates to maintenance and replacement costs. Tape measures are frequently damaged because they are used in harsh environments resulting in the measuring tape being damaged. A damaged measuring tape cannot accurately measure a distance or cannot be rewound onto a storage spool. As a result, the measuring tape must be repaired or replaced at substantial cost. In many instances, a brand new tape measure must be purchased. 
   Another problem relates to the accuracy of electronic tape measures. Many conventional electronic tape measures are inaccurate or unreliable because they don&#39;t account for the hanging radius of the measuring tape on the storage spool as the tape is unwound from the storage spool. 
   Thus, there is a need for an electronic tape measure that enables users to measure longer distances. There is also a need for a durable electronic tape measure, which allows the measuring element extended from a housing to be easily replaced, at low cost. There is also a need for an electronic tape measure that is accurate and reliable. 
   SUMMARY OF THE INVENTION 
   In accordance with certain preferred embodiments of the present invention, a digital tape measure includes a housing containing a rotatable storage spool, and a measuring line windable about the storage spool. The tape measure also preferably includes a measuring drum rotatably mounted in the housing and including a surface in contact with the measuring line, whereby movement of the measuring line causes rotation of the measuring drum. A disc is desirably coupled with the measuring drum, whereby rotation of the measuring drum due to movement of the measuring line causes simultaneous rotation of the disc. The digital tape measure also includes an optical sensor in communication with the disc for compiling data related to rotation of the disc, and electronic circuitry in communication with the optical sensor for processing the compiled data so as to calculate a length of the measuring line drawn from the housing. The housing preferably includes a visual display, such as a digital display, adapted to show the calculated length of the measuring line that has been drawn from the housing. 
   The disc is preferably a circular disc that is rotatable about a shaft. The shaft preferably links the disc with the rotatable measuring drum. The disc preferably includes a peripheral edge having a plurality of evenly spaced teeth. The teeth desirably define evenly spaced gaps therebetween. When the disc is inserted into the housing of the digital tape measure, the teeth of the disc are preferably in substantial alignment with the optical sensor. As a result, the rotating teeth are in alignment with the optical sensor. 
   The optical sensor of the present invention desirable includes a light generating element such as a light emitting diode, directed toward a first face of the rotating disc, and a light sensing element, such as a photo sensor, directed toward a second face of the disc. The light-generating element and the light sensing element preferably oppose one another and are in substantial alignment with one another. As the disc rotates, the optical sensor generates a first signal when the light from the light generating element reaches the light sensing element. When the disc rotates so that one of the teeth is aligned between the light generating element and the light-sensing element, the optical sensor generates a second signal indicating such a condition. As a result, the optical sensor generates a first pulse signal when light from the light generating element passes through one of the gaps between the teeth of the disc to reach the light sensing element, and a second pulse signal when light from the light generating element is blocked from reaching the light sensing element by one of the teeth of the disc. 
   The digital tape measure desirably includes a printed circuit board including electronic circuitry in communication with the optical sensor. The electronic circuitry is selected from the group consisting of analog circuitry and digital circuitry. The printed circuit board also preferably includes one of more microprocessors having one of more subroutines stored therein for operating the electronic circuitry. The electronic circuitry preferably operates in conjunction with software that enables a user to re-calibrate the unit for different types of line diameters and/or line materials. The calibration data is permanently stored in memory, even when battery power is out and even if battery power is not used. As a result, a user may change the measuring line to use a line having a different diameter or material, and then re-calibrate the unit to work accurately with the new measuring line. In certain preferred embodiments, the software is adapted to count pulses for a predetermined length of measuring line that passes the optical sensor. In one particular preferred embodiment, the software is adapted to count two (2) pulses for each 1 mm (0.04″) of measuring line, thereby ensuring maximum precision and accuracy. 
