The initial info of this
norcim page comes as a result of several emails about simple PPM (pulse
position modulation) radio control systems. The following notes show typical traditional
‘non-dedicated’ IC circuitry. There have been several ‘dedicated’ transmitter
encoder and decoder ICs over recent years, but these have become obsolete with
the introduction of computer-based transmitters. The following circuitry is
based on readily available, low cost, electronic components.
A (VERY) BRIEF HISTORY OF MODEL RADIO
CONTROL SYSTEMS.
Serious attempts to control models using
radio signals, began in the 1940’s with home built 27MHz ‘carrier wave’ systems. Pioneers of the day included the names of John
Wise, Jim Haddock, Dave McQue, Windy Krewlen (from
The vacuum valve transmitters of the day
were very heavy with massive 120volt dry batteries plus a 2volt lead acid
‘heater’ battery! Two people were often used to carry these transmitters from a
car to the centre of the flying space. An eight foot six inch aerial was then
erected.
Receivers used gas filled valves, which
worked with the much lower voltage of 45volts (sometimes just 22.5volts!) plus
a mini 2volt accumulator to power the valve ‘heater’. (These were often home
constructed from cutting up an old car battery!) Only one control surface of
the model could be moved (using a pre-wound elastic band in the model!). The
control surface was usually a very small rudder. The model aircraft of the day
were essentially free-flight models with loads of wing dihedral to keep them
stable. The radio control simply ‘influenced’ the flight path.
The 1950’s showed development of
electric motorised actuators (instead of wound elastic). Some actuators also
gave a limited, but difficult, control of the elevator (or throttle).
Late 1950’s showed a trend toward pulse
proportional systems, using a mechanical or electronic mark/space pulsing
circuit in the transmitter. The receiver switched a spring centred electric
motor, backwards and forwards in the model, using a fast pulse rate. This
responded to the mark/space and produced a crude but proportional control of
the rudder. A much slower pulse rate system developed by Charles Raill also
‘kicked’ the elevator of the model upwards as the transmitted pulse rate was
slowed down. This system produced a crude but extremely effective control of
both rudder and elevator. It was called the ‘galloping ghost’ system because of
the noise it produced when gliding in to land.
Other more complex systems used a
variety of audio ‘tones’ from the transmitter, which worked several control
surfaces in the model (but not proportional control).
The 1960’s began with ‘feedback’
proportional control of usually two control surfaces. Called ‘Dual proportional
control’, this system used a fairly fast variable mark/space transmission,
which was smoothed out to a voltage swing at the receiver. Analogue servos were
used with a feedback potentiometer to follow the voltage swing. The receivers
could also produce a second voltage swing by detecting a change in RATE of the
mark/space. This produced a proportional output for the second servo. Early
Analogue systems had some problems
though. Getting more than two servos working correctly proved difficult as the
control of one servo also tended to slightly effect the position of other servos.
There was also an ‘elastic’ feel to the controls too, i.e. a kind of
delay of the servos getting up to speed as the Tx control stick was moved and
the servos would slightly overshoot the command position and then quickly
bounce back to the correct position.
It was at this point in time, still in
the early 1960’s, that two NASA space engineers, Doug Spreng and Don Mathes
developed the ‘digital proportional’, radio control system.
This system was designed for use with space
satellites but the obvious and immediate application for model control was
quickly seized upon. Today 40 years on! the Mathes/Spreng
radio control system is still used by all of the worlds leading R/C
manufacturers. Even the original digital pulse timings of their system are
still used by these manufacturers!! This
has been the most significant technological input to the world of model radio
control during the last century and great credit must be given to these early
pioneers.
The Mathes/Spreng system begins at the
servo. They had developed a
servo that would sit at a centre position with a 
repetitive input pulse of 1.5
milliseconds. However varying the pulse width down to 1 millisecond or up to 2
milliseconds produced beautiful instant, accurate and precise proportional
control without the time delay or over-swing of analogue type servos. The
diagram (left) shows the input pulses used.
