it or not, there was a day when building your own electronics was
a good way to save money if your budget was smaller than your appetite
for R/C systems, radios, even TV sets. Heathkit comes to mind for
all us old-timers as a source of pre-kitted products, but like most
electronics companies of yore, they no longer offer kits; it is
much cheaper to have complete systems built overseas. Besides, modern
components - resistors, capacitors, ICs, etc., are far too small
for most people to work with successfully. Here is a two-part article
from the April and May 1972 editions of American Aircraft Modeler
showing how it was done with a custom 2-channel digital proportional
radio control system dubbed the AAM Commander. It still makes a
good read because of the theory of operation that is covered.
Part I (see
Part II, May 1972 AAM)
You can make the two-channel digital system which offers
reliability, performance, goof-resistant assembly, and low cost
by extensive use of integrated circuits.
By Fred Marks
It seemed to us two years ago that our readers would be
interested in the presentation of a two-channel digital system that
would satisfy the needs of the car, boat, glider and sport power
plane fans. AAM asked me to undertake the project and to try to
make it as simple and goof-proof as practical. The project is finally
complete and we are pleased to present the results.
accordance with the above basic requirements, the following more
specific objectives were set. The system should be as technically
up-to-date as possible; have the maximum flexibility; be the utmost
in simplicity with no frills added; and, taking advantage of inherent
digital system characteristics, it should have a growth factor for
those who would wish to expand the system later.
Using a Tektronix 435 scope, Fred
Marks checks encoder pulses. Scope is also capable of showing
modulation patterns at 27MHz.
Achieving the first objective is by far the most difficult, for
technology is a galloping horse. One must finally decide to jump
aboard and go. This process led to several recycles and ultimately
to the point that everything in the system, except the transmitter
and receiver RF
sections, utilize integrated circuits. The
encoder, decoder, and servo amplifiers are all composed of integrated
circuits. The use of bridge-type servo amplifier permits a two-wire
power pack for the receiver and three wires for each servo.
The second objective is the most satisfying of all, as the system
has been successfully tested in cars, boats, gliders and power planes.
The receiver and decoder are compatible with any transmitter on
the same frequency, from one to eight channels. The decoder simply
decodes the first two channels that it sees, usually aileron and
elevator, and ignores the rest. It will operate any servo made today
(provisions are made for battery center-tap wiring), including those
requiring a negative going pulse. It has been tested with the following:
EK MM3; Orbit PS-4; Micro Avionics; all Kraft servos; Controlaire
S4b, S4d, 5-5 and 5-6; Min-X; Digiace; MRC F-700 and F-710; Royal
Electronics; Pro-Line; Heathkit PS-9 and I C servos; Larson, and
servo has not been as extensively tested in other installations,
but thus far has proven to operate satisfactorily with systems using
IC decoders and with the Heathkit system which has an SCS decoder.
The transmitter will operate one or two channel receiver/decoders
but, obviously, is not designed to provide additional channels at
this time. It may be safely operated on from B to 12 volts with
range dependent on voltage. The nominal design voltage is 9V, which
provides out-of-visible-sight range. However, one may operate boats,
cars, or small sport airplane models on BV (seven nickel cadmium
cells give B.4V). The receiving system is designed to operate on
4.BV using four nickel cadmium cells, although alkaline energizers
may be used. (Carbon-zinc pen-cells are not recommended for this
or any other system.)
Simplicity was achieved by the use
of inexpensive ICs. They offer two distinct advantages to the home
or kit builder: (a) their use permits a drastic reduction in the
number of solder joints required, and (b) every IC is tested by
the manufacturer (this is not done for discrete components such
as transistors). The transmitter is further simplified by: the elimination
of a metering circuit; the use of an external battery charger, if
any; the necessity for only one stick assembly; and the need for
only two tuning points. The receiver is simplified by relieving
it of the normal clock and sync pulse forming functions. These functions
are performed, instead, by the IC on the decoder. The use of the
two-wire power supply eliminates approximately 10 points for potential
final objective, i.e., potential growth, is not difficult since
any digital system is modular. By building two additional servos,
changing to a new decoder board which retains all components except
for a new IC, adding a new transmitter encoder board which has one
IC, four capacitors, two resistors, and of course, a second stick
assembly, a four-channel system is produced.
