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This is only a preview of the June 1995 issue of Silicon Chip. You can view 27 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Build A Satellite TV Receiver; Pt.2":
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REMOTE CONTROL
BY BOB YOUNG
A multi-channel radio control
transmitter for models; Pt.1
This month, we introduce the new Mark
22 transmitter which is a continuation of
the series which has featured the Mark 22
receiver & 8 & 16-channel decoders. This new
transmitter is right up to date but employs
discrete components rather than a custom
microprocessor.
In response to all those who must
have missed the first article in this
series and have rung or written with
“the” question, I am happy to state that
“Yes Virginia, there is a transmitter”.
Here it is in all its glory.
What we are presenting is a 4-channel transmitter in the standard modelling layout with two dual-axis control
sticks. The toggle switch on the top
left is the dual control change-over
switch. The dual control socket and
master select switch is on the bottom
of the case. The ON-OFF switch is
under the black cover between the
two joysticks and the charge socket is
just below. The charge socket plays an
important secondary role, as we will
soon see. The trim levers are located
in the traditional spots on the joystick
surrounds and a meter rounds out the
complement of displays and controls.
In subsequent articles, I will present
photographs of vari
ous transmitters
of up to 32 channels and the circuits
for 24 channels. The encoder module
simply strings together so that you
can have multiples of 8, 16, 24 and
32 channels or more if required. Remember here that servos start to slow
down after 24 channels unless modified. Construction details will not be
presented for transmitters above eight
channels.
72 Silicon Chip
I have to tell you that with the
normal difficulties encountered with
electronic development and suppliers
breaking their promises, the path for
manufacturers is far from smooth.
(And I might add, the playing field is
far from level). No wonder Australian
manufacturers long ago learned the
value in picking up the phone and
ordering their equipment complete
and off-the-shelf from overseas.
This time around, I have run into
problems with the second harmonic
on the transmitter output being higher than legally permissible which, of
course, has prevented publication of
the circuit until the levels are correct.
At the time of writing, this problem
has finally been overcome but sadly
too late for publication this month.
Table 1: Channel Functions
Channel
Function
1
Throttle
2
Aileron
3
Elevator
4
Rudder
5
Retracts (Toggle Switch
6
Aux. Slide 1
7
Aux. Slide 2
8
Toggle Switch 2
That leads into the first discussion
for this month and that is the final format of the RF module. I had intended
to make this module available as a
kit but its tuning really does require
a spectrum analyser to meet the legal
requirements, so I have decided to
supply the RF module as a finished
and tuned module only. The RF circuit
will still be published and the encoder
will still be available as a kit, as will
the mechanics.
As you can see from the photos, the
new transmitter is a true modular system which will facilitate servicing in
the field, with a change-over fee being
charged for module replacements.
The same applies to the receiver and
also do not forget, all of the components are available in Australia and the
circuits readily available. All of this
should go a long way to alleviating the
service problems commonly encountered in the model business, as this
system should be within the capability
of any competent serviceman.
If you have a look at the decoder
board in the photos, you will notice
that there are seven rows of header
pins on the righthand side. These
pins are the connectors for the control
potentiometers. These pins perform
an important function in the overall
design. Firstly, they allow the module
to be easily removed from the transmitter. Secondly, they provide the servo
reversing function.
Each set of three pins is arranged
with the wiper on the centre pin and
the positive and negative supply on
the outside pins. Thus, by rotating
the connector through 180°, servo
reversing is achieved. Thirdly, they
provide channel shuffling, a very important feature in the Mk.22 system.
following reasons: (1) incoming noise
affects the first channel more than any
of the others; and (2) as the encoder is
a sequential pulse generator, a failure
after channel 5 will still leave the main
flying controls operational.
Other manufac
turers have other
ideas and Futaba, for example, use
Aileron, Elevator, Throttle, Rudder,
Retracts, Aux 1, Aux 2 and Toggle
2; that is, when they are not mixing,
matching or mode changing. In this
case, anything can be anywhere. For
example, they recommend leaving the
channel allocation untouched during
mode changes which means throttle
and elevator can be reversed.
At Silvertone, we insist on the
channel allocation remaining constant
when changing modes, for reasons
which will be explained later.
This is the prototype Mk.22
transmitter, a standard
4-channel system with two
dual axis control sticks. One
of these is a ratchet type for
the throttle while the other
is spring-centred. The final
version of the transmitter will
have a professional front panel
to give it a more up-market
appearance.