But how could several of these
‘super-servos’ be controlled in a model at the same time? Well Mathes/Spreng had already got this
one sussed too! They would use the transmitter to send out control pulses for
each servo in sequence, i.e. servo 1 pulse would be transmitted first,
followed by servo 2 pulse…followed by servo 3 pulse….etc. And the transmitter
would keep repeating this sequence of pulses over and over again. They settled
on sending ‘frames’ of servo pulses 50 times every second! With their servos
being told their control position at such a fast rate, helped with radio
interference. The transmission for a typical four-
servo system is shown in the diagram
below. Note that there are five pulses of the carrier (AM or FM) to produce
four servo controls. It’s the time between the transmitted pulses that produces
the servo pulses in the receiver. Note also that the bursts of servo pulses are
separated by a dwell period. (see next text). As the pulses were generated with
separate timing circuits (see later circuit) there was no interaction of servo
positions as with the analogue systems. Note also that the 20 millisecond frame
rate used by Mathes/Spreng allowed up to eight servos to be controlled.
They also developed the first receiver
that could count! As the servo pulse information was received,
a counter circuit directed the first pulse (1) to the first servo output pin of
the receiver. The counter immediately shifted the next servo pulse output (2)
to the second servo output pin. And so on. Each burst of pulses was followed
with a delay which was called the ‘Synchronising Period’ 
this delay caused the counting circuit
in the receiver to reset to zero ready for the next burst of servo pulses.
The transmitter pulse circuitry, (called
the ‘encoder’) was
delightfully simple and shown left. Q1 and Q2 formed an astable multivibrator
running at 50 cps. The ‘half shots’ Q3, Q4, Q5, Q6 sequentially fired one after
the other as Q2 switched ON. The outputs A B C D and E were fed to the
modulation transistor, creating the pulses in the transmission. This discrete
component ‘multivib’ circuit followed by ‘half shots’, was still used by many
manufacturers even at the end of the 1970’s when 35MHz FM radio control had
been introduced in Germany and the UK. Later versions used special design
integrated circuits, (from Signetics and Toko) which did the same thing with
fewer external components. The 1990’s saw the introduction of computer (or PIC
programmable integrated circuits) to the model radio control scene and the
demise of the special dedicated Encoder and Decoder ICs.
http://www.rcsail.com/hksoaringhistory.htm
http://www.rc-airplane-world.com/spread-spectrum.html
http://en.wikipedia.org/wiki/Radio_control
THE FOLLOWING RADIO CONTROL TRANSMITTER ENCODER
CIRCUIT, USES BOG STANDARD COMPONENTS!
This encoder circuit is capable of
generating proportional controls for up to eight servos and is voltage stable
from below 5 volts to over 10 volts. Current consumption is miserly at less
than 2 milliamps! The joystick control pots work with the wipers at centre
position so ‘servo reverse’ can be achieved via a reversible three-pin plug
from the pots. The free running transistor multivibrator is used to clock a
Cmos 4017 counter chip. As it does this, the outputs of the 4017, sequentially,
inject an additional timing component (via T1 to T4) to just one half of the
multivib. The result is a sequence of modulation pulses (of up to eight 
channels). The space
between each individual pulse is variable from 1 to 2 milliseconds via the
position of the control pots. (note that only the centre 60 degrees of the
track is used to achieve this to suit typical joysticks). After all the control
channels have been generated, Q0 via TS, produces a long 8millisecond space (to
let the ‘receiver decoder’ reset) before the next train of control pulses. This
suits all radio control servos. The circuit is drawn for four-servo operation
but further control pots can be added to the available 4017 outputs. The small
‘diode pump’ circuits T1 to T4, which accompany each control pot are shown in
the small diagram. The diodes are 1N4148s. C1 is 47n 5% for T1 to T8. The 8
millisecond space is produced by TS which is the same circuit but C1 is u15
value. R11 presets all servos to centre. R10 presets the throw of all servos.
(note that R10 and R11 are interactive so some juggling of the two is
necessary).
This encoder circuit was
designed to work with the 35/40 MHz transmitting section covered in page 3 of
the norcim web site. Simply joining the two circuits together produces a
With chatting to
Pete, some additions to the circuits above have come up. Firstly, a 102
capacitor is necessary across each of the ‘D1’ diodes in the above small
circuit. These simply ground RF from the transmit section. (which got forgotten
when the circuit was drawn!) The second item that came up was the use of the
SLM joystick circuit outlined in page3. This presented Pete with some problems.
The coder circuit shown was designed using simple 5K mechanical trim joystick
pots which give no problem. The SLM Electronic trim joysticks (see Radio3)
however present too much load to the outputs of the 4017 chip resulting in low
servo throw and some problems with rate switch operation. The picture shows Pete’s coder (top)
with his decoder)
This brings up a possible mod to
the main coder circuit above. Normally if using mechanical trim joystick units
then the common control pot wiring (listed ‘see text’) should go to ground (
CHANNEL MIXING FOR USE
WITH DELTA TYPE AIRCRAFT AND THOSE MODELS WITH A ‘V’ TAIL-PLANE LAYOUT IS
POSSIBLE USING THE 
SIMPLE FIVE
COMPONENT PLUG-IN MODULE SHOWN.