To insure that
the system was designed properly and that performance was optimized,
the following procedure was employed. The basic system was developed
by first designing the decoder, interfacing it with a well-known
and tested receiver operated by a six-channel transmitter. The transmitter
was breadboarded and verified on the bench; a P.C. layout was then
made and three prototypes constructed. This setup was tested extensively
using existing servos.
considerable amount of effort was expended in the development of
a suitable servo amplifier. Center-tap amplifiers using an ML 85
IC were tried and rejected, as was a design using a Darlington amplifier
in each drive leg:
Amplifiers using a Schmitt trigger in each
drive leg were tried but we couldn't achieve the goal of placing
the servo amplifier on a single board in the smallest servos. Finally,
the World Engines IC chip was selected for the servo. This seems
to be ideal and only has the disadvantage of requiring an 11 ohm
final problem attacked was the receiver design. We first attempted
to use the ready-made ACE Micro Gem receiver modified to add AGC.
This performs quite satisfactorily in "cars and boats but does not
provide a sufficient margin of performance for planes. It then became
necessary to begin the design of a new receiver which could take
advantage of the IC decoder. This done, three prototype units were
tested extensively in a two-channel 0.10-powered model called the
Flexible Flyer. In order to insure repeatability of the system,
artwork and construction instructions were completed and a "pilot"
run of ten systems was performed by local modelers.
the preceding cannot absolutely assure that one will never experience
a glitch, it is felt that system performance is well verified.
Another important point, not only for this, but for any digital
system whether kit or not, is "In case of difficulty .. ." Except
for checks of DC voltage, presence of RC output, and verification
of transmitter operation using a monitor, an oscilloscope must be
available to the builder of any digital system if he is to perform
his own troubleshooting. Actually, troubleshooting is quite simple
if one can use a scope and knows what the traces should look like.
Insofar as practical, this information will be presented, not just
to help the builder of the AAM Commander, but also as a means of
showing the reader how digital systems work.
can be hoped that a spark may have been kindled to assemble a complete
system (transmitter, receiver, decoder, and two servos) or part
of the system for use with an existing system. Here is the planned
sequence for the "AAM Commander: I n this first part of the series,
we have described the system in general, and will now present the
printed circuit layout, list the components required, describe how
they may be obtained, give the drawings for the subassemblies which
may be readied for later use, and provide other information useful
for getting started. The second of the series will present the schematics,
block diagrams, design information, and instructions for assembly
of the transmitter "and the servo amplifier. The third part of the
series will present the schematics, block diagrams, design information,
and instructions for assembly of the receiver and decoder. At the
completion of these, an entire flyable system will be achieved.
To aid in better understanding the system and, as an informative
item, the final article will: present system integration procedures;
show how parts of the system may be used with other systems; provide
trouble-shooting procedures; and indicate changes required for expandability.
1 presents a full-size print of the printed circuit layout for the
complete system. I n order to make the necessary boards, have a
film negative made of the P.C. layout. This is done by photographing
the P.C. drawing layouts on page 53 and having a film negative of
this made to the exact same size as presented in the magazine. Special
arrangement has been made with AAM's plans service to supply the
film negatives for $1.00 per set. These may be ordered by writing
to me c/o AAM or directly to the Editor. Purchase one sheet of 6"x6"
pre-sensitized P.C. board (Kepro S66G was used to make the pilot
systems). Also needed will be one pint of P.C. emulsion developer
(trichlorethylene) and etchant. Either ferric chloride or ammonium
persulphate may be used. These three chemicals may be purchased
from a chemical supply house-perhaps the local druggist can suggest
one. One may also purchase the Kepro kit for P.C. boards, which
has the necessary chemicals, or purchase them packaged separately
The sensitized P.C. board is not extremely sensitive
to light but must not be exposed to direct light. Use a red lamp
while working to set up for exposure. Make everything ready (except
for actual board exposure) before opening the package of sensitized
board. A lamp socket fitted with a 750-watt photoflood bulb, a pane
of glass, and two weights will be needed. Pour the developer (not
the etchant) into a shallow pie tin. Turn off all lights except
the red lamp. Place the P.C., board on a table (copper side up),
and lay the P.C. film negative over it-the letters FM must read
properly or the boards will be made backwards. Place the pane of
glass over the negative and hold in place with weights. Turn on
the 750-watt photoflood and expose the board for five minutes from
a distance of 7 to 10 in. Place the board in the developer (trichlorethylene)
for one minute; agitate while developing. Remove and permit the
board to dry for at least 10 minutes. Do not touch and do not blow
on the board.