Stick modes
As there is no standardisation on the
arrangement of the channel numbering between manufacturers, channel
shuffling allows the transmitter to be
tailored to suit any brand of receiver
you may own.
Channel allocation
This is an important point if you
are using a Mk.22 transmitter with an
aircraft that is already set up. Thus,
there is no need to disturb the servo
connectors in the model – the correct
channel allocation can be set up in
the transmitter instead. Note that the
encoder module shown is an early
development module and not the
production version.
I recommend the channel allocation
shown in Table 1.
These are not arbitrary allocations.
They are specified this way for the
Stick modes are another contentious
point and much ink has been spilled
over which stick mode is “the best!”.
By stick mode I mean the arrangement
of the controls on each dual axis joystick assembly. There are two basic
stick configurations, spring-centring
and ratchet.
The spring return sticks are used
for the flying controls. (I will refer
here to the flying controls be
cause
they are usually more numerous than
the steering controls on a car or boat).
The ratchet configuration is used for
throttle or any non-centring control.
Now the fun begins when you try
to decide on the grouping of these
controls on the two stick assemblies.
Modelling conven
tion has defined
Mode 1 as Throttle and Aileron on the
right hand stick and Elevator and Rudder on the left hand stick. Mode 2 is
generally defined as Aileron/Elevator
on the right and Rudder/Throttle on
the left. Even here you will encounter
some conflict as it is sometimes defined in reverse.
The purist will insist that real aircraft are flown with Mode 2 and that
models should be flown likewise. For
a great many other reasons, all valid,
there are others who insist that the
two primary flying controls should
be separated, as we use our thumbs,
not our wrists.
As a general rule, aerobatic and
pylon fliers will fly Mode 1 and scale
buffs Mode 2. Most beginners are heavily influenced by their instructors and
often a club will show a preference to
June 1995 73
The two boards in the
Silvertone transmitter
are the transmitter
itself (at left) & the
encoder. Note the rows
of header pins which
allow for easy servo
reversing & channel
shuffling.
one mode as a result of the availability
of instructors. I think these days that
Mode 1 is more common but choice
of mode is a very personal thing and
best left to the individual to decide
on. I began by flying Mode 2 as a result of my instructor’s influence but
never felt comfortable on this mode.
I subsequently changed to Mode 1,
with a dramatic improvement in my
standard of flying.
The Mk.22 Tx is very simple to
change modes on and when we come
to the mechanical assembly I will present the details. The channel shuffling
facility removes any need for soldering
in this process. In some transmitters,
mode changing is a tricky business, not
to be undertaken by the fainthearted
or unskilled.
Dual control
The stick mode problem rears its
head again when the dual control facility is being designed. Dual control is a
very valuable asset in any transmitter,
particularly in clubs where training is
a big item.
Model aircraft are very difficult to
learn to fly and some form of instruction is desirable, at least in the early
stages. The MAAA (Model Aeronautical Association of Australia) has now
adopted the RCAS (Radio Control
74 Silicon Chip
Aircraft Society of NSW) flight training
system (the “buddy” system), so all
clubs in Australia now have a unified
flight training system. Drop outs due to
the difficulties in learning to fly have
been greatly reduced as a result and
clubs are now at record membership
levels.
What has not been unified is the
dual control system and, in particular, the difficulties of mixing two
transmitters on different modes. The
problem arises because most dual
control systems only allow the pupil
to use the slave (non-radiating Tx)
which means that if the instructor
does not fly the same mode as the
pupil, he is stuck with a Tx on the
wrong mode.
There are ways around this problem
but they require prior planning. The
Mk.22 dual control system overcomes
this problem in that it allows mixed
mode operation (Mode 1 and Mode 2)
as well as master/slave configuration, a
feature not found on any other system
to my knowledge.
This now opens the way to instructors being able to teach people to fly
on the opposite mode. I should mention that it is very difficult to fly both
modes, as reflex action gets in the way,
due to the speed at which the models
fly. Most people will not fly a model
on the wrong mode. This includes a
lot of instructors. I used to fly both
modes but there is a third mode which
I could never master, which is aileron/
elevator with a knob on top of the stick
for rudder. This was a true three-axis
system, commonly known as single
stick. It is not seen too often on fields
these days.
Now it becomes obvious why Silver
tone insists on the channel allocation
being constant, if we are going to mix
transmitters on different modes and
in the master/slave configuration.