The circuit can be assembled on
Veroboard and plugs in-between the Coder and aileron and elevator fly-leads
from the stick units. C1 at 47n gives 50/50% movement of aileron to elevator
effect. Varying this capacitor value gives different mixes. 22n gives a 20/80
mix while a n15 cap will produce a 60/40 mix. Using suitable sockets the
different value capacitors can be plugged in to suit the % mix required for the
aircraft. D1 and D2 are common 1N4148 silicon diodes or similar. The 102 caps
across each diode get rid of RF pick-up from the transmit circuitry. As shown
the mixer will mix the aileron and elevator channels of the Tx but it will mix
any two other channels. Cost of components alone of this Tx mixer circuit is
less than 50p!!!
Note that this mixer circuit will only work with the
above coder circuit. It will not work with owt else!
THE FOLLOWING RADIO CONTROL RECEIVER DECODER CIRCUIT
USES BOG STANDARD BITS!
The components of this R/C receiver
‘decoder’ circuit will set you back less than £1.00 
pulses from the
receiver. The leading edge pulls pin 15 reset Low, allowing the trailing edge
of each channel pulse to clock the IC, giving servo output pulses sequentially
from Q1…Q2….etc. During the 8 millisecond rest period, the charge across C1
rises and the IC resets ready for the next burst of control pulses. Some older
manufacture of Cmos 4017N chips, produce spikes on the outputs of the IC,
during the relatively slow ramp reset. R1 prevents this by slowing down the
internal switching speed of the IC. Newer 4017 ICs will not need R1.
It is possible to use the encoder
circuit to drive directly the decoder circuit without the use of radio. One
application has been a submersible unit with electric motor propulsion with on
board camera for underwater inspection of off shore oil rigs. The wiring
between the two circuits can be quite long before capacitive effects of the
cable round off the pulses too much. 
The cable between the
encoder and decoder in this application is called the ‘umbilical’. The
umbilical wiring needs only to be a two wire cable, negative and signal. For
this application both the encoder and decoder should use the same but
independent supply voltage. 5 volt supply for the encoder and at the other end
of the umbilical, a 5 volt supply for the decoder and servos etc. The picture
shows Pete’s neat Veroboard version of the decoder circuit surrounding a 27MHz
AM receiver. It shows just how compact a simple seven component circuit can be
achieved using Veroboard. For more complex circuits however, such as the coder,
a PCB is the only way of keeping size to a minimum.
For those who have
not noticed! The input to the decoder needs inverting for the ‘umbilical’
application. A single transistor stage at the encoder end or the decoder end
will do the trick. A suitable inverter stage is shown. Because of the high
value of R2 in the receiver decoder circuit, a mosfet is unsuitable. (mosfets
do not completely switch off).
THOUGHT THAT
THE ABOVE R/C ENCODER WAS SIMPLE!..THEN TAKE A LOOK AT HARRY’S VERSION!!
Existence of this PPM
transmitter coder circuit was emailed to the norcim site by fellow enthusiast
Bruce Johnson from down under. (Bruce has no snowflakes around Christmastime!….Ahh!).
The circuit is extraordinarily simple and again uses ‘bog standard, easy to get
components’. I recon at a good electronics store, all the components could be
picked up for around a $. The six joystick control pots are all 100K value and
work at an ‘off centre’ wiper position to produce the 1 to 2 millisecond swing
for all channels. Diodes are 1N4148 or 1N914 or similar. More detail is
available at Harry’s website:- http://web.telia.com/~u85920178/use/rc-prop.htm
HARRY HAS A DECODER CIRCUIT TOO!!
It uses a ‘floating input stage’.
This type of circuit is superior to the simple decoder circuit shown 
above in that it
automatically ‘follows’ mild voltage level changes of the receiver’s recovered
audio output. These changing DC levels of the receiver audio output voltage
levels result mainly with AM receivers and can be a problem at mid range. (they
can be the cause of ‘glitches’, i.e. unwanted instant changes of direction of
the model) FM receiver radio circuits can also exhibit a similar characteristic
but usually only at very close range to the transmitter. I.e. flying at speed
past the transmitter. Harry’s circuit would handle this situation better. The
PPM decoder circuit is shown left and is capable of driving six servos. A 4v8
supply voltage from the more typical receiver flight pack should be perfectly
sound. More info is available on Harry’s website:- http://web.telia.com/~u85920178/use/rc-prop.htm
Many thanks to Harry/Bruce and Pete for the input
above…….’Tis Good To Share’.