the board in ferric chloride or a solution of 3 oz. of ammonium
persulphate per pint of water. (The latter is preferred because
the solution is nearly clear, whereas ferric chloride is quite opaque.)
The process may be speeded considerably by heating and by agitating
the solution. Up to 1800 is adequate-it should not boil. About 15
minutes etching time should suffice. It takes much longer at room
temperature. Inspect the boards carefully for unetched material
which might cause a short. If patience permits, a check between
each land and all adjacent lands with an ohmmeter will insure absolutely
against such shorts.
The boards are glass epoxy so any attempt
to drill with anything less than carbide steel bits is a waste of
time. A No. 64 carbide steel drill bit can be used if a very true
running collet chuck is used in a Dremel or other high-speed tool.
However, bit breakage is a problem. It was found that a No.1 carbide
steel, round dental burr fits one of the Dremel collet chucks perfectly
and is at least an order of magnitude better than regular drill
bits because the shank is about 3/32 in. diameter for excellent
stiffness. One bit should drill all the holes needed. Such bits
can be obtained from your dentist's supplier. Larger holes will
be drilled in the few places needed during construction.
Shear the boards, if possible, on a shear used for P.C. boards
or on a foot shear used for sheet metal. As a last resort, tin shears
will do. Work slowly and be sure the outside line is used for cutting.
Haste will make a handful of wasted board! File and/or sand carefully
to the final shape. This completes the P.C., boards. Set aside in
paper toweling or plastic, where they can't be marred, until needed.
procured parts for the pilot models, we have an excellent feel for
the job of rounding them up. It is asking for trouble to substitute
parts even if
you think you know what you are doing, particularly
in the case of the semiconductors. No matter what anyone says, the
general replacement lines of transistors will not work. Don't buy
them! The most general source of supply is through the Allied Industrial
Catalog, if there is no industrial supplier in your vicinity. Most
of the transistors used are made by Motorola, as are the IC, and
are generally available from an electronic wholesale firm.
We are happy to report that arrangements have been made with
ACE R/C Inc., Higginsville, Mo. 64037, to kit the system. In addition,
ACE will include all the individual components in their 1972 catalog.
While one may procure all parts and build the system from scratch,
the chore is made much easier by obtaining the kit units. The OEM
manufacturer for items is identified on the parts list for those
able to obtain parts directly through a distributor.
following information may help in obtaining components for any project,
and to aid in building kits. Resistors
resistors used in the system are 1/4 watt composition types that
are 10 percent tolerance. There is only one exception-one 1/8 watt
resistor is used per servo. There are four colored bands around
the bodies of all the resistors. These will be discussed as the
first, second, third, and fourth colors. The fourth color is always
silver or gold, while the first color is never silver or gold. Thus,
the first color band is quite simple to distinguish. In addition,
the first color band is usually closer to the end of the resistor
body than the fourth color.
The fourth color identifies
the tolerance of the resistor; gold is 5 percent and silver is 10
percent. All the resistors used in the : AAM Commander have a silver
band. The colors of the other three bands are identified by the
following resistor color code:
The preceding identify the value of the resistor, in ohm's,
as follows: (a) The first band gives the first digit of the resistance
value; (b) The second band gives the second digit of the resistance
value; (c) The third band gives the multiplier for the value, i.e.,
the number of zeroes which must be added.
