MODEL RADIO CONTROL ELECTRONICS 
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.
BRIEF HISTORY
OF MODEL CONTROL SYSTEMS
A ‘PPM’ TRANSMITTER ENCODER CIRCUIT USING EASY TO GET
BITS!
A ‘PPM’ RECEIVER DECODER CIRCUIT USING EASY TO GET BITS!
MORE ‘PPM’ ENCODER AND DECODER CIRCUITS from fellow enthusiast Harry.
FURTHER PPM/
PCM AND OTHER THOUGHTS FROM Dave McQue…… all of which are worth reading!
A THOUGHT OR TWO
ABOUT DUAL CONVERSION RECEIVERS……..swings and roundabouts for the 35 MHz band.
NORCIM 2
40 MHz Tx/Rx notes, PCM system, Speed Controller, Tony’s
website
NORCIM 3 Tx circuitry, electric glider, idea from NASA, Multi Chan Failsafe
NORCIM 4 simple circuits of R/C interest
NORCIM 5
commercial receiver testing by R/C Guru Dave McQue UK.RCC
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
Mathers 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 Mathers/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 Mathers/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 Mathers/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 Mathers/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.
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
Thoughts
about 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 on
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.
TBT