Comments on Airborne Radio Control equipment
usage (By Dave McQue)
The current standards
applicable to model radio control gear for use in the UK and incidentally the
rest of the EU are ETSI EN 300 220 and EN 300
683 for EMC, for the 35 MHz band 10 kHz channel spacing is well defined.
In the
This long time coding method
provided in all R/C transmitters is Pulse Position Modulation (PPM). Here the
first pulse denotes the start of the first servo control signal while the next
and each subsequent signals the end of one control signal and the start of the
next until the last which terminates the last control. A gap of at least 4
milliseconds without pulses then follows used to indicate that the next pulse
is the start of control signal one. With up to 8 servo channels with their
control signals varying from 1 to 2 millisecs, a frame repetition rate of 50
per second is normal.
Most current equipments
have their frequency controlled by a crystal resonator (‘Transmitter Xtal’)
which has to be replaced to effect a frequency change. The frequency tolerance
requirements are such that only the R/C manufacturers specified crystals can be fitted to give a correct transmission
channel frequency. This applies both to receivers as well as transmitters.
What is not generally
appreciated is that crystals can age. It is common for some 20 transmitters to
fail a frequency check out of some 300 checked at the BMFA Nationals. New units
using ‘frequency synthesis’ are now appearing, these have a single reference
crystal built in and is set by the mmnufacturer to high accuracy by means of a
trimmer capacitor. If any frequency drift is detected or suspected the unit
should be sent to an authorised service centre for recalibration against a
precision frequency standard.
While modern gear is very
reliable that does not mean that no faults can occur. Transmitter controls can
wear out and component failures happen. A fellow club member had one where his
transmitter modulation circuit failed, although it still transmitted a carrier.
I had a receiver fail on switching on for the next flight. Any receiver showing
signs of ‘glitching’ in the air despite apparently having survived a prang
should be scrapped. Transmitter whip antennas should be kept clean of fuel
deposits and replaced if loose or otherwise damaged. Metal to metal linkages in
models should be avoided unless bonded.
The benefits of using FM rather than AM
include the rejection of interference spikes and the ‘Capture Effect’ where the
receiver responds to the strongest signal on its frequency and is not affected
so long as the wanted signal at the receiver is more than about four times
stronger than the interferer. From some recent tests we have been able to
confirm that for models flown no further from the pilot than 500m and no higher
than 500 ft, a transmitter on the same frequency channel 2miles or more away is
unlikely to have any effect. It is more likely to be someone on your field who
is not on the channel he thinks he is.
Range checks are commonly
conducted with the transmitter antenna retracted to reduce the radiated power
by something like 100 times, typically from 50mW to 0.5mW. With a good receiver
properly installed a range of nearly 100m can be expected before servos
noticeable chatter when using PPM or when on PCM the servos move to the
failsafe position. Rarely does anyone conduct a full power, antenna up, check
at ground level. With 446 radios for comms it is possible and instructive. I
will be surprised if you get to 400m especially if you point the transmitter
antenna directly at the distant receiver. From free space considerations one
would expect 10 times the ant down range. But below an angle of about 15
degrees to the horizontal ground losses will be greater.
Another little realised
property of PPM is that long before the servos are noticeable affected by noise
the servos are drawing appreciable currents. These could overcome the limits of
a BEC or a weak battery. (See RX tests). This does not happen with PCM which is
a clear advantage. Some PPM receivers incorporate some ‘processing’ to reduce
the effect of noise but none is as good as PCM. It is a pity that nobody
devised an open PCM system that all manufactures could offer as a common
alternative to PPM. Instead we have a proliferation of proprietary non
compatible PCM systems many offering resolutions well beyond the need and
capability of ordinary servos and linkages. So PPM for all its age and
limitations remains as the only common Standard.
Over the last 8 years I
have looked into many cases of possible interference and only in one have I
found a certain cause. While watching a glider flying using a single conversion
35 MHz receiver, I saw it twitch while at the same time I heard tones on the 34
MHz image frequency. The image frequency of a single conversion receiver is 910
kHz lower than the selected channel frequency. For example for a receiver on
channel 66 = 36.060 MHz will have its crystal on 34.605 MHz so that a signal at
the receiver on 34.150 MHz will also produce an IF of 455 kHz. If you are near
a military site it could happen to you unless you use a dual conversion
receiver.