an example, consider a 270 ohm resistor; the first band is red,
for a two, the second is violet, for a seven, and the third is brown
for addition of one zero. A 15,000 ohm resistor is identified by
brown (one), green (five), and orange (three zeroes). In the schematics,
large values may have the multiplier 1000 identified by a k, such
as 15k for 15,000 ohms, etc. Capacitors
The marking of the tantalum capacitors is quite clearly
stated; however, disc capacitors may have varying markings. The
capacitor will always have two, three, or four digits indicating
the value of capacitance. However, there is inconsistency in the
characters that follow the digits. For example, a 10 picofarad (10pf),
which is the same as 10 micromicro farads, may have only the number
10 printed on the body. A 2S0pf may be marked "250Z," a 47pf may
be marked 47k and a 0.05 microfarad (mf) may be marked 0.05. Nevertheless,
the basic value is always marked; the difficulty is in determining
the multiplier, if any. The following will be useful in determining
Value in pf Same Value
Most of the values which are .01 or larger are simply
identified that way. Quite often, disc capacitors will have markings
to identify the voltage rating such as 10v or 1 kv: do not confuse
this with the value of the capacitors.
The physical size
of the capacitor will be determined by its capacitance and the voltage
rating. It is desirable to stick to the physical sizes to be shown
on the overlay drawings, otherwise things won't fit.
The basing arrangement for transistors varies. Figure 2
presents the basing for transistors which will be used in the system.
It is hoped that the preceding data will be posted above the work
bench during construction.
As indicated earlier, the transmitter
and servo will be constructed first. There are some preassemblies
that can be done before transmitter construction is started. These
subassemblies are shown in Figure 3.
Wind L4 and LS on the
CTC 2173-3-3 coil form as shown in Figure 3. The technique to be
used is as follows: Using two pair of pliers, grip the ends of a
24-in. length of No. 22 enameled wire and stretch slightly to straighten
and "set" the wire. Scrape 1/4 in. of enamel from one end of the
No. 22 enameled wire and solder to terminal No.1 of the coil form.
Strip 1/4 in. of insulation from one end of a 24-in. length of No.
22 stranded hookup wire. Solder to terminal No.3 of the : coil form.
Coat the area of the-coil form onto which the coils are to be wound
with S-minute epoxy. Wind simultaneously, Clockwise viewed from
the top, both the enameled and hook-up wire as shown in Figure 1.
Note that the hookup wire proceeds 1/2 turn to terminal No.1 before
winding of L4 starts. Wind both until about 8 turns are on the form.
Hold the coils in place until the epoxy sets; the wires can be unwound
as needed with no difficulty.
Working carefully, unwind
the hook-up wire until 5-3/4 turns are on the form, i.e.,
count up 5 turns from terminal No.3, then on around 3/4 turns to
terminal NO.4. Bring the hook-up wire straight down, strip 1/4 in.
and tin, then solder to terminal No. 4 to complete L5. Unwind the
enameled wire until 6-3/4 turns remain, i.e., count up 6 turns from
terminal No.1, then proceed 3/4 turns more to terminal No. 2 for
a total 6-3/4 turns. Bring the wire straight down, scrape 1/4 in.
of enamel at the junction to terminal No.2 and solder. This completes
L4. Recheck the completed coils against Figure 3. The epoxy will
hold the coils in place.
Wind L7 as shown in Figure 3. The
middle size X-acto knife handle is exactly the right size (approximately
1/2 in. diam.) form to wind L7 on. Wind exactly 7 turns of No. 18
enameled or bare wire, bend the ends straight down, and clip to
1/4 in. Remove from the X-acto handle and carefully spread all turns
evenly until the coil is exactly 3/4 in. long. Add the tap at 2-1/2
turns from the end as shown in the overlay drawing. Set aside until
Prepare the antenna trimmer C 18 as' shown in Figure
3. This is done to provide solid mounting for C18 with the adjust
screw accessible from the pc side of the board. Set aside until
Higginsville, Mo. 64037
Royal-Electronics Corp. 2119 S. Hudson St.
Denver, Colo. 80222
World Engines, Inc. 8960 Rossash
Ave. Cincinatti, Ohio 45236
ACE R/C. Royal Electronics, World Engines, or from Doss Electronics,
6660 Security Blvd.
Baltimore, Md. 21207
has a $25 min. order)
Sprague, Motorola, Fairchild, GE,
CTC, Switchcraft, Elemenco, CRL, Belden, Miller-National
though Allied Radio)
D & R Products
27635 Forbes Rd.
Laguna Niguel, Calif. 92677