   The measuring line drawn from the storage spool is preferably flexible. In certain preferred embodiments, the measuring line is made from a material selected from the group consisting of cotton, metal and synthetic materials such as nylon and polyester. In one preferred embodiment, the measuring line is string. In another preferred embodiment, the measuring line is made of stainless steel. The measuring line is wound about the storage spool and has a length of approximately 750 to 1500 meters. Although not limited by any particular theory of operation, it is believed that using a measuring line, as opposed to a wide measuring tape, enables a longer length of measuring material to be wound about a storage spool. As is well known to those skilled in the art, a conventional measuring tape has substantial thickness and width that occupies a substantial amount of space on a storage spool. This dramatically limits the length of measuring tape that may be stored in a hand carried tape measure. In contrast, the present invention provides an elongated, flexible measuring line that has minimal width and thickness. As a result, a substantially longer length of material may be wound about a storage spool, thereby maximizing the distance that can be measured using the present invention. 
   In certain preferred embodiments, the housing includes a hingeable front cover having a button that may be depressed for allowing the front cover to be swung from a closed position to an open position. In the open position, an interior region of the housing may be accessed. The present invention also desirably includes a crank assembly including a crank handle. The crank assembly is coupled with the storage spool for providing an element for rewinding the measuring line about the storage spool. 
   In certain preferred embodiments, the housing includes a well adapted to receive the storage spool and a removable cover securable over the well. The side cover includes a lock for selectively uncovering the well. As a result, a storage spool having worn or damaged measuring line wound about it may be replaced with another storage spool having new or repaired measuring line. Thus, the removable side cover enables a replacement storage spool with measuring line to be quickly and easily installed in the housing. 
   The present invention also preferably includes a measuring drum assembly including the measuring drum, whereby the measuring drum is mounted on a shaft carrying a first gear that is mechanically coupled with the rotating disc in communication with the optical sensor. The measuring line is threaded through the measuring drum assembly so that movement of the measuring line causes simultaneous rotation of the measuring drum. Due to the gear and shaft linkages described above, rotation of the measuring drum causes simultaneous rotation of the disc. In certain preferred embodiments, the measuring drum includes at least one stationary guide, at least one roller guide, and at least one cleaner guide for cleaning an exterior surface of the line. For example, if the measuring line is made of cotton, the cleaner guide may remove extraneous matter present at an outer surface of the measuring line. Thus, the measuring drum assembly defines a threaded path for the measuring line as the measuring line passes between the storage spool and the opening in the hingeable front cover of the housing. The measuring drum assembly preferably maintains at least a section of the measuring line in non-slip engagement with the surface of the measuring drum, and maintains the measuring line under tension. As used herein, the term “non-slip surface” means that there will be no slippage between movement of the measuring line and the outer annular surface of the measuring drum. 
   Other preferred embodiments of the present invention disclose an electronic measuring device having a flexible measuring line including a housing having an opening for the measuring line, a storage spool rotatably mounted in the housing for storing the measuring line, and a measuring drum rotatably mounted in the housing and including a surface in contact with the measuring line, whereby movement of the measuring line through the opening of the housing causes rotation of the measuring drum. The electronic measuring device also desirably includes a disc coupled with the measuring drum for rotating simultaneously with the measuring drum, an optical sensor in communication with the disc for generating signals related to rotation of the disc, and electronic circuitry in communication with the optical sensor for processing the generated signals so as to calculate a length of the measuring line passing through the opening of the housing. 
   Other preferred embodiments of the present invention include an electronic tape measure comprising a housing adapted to store measuring line, the housing having an opening for drawing the measuring line from the housing, and a measuring drum mounted in the housing and including a non-slip surface in engagement with the measuring line, wherein movement of the measuring line rotates the measuring drum. The electronic tape measure also includes a disc coupled with the measuring drum for rotating simultaneously with the measuring drum, an optical sensor in communication with the disc for recording data related to rotation of the disc, and electronic circuitry in communication with the optical sensor for processing the recorded data so as to calculate a length of the measuring line passing through the opening of the housing. This particular embodiment also preferably includes a storage spool rotatably mounted in the housing for receiving the measuring line, and a crank assembly connected with the storage spool for rotating the storage spool so as to wind the measuring line about the storage spool. 
