Use one or two of these simple memory walkaround throttles in
place of your MRC walkaround throttle. They will work with your Command
2000 whether it is straight from the box or has been modified for use with
|by Jim Banner
photos by the Author
Model Rectifier Corporation's Command 2000 digital command
control system can simultaneously operate up to five independent trains,
each with its own throttle. These throttles will not increase
that number because the microprocessor which is at the heart of the Command
2000 system has been factory programmed to read only five throttles.
If your layout requires more than five active throttles, consider adding
a second Command 2000 command station and a Y-adapter or upgrading to a
system capable of reading more than five throttles. Often the decision
will depend on whether or not you are presently using a booster with your
Command 2000. If you are, then the cost of a second command station
plus a Y-adapter is probably not excessive. If you do not have
a booster, then the cost of a second command station plus a Y-adapter
plus a booster plus the resale value of your present system may well exceed
the cost of a total new system capable of meeting your requirements.
If the walkaround throttles presented here cannot increase the total
number of throttles, then why build them? Three reasons come to mind
- cost, independent operation, and improved low speed control. Two
of these throttles cost less than buying one MRC walkaround, yet allow
two people to independently operate a train each, which a single MRC walkaround
does not. Choice of parts allows extended low speed control, enabling
slow, realistic starts and stops. The Command 2000 with or without
an MRC walkaround tends to have most of its speed control range crowded
into the first third of the control movement, making it difficult to avoid
jack rabbit starts and stops. A fourth reason for building
these throttles, at least for some of us, is that they use a rotary
knob rather than a slider.
start off by looking at the circuit of an MRC Walkaround, shown at right.
It connects to the command station via a four wire telephone cable, which
has yellow, green, red and black conductors.
The yellow wire is ground (common return) for all the circuits.
The green wire lights an LED which is on when group A is selected, off
when group B is selected, and flashes when the walkaround throttle is turned
off (i.e. when set 2 is selected.)
The red wire conveys throttle #5 speed information as well as direction
5, light and accessory push button data to the command
station. In the command station, there is a micrologger (analogue
multiplexer plus A/D converter plus microprocessor) that reads the voltage
on the red wire. If the voltage is between 1.9 and 5.9 volts, then
the micrologger interprets it as a speed signal with 1.9 volts being
stop, 5.9 volts being full speed. If the direction 5 button
is pressed, the red wire is connected directly to ground, the micrologger
reads 0 volts and interprets it as "toggle* the direction".
If the light button is pressed, the red wire is connected
to ground via one diode, the micrologger reads 0.6 volts and interprets
it as "toggle the light." If the accessory button is pressed,
the red wire is connected to ground via two diodes, the micrologger reads
1.2 volts and interprets it as "toggle the accessory."
If the walkaround is unplugged, the voltage on the red wire (at the command
station end) rises to 10 volts and the micrologger interprets this as a
"no action required" signal and does not change either speed or direction.
The black wire conveys throttle #4 speed and direction 5
and group select data in much the same way. In this way, eight
functions (seven inputs and one feedback) can be carried on just
*note: toggle in this sense means determine the present
state of a circuit or function that has two stable states then set it to
the other state. Sort like a push-on-push-off switch.
can select from the above diagram only those components that are necessary
for the operation of a #4 walkaround and redraw them as at left.
we can select only those components necessary for a #5 walkaround and redraw
them as at right. Looking at both these diagrams, each one has a
throttle to control speed and each has three push buttons to control direction,
lights and accessory. Each walkaround even has its own LED to indicate
whether it is active. But neither walkaround has a group select
push button - these walkarounds, like the MRC version, all have to operate
in the same group as throttles 1 to 3 on the command station anyway.
They can of course be added if the builder so desires.
These walkarounds each have one added component compared to the MRC
type - a 2.2k resistor in series with the LED to compensate for mismatched
between LED's. If your LED's are all the same type and colour, you
probably will not need these resistors.
||A typical walkaround is shown under construction
at left. The push buttons, LED and connector are simply hot glued
into a plastic box. The small components (two diodes and a resistor)
are enclosed in clear shrink tubing which protects them, insulates the
connections to them, and still keeps them visible.
The 10k potentiometer can either be linear taper to give a control range
much like the MRC units, or they can be 10k logarithmic taper to give an
expanded low speed range.
The lower photo at left shows the front of the walkaround still with
temporary markings on the push buttons. These will be replaced with
dry transfer letters brushed with Floquil Crystal Coat for protection.
The numbered knob, which uses the LED for a reference mark, gives a relative
indication of speed.
push buttons used in this project were salvaged from an old VCR bought
for $5 at a garage sale. One is shown at right perched on my keyboard.
These are known as "membrane" push buttons and were common in old 12 channel
VCR's as channel buttons, programming buttons, etc. What makes them
ideal for this project (and why MRC used a form of membrane push buttons
in their walkaround) is that their output is virtually bounce free.
"Bounce" is when a push button makes contact more than once when
you push it. Bounce has no importance at all when you, for example,
throw a switch machine. But bounce is extremely important when dealing
with a circuit that can toggle a function for every contact. If you
have this type of switch, cut off the frame connection (the part extending
down the left side of the switch in the photo.) Then choose two pins
that are cater corner from one another and connect to them. This
saves having to figure out which way the connections are paired.
Although I have not tried this personally, Russ Widom tells me that
ordinary push buttons with 4.7 µf capacitors wired across them work
as well. Just be sure that the negative leads of the capacitors are
connected to the ground (common return) side of the pushbutton. Although
this results in a slightly higher parts count, these push buttons and capacitors
are available from Radio Shack.
|A socket for the walkaround can be made from
a coupler as shown at right. It has a joint (red arrow) that can be opened
by grabbing both sides and bending.
The lower photo shows an opened coupler. The left hand half is
colour coded correctly for our walkarounds. It can be readied for
use by clipping the wires in half and extending them with wires of similar
colour. Of course, the right half could also be used by extending
the wires with correct colours. Close inspection of the #4 walkaround
in the blue box above will show that this is exactly what was done for
Mounting half a coupler is simply a matter of cutting a rectangular
hole with a drill and small square file, then sticking it in place with
lots of hot glue or epoxy
The only remaining item is a cord to connect the walkaround to the command
station or a jack connected to it. This could be an MRC cord, a made
up flat cord, or a handset curly cord. Handset cords normally come
with 4 pin plugs but these are easily cut off and 6 pin plugs installed.
Just be sure when installing plugs that they are oriented
properly. Changing plugs or making up new cables will require
a tool to crimp the plugs. These tools are available from Radio Shack
and other sources. The cost of this tool should be weighed against
the cost of at least four cords, one for each walkaround and a couple of
spares (these are high maintenance items, whether you make them yourself
of buy them from MRC.)
VISIT THE OTHER PAGES IN THIS SERIES
Modifying MRC Walkarounds
An easy modification of your MRC Command 2000 Walkaround throttles will
allow them to be used in pairs without interfering with one another
Modifying the Command Station
Some simple changes to the command station allow you to use two additional
Building Tethered Walkarounds
Use two of these with your modified command station to move throttles two
and three elsewhere on your layout. Optional extended low speed control
is ideal for switching.
Cords and Cables
Some guidance on extension cords and installing throttle stations.
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