Electronics Forum / Basics / October 2006

Hi, Ken.  Easiest way is to use a gearmotor with a cam to trigger a
microswitch to stop the motor after every turn.  You can use a relay to
override the cam to start for the next turn.  I can assure you a
toymaker would look at this option first -- it's cheapest and most
reliable.

If you want something a little more esoteric, you might want to place
magnets on each floor, and have a hall-effect sensor in the elevator
(or at slightly greater expense, do the reverse).  You might also have
a separate sensor or switch for the floor (home position).

If those don't do it, your encoder setup might be relatively simple.
By reading quadrature, you can tell distance *and* direction.  Again,
you'll need a home sensor.

Another possibility would be using a long threaded bolt with a riding
nut to get the distance.  If you use a metric pitch, your 5mm could be
just a certain integral number of revolutions.  Again, a switch or
sensor to determine revolutions, and one for the home position, and
you're done.  This means gives you a great mechanical advantage, which
can be a blessing or a curse.

For any of this, counting revolutions may be a hassle.  If so, you
might want to use a stepper motor instead of a gearmotor -- it has a
set number of steps per revolution, and you can just count those.
Steppers are easy to interface to digital logic or microcontrollers

You'll notice I'm kind of edging away from your original suggestion.
The reason is your use of the word, "precise".  Resistive tape doesn't
exist, and even if it did, it would probably wear out right away from
the force and friction of the wiper.  But it's not very likely you'll
be too satisfied with the precision of any resistive solution that's
relatively inexpensive.  Various solutions might include gearing to a
potentiometer, using resistive wire element with a wiper and a current
source.  No linearity.  Using these to accurately gauge distance won't
work.  Your results might be repeatable, but they definitely won't be
accurate.

The expensive means is to use a linear potentiometer, but they're too
expensive for toys.  Don't go there.

Hope this has been of help.  If one of these ideas sounds promising,
and you'd like to know more, feel free to post again.

Cheers
Chris