   Still other preferred embodiments of the present invention include a digital tape measure having a housing containing a measuring line, and a rotatable measuring drum in engagement with the measuring line so that movement of the measuring line causes rotation of the measuring drum. The digital tape measure also desirably includes a rotatable disc coupled with the measuring drum for rotating simultaneously with the measuring drum, an optical sensor in communication with the disc for compiling data related to rotation of the disc, and electronic circuitry in communication with the optical sensor for processing the compiled data so as to calculate a length of the measuring line drawn from the housing. The optical sensor desirably includes a light-generating element opposed by a light-sensing element that is adapted to sense light emitted from the light-generating element. The optical sensor desirably generates a first signal when light from the light-generating element reaches the light-sensing element, and a second signal when the light from the light-generating element is blocked by one of the teeth of the disc. 
   These and other preferred embodiments of the present invention will be described in more detail below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a side view of a digital tape measure, in accordance with certain preferred embodiments of the present invention. 
       FIG. 2  shows a partially exploded view of the digital tape measure shown in FIG.  1 . 
       FIG. 3  shows a cross-sectional view of the digital tape measure shown in  FIGS. 1 and 2 , including an optical sensor secured in a housing. 
       FIG. 4  shows a front end view of the optical sensor shown in FIG.  3 . 
       FIG. 5  shows a bottom view of the optical sensor shown in FIG.  4 . 
       FIG. 6  shows a sectional view of the optical sensor taken along line VI—VI of FIG.  5 . 
       FIG. 7A  shows an exploded view of measuring drum assembly for a digital tape measure, in accordance with certain preferred embodiments of the present invention. 
       FIG. 7B  shows the measuring drum assembly of  FIG. 7A  in a partially assembled state with a measuring line threaded therethrough. 
       FIG. 7C  shows a perspective view of the measuring drum assembly of  FIG. 7A  in a fully assembled state. 
       FIG. 8  shows an expanded view of a stationary guide for a measuring line, in accordance with certain preferred embodiments of the present invention. 
       FIG. 9  shows a top plan view of a control panel for a digital tape measure, in accordance with certain preferred embodiments of the present invention. 
       FIG. 10  shows a perspective view of the digital tape measure shown in FIG.  9 . 
       FIGS. 11A-11D  show subroutines for operational circuitry of a digital tape measure, in accordance with certain preferred embodiments of the present invention. 
       FIG. 12  shows a schematic of electronic circuitry of used to perform the subroutines shown in FIGS.  11 A- 11 D. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows a digital tape measure  20  including a housing  22  having a front  24 , a rear  26 , atop a bottom  30 . In certain preferred embodiments, housing  22  is made of molded plastic. Housing  22  includes a hingeable front cover  32  movable between open and closed positions. Hingeable front cover  32  includes a cutting element  36  attached thereto for selectively cutting or trimming a measuring line (not shown). Hingeable front cover  32  includes an opening  38  through which the measuring line can be drawn from housing  22 . 
   Housing  22  also includes a handle  40  that extends along rear  26  between the top and bottom  28 ,  30  of housing  22 . Handle  40  is preferably integrally molded with the housing  22 . A gripping surface  42  is preferably secured over an exterior surface of handle  40  for improving the gripability of the handle. The handle  40  also preferably includes one or more finger grooves  44  for enhancing a user&#39;s grip. 
   Housing  22  preferably includes a first molded part  46  and a second molded part  48  that are assembled together and held together by screws  50 ,  50 A,  50 B and  50 C passed through openings  52 A,  52 B and  52 C formed in first molded part  46 . Screws  50 A- 50 C are preferably anchored in threaded apertures (not shown) formed in second molded part  48  for reliably securing first and second molded parts  46 ,  48  together. 
