fig1 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 . fig2 shows a partially exploded view of the digital tape measure of fig1 , after hingeable front cover 32 has been opened and side cover 56 ( fig1 ) 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 fig2 . 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 fig3 , 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 fig3 , 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 . fig3 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 . fig4 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 . fig6 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 fig6 , 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 . fig7 a 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 ( fig2 ). 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 fig2 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 . fig7 b 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 ( fig3 ). referring to fig7 a 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 fig2 . fig8 shows an expanded view of the stationary guide 78 shown in fig2 . 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 . fig9 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 fig9 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 . fig1 a - 11d show preferred logic subroutines used for operating the digital tape measure . fig1 shows electronic circuitry mounted atop the printed circuit board ( fig2 ) for performing the steps shown in the logic subroutines of fig1 a - 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 .