With channel shuffling, the problem
becomes academic anyway, because
the channel allocation can be very
quickly changed on the field.
Basically, the Mk.22 dual control
system consists of a sock
et, slide
switch and toggle switch. The two
transmitters are hooked together by an
umbilical cord which plugs into this
socket on each Tx.
The umbilical carries the data from
the encoders. The slide switch selects
which encoder and which RF module
will be paired. The toggle switch on
the top of the transmitter is a spring
loaded OFF type. Thus, the instructor
hands over control to the pupil with
the toggle switch and if things start
to go pear-shaped, then he just grabs
for his controls and the spring toggle
automatically returns control to his
transmitter.
Thus, not only can mixed mode operation be achieved but the pupil can
be given the transmitter with the antenna from the very beginning, thereby
teaching him to position the antenna
for the best radiation match with the
receiver antenna from the outset – a
very important point in flight training.
This arrangement with the instructor on the transmitter without the
antenna is most unusual and is called
master/slave mode. It also allows the
instructor to take the master transmitter (with antenna) if he prefers it that
way, and he flies the same mode. As I
am running out of space, I will leave
the description of the mixing aspects
of dual control for a later issue.
Frequency interlock
Another unusual feature of the
Mk.22 Tx is the frequency interlock
system. In 1969, Silvertone pioneered
narrow-band spacing (15kHz) in Australia and to control these frequencies
we had to develop the Silvertone
Keyboard. The original keyboard featured 57 slots at 5kHz spacing. This
was to allow mixing of all the known
frequency spacing systems available
from anywhere in the world.
At that time, all countries had
allocations on 27MHz but there was
no standardisation of the frequency
spacings. Thus, we had sets of crystals on 10, 15, 20, 25, 30 and 50kHz
spacings, all appearing on the field at
the same time. We also had 1.5", 2",
3" and 4" keys in the board at any one
time. I can clearly remember the day
in 1969 when we had 16 aircraft in the
air at one time. This is common enough
these days but unheard of then. The
Mk.22 system, incidentally, is cleared
for 20kHz spacing (2" key).
Frequency control had degenerated
into a nightmare and thus we were
forced to develop the keyboard. Basically the modern keyboard consists
of a graphical display of the frequency
allocation on a 1" = 10kHz grid. The
original keyboard was designed on
a 0.5" = 5kHz grid which fell by the
wayside when frequency spacings
were standardised on 10kHz.
A frequency key whose width is
proportional to the band
width of
the system in use, is slid into the
keyboard, thus reserving the frequencies required for safe operation of
that system. Nothing new here, most
clubs have been using this for years
and it is now the system required
for all MAAA-sanctioned events in
Australia.
At the time of its introduction, however, it was the most democratic and
revolutionary system seen on flying
fields anywhere in the world.
The Mk.22 Tx, however, carries this
concept to its logical conclusion. If
each modeller on the field has his own
personal key, why not plug it into the
transmitter when this key is not in use
and cut off all power to the transmitter?
This renders the transmitter inoperable at all times when the key is not in
the keyboard. Thus, we now have a
true frequency interlock system – end
of accidents involving transmitters left
on inadvertently in transmitter pound,
a not too infrequent occurrence.
In the Mk.22 TX the charge socket
doubles as the frequency interlock.
Thus each frequency key is fitted with
a plug which plugs into the charge
socket, thereby cutting off power when
the Tx is not in use.
Again, there is nothing new here.
I introduced this concept with the
original keyboard in 1969. The mistake
I made then, however, was to patent
the system. This meant that had the
system been adopted, all sets imported
into Australia would have been forced
to pay a royalty.
The importers went berserk. The
system was the subject of a campaign
which kept it out of use until the patent expired. After that, the keyboard
was adopted as Australian standard
and offered for sale by the same importers who so vehemently opposed
the system whilst the patent held up.
Unfortunately, the frequency interlock
fell by the way. However, that does not
stop me from using it and all Silvertone
transmitters built from 1969 onwards
have had it built in as standard. Of
such stuff is history made.
I have tried to design the Mk.22
system so that it does not compete
head-on with imported equipment. By
taking well-developed concepts that
we pioneered in the past and combining them with modern concepts and
technology, as well as building in the
utmost flexibility and serviceability, I
believe that I have achieved this goal.
The Mk.22 is a unique and interesting
system and one that will find many
uses in the field of hobby, sporting
and commercial radio control. Next
SC
month, the circuit. I promise.
June 1995 75
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