   Housing  22  also preferably includes a battery compartment cover  54  that selectively closes a battery compartment opening  55  at a lower end of handle  40 . Housing  22  also preferably includes a removable side cover  56  that covers a well formed in housing  22  that is adapted to receive a measuring line storage spool (not shown). Removable side cover  56  includes a slidable lock  58  that may be urged towards a central knob  60  for releasing side cover  56  from its attachment with housing  22 . 
     FIG. 2  shows a partially exploded view of the digital tape measure of  FIG. 1 , after hingeable front cover  32  has been opened and side cover  56  ( FIG. 1 ) has been detached from housing  22 . The well  62  is adapted to receive a storage spool  64  having measuring line  66  wound thereon. Spool  64  has a central opening  68  that is mounted over spool gear  69  having a central shaft  70  and spaced projections  72  that are engageable with similarly shaped, spaced depressions (not shown) formed on an underside of storage spool  64 . The spaced projections  72  provide positive alignment of storage spool  64  with gear  74 . 
   Digital tape measure  20  also includes a measuring drum assembly  76  insertable into the front  24  of housing  22  when hingeable front cover  32  is in the open position shown in FIG.  2 . As will be described in more detail below, measuring drum assembly  76  is designed to receive a free end of measuring line  66  supplied from storage spool  64 . Measuring drum assembly  76  includes a series of guides for threading the measuring line  66  therethrough. Housing  22  also includes a stationary guide  78  having an opening  80  extending therethrough for engaging and guiding the measuring line  66  after the measuring line has passed through a front end of measuring drum assembly  76 . 
   The digital measuring tape also includes an optical sensor  82  including a light generating element  84  and a light sensing element  86  that are directed towards one another on opposite faces of a rotatable disc  88  having teeth  90  at the periphery thereof and gaps  92  between the teeth  90 . The teeth  90  are preferably evenly spaced from one another. 
   Hingeable front cover  32  includes a depressible button  96  and a projection  98  integrally connected to button. When button  96  is depressed, projection  98  moves in an arcuate path shown by arrow A 1 . When hingeable front cover  32  is moved in a clockwise direction toward the top  28  of housing  22 , projection  98  is securable under locking bar  100  for holding hingeable front cover  32  in the closed position. In order to open hingeable front cover  32  once again, button  96  is depressed so as to uncouple projection  98  from engagement with locking bar  100 . 
   Digital measuring tape  20  also includes a printed circuit board  102  secured within housing  22 . The printed circuit board  102  preferably includes microelectronic elements (not shown) such as microprocessors and memory devices for controlling electronic circuitry coupled with printed circuit board  102 . The printed circuit board  102  is preferably in communication with optical sensor  82  for obtaining data from optical sensor  82  when disc  88  rotates as measuring line  66  is drawn through the opening  38  of hingeable cover  32 . 
   Referring to  FIG. 3 , digital tape measure  20  also includes a brake  104  having a pivotable brake knob  106  with an underside  108  coupled with a first end  110  of a first lever  112 . First lever  112  also includes a lower end  114  coupled with a second lever  116 . The second lever  116  has a lower end with teeth  118  with teeth that mesh with crank assembly gear  120  having teeth  122 . The brake knob  106  is pivotable between a first unlocked position and a second locked position. In the first unlocked position, the teeth  118  of second lever  116  are remote from the teeth  122  of crank assembly gear  120  so that crank assembly gear is free to rotate. In a second locked position shown in  FIG. 3 , the teeth  118  of second lever  116  engage the teeth  122  of crank handle gear  120  so that crank handle gear  120  cannot rotate. In operation, the brake  104  is movable back and forth between the first unlocked position and the second locked position for selectively locking movement of the crank handle gear  120 . 
     FIG. 3  also shows rotatable disc  88  mounted in housing  22  for rotating in response to movement of a measuring drum, as will be described in more detail below. The teeth  90  at the periphery of disc  88  are designed to pass between light generating element  84  and light sensing element  86  of optical sensor  82 . Optical sensor  82  is preferably in communication with printed circuit board  102  via communication line  124 . 