A full size half wave
antenna for 35 MHz spans 4.3m clearly the 1m whip on the transmitters and the
1m wire on receivers is barely an 1/8 wave. In both cases inductances, coils
are used to compensate. In the case of the transmitter there are losses.
Typically one watt of dc power is used to generate ½ watt of RF of which some
50 milliwatts is radiated. Note 100 mW is the max permitted effective radiated
power. Note also that you act as the bottom part of the antenna system. For the
receiver the battery, servos and wiring are the bottom part. Then to get the
maximum capture area you have to route the antenna wire as far as possible from
them. In most cases that means to the top of the fin. For a Delta, out to a
wing tip and up a fin. For some foamies underneath and then trailing can be
best.
Strong local sources of RF
on frequencies far away from 35 MHz can interfere with both receivers and
servos. At 100m or more from a mobile phone mast the maximum signal in the
boresight, about 3 degrees down from the horizontal, is well below the level
likely to cause trouble. Radars have ERPs in the megawatt range so do not get
too close! Although I have not been able to confirm it with my personal
equipment there is evidence of transmitters having their model memories upset
by personal mobile phones.
Microwave links using dish
antennas have a narrow beam and can be avoided.
Batteries should be
checked on load for any defects in wiring or switches can cause a voltage drop
when the servos operate causing the receiver to malfunction. When an aircraft
uses a lot of servos it can be prudent to power the receiver and servos from
separate batteries. In the case of large models the use of opto-isolators is
advised.
Dave McQue
SOME THOUGHTS ON SINGLE
AND DUAL CONVERSION R/C RECEIVERS
Single conversion receivers using the 35 MHz band
offer the simplest circuitry for use with plug-in crystals. A technical drawback
that is well recognised is their ability to reject transmissions that may
occur on their image frequency band below their receiver crystal. That means
any transmission activity on the 34 MHz band could cause havoc to these
receivers. Their ability to reject 34 MHz transmissions is minimal. Frequency
monitors used in club situations would not necessarily pick up this
interference as they are monitoring just the 35 MHz model band. The rejection
of a single conversion receiver to a transmission on their direct image
frequency (34 MHz) is virtually zero and can prove catastrophic.
It was for this reason that Dual conversion R/C
receivers were adopted. With the miniaturization of electronics, the extra
circuitry involved is easily digested by even the smaller receivers. Dual
conversion circuitry allows much better front end rejection of ‘image
frequency’ transmission. In fact the DC receiver can reject its image frequency
thousands of times better than the simpler single conversion type.
However an unfortunate coincidence occurs here in the
TBT
FURTHER THOUGHTS ABOUT
SINGLE CONVERSION R/C RECEIVERS
Single conversion FM receivers using the 35 band have
another interesting feature. The plug-in local crystal controlled oscillator is
just 455KHz away from the incoming transmitter signal. If the on-board
oscillator is fairly active, it can unfortunately be picked up by the receiver
antenna. These receivers commonly use only one antenna input coil to select the
35MHz band and the rejection of signals just half a megahertz away is not good.
The result can be that the receiver happily picks up it’s own Xtal osc at range
and becomes deaf to the transmitter signal! The answer seems to be the use of a
low activity oscillator….or careful screening of the oscillator circuitry. As
receivers become ever smaller with surface mount components, screening presents
a practical mechanical problem. Micron used an interesting method of getting
around this problem by avoiding a tuned circuit in their oscillator. They used
a ferrite bead with a couple of turns of wire which was simply selective of the
third overtone frequency of the receive Xtal rather than it’s fundamental. The
resulting circuit was relatively low activity compared with a series tuned type
(or other tuned type). The use of the bead seemed also to ‘absorb’ much of the
RF. Reliable oscillator start-up (on correct frequency) was from as low a
voltage as 1v5 and OK to 6v. The receiver crystal body is also one of the main
emitters of RF. Some form of neg earth grounding via a side spring clip as the
Xtal is plugged in considerably helps. As a crude test….the range of the
receiver should not diminish with one turn of the antenna flex around the Xtal.
Double tuned coil front ends and the use of Toko screened oscillator coils in
Micron days did not appear to solve the problem. The reduction in range due to
this phenomenon can also be made worse as servos and other bits are plugged in.
The following link by
Google is well worth a click
THANKS FOR READING!
A change of subject ?.... The energy saving light bulb Myth. Do they work and at what cost!