Almost without thinking, we explain our relationship to time through many different metaphors. Metaphors for time – its passing and measurement – are apparently the most commonly used metaphors in the English language. We are taught how to represent time – both elapsed and absolute – using a circular clock face, and to use phrases like “the top of the hour” that reference this metaphor. The metaphor seems apt considering the repeating and “circular” nature of our daily perception of time, and the use of the clock face to display time is ubiquitous.
But then again we also reference the linear metaphor of a line stretching into the distance, when we talk in terms of lengths of time, such as the question “how long did that take”? So, why not design a clock that exploits this notion of “length(s) of time”?
Introducing the “Length of Time Clock”… a novel clock that measures time – literally!
Yep. The “Length Of Time Clock” displays the time through the linear measurement of extending tape measures… You literally measure time. As a 24 hour clock – and being at one with Imperial and metric units of length – hours are measured in inches while both the minutes and seconds are measured in centimeters.
Each of three STANLEY measuring tapes are extended and retracted by a toothed belt driven by a NEMA17 4-wire stepper motor. The extension of the tape – it’s length – reflects the current time in hours (in inches), minutes and seconds, both in centimeters.
The unit is constructed of two wooden boxes, connected together by 1/4×20 threaded rod. One box houses the stepper motors and electronics while the other houses the tape measures, pulley wheels and components for the homing switches.
Each of the the stepper motors is driven its own stepper driver (DRV8825) which is controlled through a pair of pins – direction (DIR) and step (STEP). These pins are connected to an Arduino Nano and uses the AccelStepper software library. The software converts data from a real-time clock to the appropriate control to each of the steppers to control their movement and absolute position. The drivers have three pins to determine different step-sizes and in this application, the smallest step-size is selected by tying all three pins HIGH. The enable pin of each driver (active LOW) is connected together to a pin on the Nano. During testing, this was also connected to a “kill switch” used in case the steppers did not respond as desired and drove the tapes too far in either direction. Following debug, this switch was used as a reset button to force the software to execute the homing routines.
Small rare-earth magnets and A3144 Hall-Effect sensors (not shown) are used to “home” each tape measure to a known position so that the software can then control the “absolute” position of each tape in sympathy with the time of day. The small magnet is glued to the underside of the tape-measure tab, while the A3144 is glued to the underneath of the tape measure shell, such that when the tape is fully retracted, the sensor’s output goes LOW. The three connections for each sensor (Vcc, GND and output) are wired to a small junction board and the three sensors are connected (via a ribbon cable) to three inputs of the Nano.
So now, when anyone asks you some question beginning with “how long until … “, remember to first clarify “Imperial or Metric?”