     FIGS. 4 and 5  show a front end view of optical sensor  82  including light generating element  84  and light sensing element  86 . Disc  88  is provided between light generating element  84  and light sensing element  86 . Disc  88  has a first face  126  facing toward light generating element  84  and a second face  128  facing toward light sensing element  86 . The teeth  90  at the periphery of disc  88  are adapted to pass between the light-generating element  84  and the light-sensing element  86  as disc  88  rotates. 
     FIG. 6  shows the teeth  90  of disc  88  passing by light generating element  84 . When one of the gaps  91  of disc  88  is aligned between the light generating element and the light-sensing element, light generated by light generating element  84  is able to pass through the gap  91  and be sensed by light sensing element  86  (not shown). When one of the teeth  90  is positioned between the light generating element and the light sensing element, as shown in  FIG. 6 , the light emitted from the light generating element  84  is not sensed by the light sensing element. In operation, optical sensor  82  generates a first signal when the light passes through one of the gaps  91  and a second signal when the light is blocked by one of the teeth  90 . The first and second signals are processed by the operational circuitry to determine the length of the measuring line drawn from the housing. 
     FIG. 7A  shows an exploded view of a measuring drum assembly  76 , in accordance with certain preferred embodiments of the present invention. Measuring drum assembly  76  includes a base member  130  and a cap  132  securable over the base member. The base member  130  includes a centrally located well  134  adapted to receive measuring drum  136  having an annular surface  138  adapted to engage the measuring line unwound from the storage spool (FIG.  2 ). The measuring drum  138  includes a central hub  140  having an opening  142  adapted to receive shaft  144 . Shaft  144  has a lower end (not shown) passable through an opening  146  in the bottom of well  134  for supporting rotation of measuring drum  136 , and an upper end  148  having a drive gear  150  mounted thereon. The drive gear  150  rotates at the same rate as measuring drum  136 . A bearing  152 , mountable at the upper end  148  of shaft  144  is insertable into a top wall aperture  154  of cap  132 . 
   Referring to  FIGS. 2 and 7A , base member  130  includes an upstream end  156  and a downstream end  158 . When the measuring drum assembly  76  is secured within the housing  22 , the upstream end  156  is positioned adjacent the stationary guide  78  and the downstream end  148  is positioned adjacent the opening  38  of hingeable front cover  32  when the hingeable front cover is in the closed position. Base member  130  includes a first stationary line guide  160  securable in U-shaped groove  162  and a first line cleaner guide  164  securable in cup  166 . Bottom assembly  130  also includes two roller guides  168 ,  170  for guiding movement of the measuring line. Base assembly  130  also includes a second stationary guide  172  securable in second U-shaped groove  174 , a second line cleaner guide  176  secured in a second cup  178  adjacent the downstream end  158  of base member  130 , and a third stationary guide  179  between the second stationary guide  172  and second line cleaner guide  176 . 
     FIG. 7B  shows measuring line  66  threaded through the base member  130  of measuring drum assembly  76 . The measuring line is drawn from storage spool  64 . The measuring line is threaded in the following pattern: first stationary line guide  160 , first line cleaner guide  164 , first roller guide  168 , wrapped approximately one and two-thirds times around measuring drum  136 , around second roller guide  170 , through second stationary line guide  174 , through third stationary line guide  175  and through second line cleaner guide  176  at downstream end  158 . The measuring line  66  is then passed through the opening  38  in hingeable front cover  32  (FIG.  3 ). 
   Referring to  FIGS. 7A and 7C , after the measuring line  66  has been threaded through the measuring drum assembly  76 , the top cover  132  and base member  130  are assembled together, such as by snap-fitting the top cover  132  and base member  130  together. The fully assembled measuring drum assembly  76  is insertable into the front end of housing  22  as shown in FIG.  2 . 
