A simple solution is to attach a
magnet to either the arm or cable and
locate reed switches so that the magnet moves by them thus closing their
contacts. The number of reed switches would be determined by the resolution required of the readout. This
approach has the advantage of not
putting any load on the arm or cable.
If the motion is just a few inches,
one could attach the slider of a linear
slide potentiometer to the control
arm (or a regular potentiometer to
the shaft if the control arm rotates)
and read the position with a voltmeter. The potentiometer would be
wired as a voltage divider. This has
the advantage that it would require
only two wires to the remote location
rather than the number of wires for
James Schmidt, Deer Lodge, MT
#2 There are several different ways
this can be accomplished. Each
method has benefits or disadvantages, so let's start with the
A. Analog feedback method: A
potentiometer is coupled to the arm
and provides a voltage signal to a follower for signal conditioning (low
output impedance) and is then fed to
a variable current source. You can
find these circuits in any good Op-amp Cookbook with the necessary
explanations. The current signal is
then dropped across a resistor on the
other end and you will have a voltage
signal to work with. Another follower
with a few comparators will give you
the signals for the LEDs or you can
use a digital or analog instrument. A
signal of 0/4-20 mA is commonly
used. This has the advantage that
cable length is not an issue and the
remote circuit (transmitter) needs
only two wires if carefully designed.
B. Simple digital feedback
method: A quadrature encoder is
used at the arm with an optional
index hole. This is typically a disk
with a series of slots and two slotted
optical switches, slightly shifted so
that one activates before the other.
The shift is exactly 90
(therefore its name)
and allows the steps to
be counted in both directions. You can find this type of circuit
in any mouse (there are actually two
circuits in a mouse for X and Y direction, but no index) or on motion control systems. The index hole with its
own optical switch serves to determine a home position. A microcontroller counts the steps and transmits
the signal serially to another micro to
turn on the appropriate LEDs. There
is good literature up on Microchip's
website (AN894) and on National
C. Digital feedback with an
absolute encoder: An encoder to
provide position in binary or gray
format is connected to the arm and
then transmitted to the remote site
and used to display the position. This
could be as simple as a three-bit (
giving you eight different positions) or
as complex as necessary.
Walter Heissenberger, Hancock, NH
“Now With 5x More Products!”
We’ve added thousands of money saving
Jameco ValuePro™ and Jameco ReliaPro™ products.
These products are manufactured for us and shipped
directly to you, eliminating costly supply chain layers.
We’re also finding unique ways to help you save even more on name-brand
products. Rather than print unneeded information in our catalog, we’re printing just
enough information to let you know what we offer and where to find it on our web site.
Less space means less paper, less postage, less waste and less costs.
A simple and potentially big savings we can
pass on to you!
Take a look
at some of our
Great Products. Awesome Prices.
Call 1-800-831-4242 for your free catalog—Or go directly to