     FIG. 8  shows an expanded view of the stationary guide  78  shown in FIG.  2 . The stationary guide  78  includes an opening  80  that provides a guide for the measuring line (not shown) between the storage spool and the upstream end  156  of measuring drum assembly  76  when the measuring drum assembly is inserted into housing  22 . 
     FIG. 9  shows a plan view of the top  28  of housing  22 . The housing  22  includes hingeable front cover  32  and depressable button  96  for enabling hingeable front cover to be swung away from a front end of housing  22 . Hingeable front cover also includes opening  38  that enables measuring line  66  to be drawn therethough. 
   The housing  22  has a visual display  180  such as a liquid crystal display. Visual display  180  includes a main readout  182  providing a six-digit numeric with comma and decimal point for decimals. The main readout  182  provides current measuring line readings and displays the value of memory and recall locations. Visual display  180  also includes a negative sign  184  that is displayed when the value on the main readout  182  is negative. The negative sign indicates that the measuring line is retracted from a “rezero” position. Visual display  180  also includes a memory indicator  186  that is displayed when memory is selected. The memory indicator  186  indicates which of the 15 memory storage locations is being used and when all of the 15 memory locations are “full.” At that point, a prompt “overwrite? Y/N” indicator  188  will illuminate. If a user desires to overwrite the previously stored memory at that memory location, the user will press the yes button below. 
   The visual display also includes a recall indicator  190  that is displayed when recall is selected. The recall indicator  190  indicates the last stored measurement and the corresponding memory location ( 1 - 15 ). Pressing the recall button again will step through to the next measurement stored in memory. Visual display  180  also includes location indicator  192  that displays the storage location of the saved value or the recall value in memory. Visual display  180  also has a sound activated indicator  194  that indicates when the sound function has been toggled to the “on” position. Unit indicator  196  denotes the type of units that are being displayed in the main readout  182 . Rezero indicator  198  displays when the “On/Zero” button is pressed to rezero the display. Overwrite option indicator  188  is illuminated in the event that a user attempts to save a measurement in an occupied memory location. At that point, the user is prompted to confirm the action with the overwrite option indicator  188 . Once a “yes” or “no” is selected, the overwrite option indicator is turned off. 
   The visual display  180  also includes an add indicator, represented by the “+” sign  200 , that confirms that the user is in the “add” mode. Low battery indicator  202  illuminates when battery voltage is below a predetermined level, such as 4.8 volts. Overspeed indicator  204  illuminates when the digital tape measure will not function properly due to the measuring line  66  moving at a speed of over 10 feet per second. If the overspeed indicator  204  illuminates, a user should remeasure a length, while insuring that the measuring line does not move at a speed of over 10 feet per second. The calibration indicator  206  is displayed when the “On/Zero” button is depressed and held for five seconds. At that point, the digital tape measure  20  enters calibration mode. 
   Digital tape measure  20  also includes a series of data entry keys for controlling operation of the device. The data entry keys include add button  208 , Clear/No button  210 , unit button  212 , recall button  214 , Enter/Yes button  216 , sound activating button  218 , memory storage button  220  and On/Zero button  222 . The housing  22  also includes brake button  106  that may be toggled back and forth between a locked position and an unlocked position in which the measuring line  66  can be drawn from the housing  22 . 
   Referring to  FIGS. 9 and 10 , the tape measure also includes a crank assembly  224  having a crank handle  226  that is coupled with the storage spool gear  69  for rotating storage spool  64  to wind measuring line  66  about storage spool  64 . When the brake assembly  106  is moved into the locking position, the crank assembly  224  is locked in place, thereby preventing further movement of the measuring line. 
     FIGS. 11A-11D  show preferred logic subroutines used for operating the digital tape measure.  FIG. 12  shows electronic circuitry mounted atop the printed circuit board ( FIG. 2 ) for performing the steps shown in the logic subroutines of  FIGS. 11A-11D . 
   Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the embodiments described herein and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.