Page 6 



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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.




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 Holland) and Mr Hill of the superb ‘two valve receiver’.


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 UK pioneer, Doug Bolton, from Nottingham, developed several analogue servo designs, including one that used only three transistors! And it worked real well! A later version of the simple Bolton servo was also used with ‘digital proportional’ systems giving a unique feature that it returned to centre with no Tx signal! The best of the analogue systems was produced in the UK under the name of Flight Link Systems, which offered four servo operation with super precision coreless electric motor servos and a superb triple axis joystick transmitter.


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.


The great pioneers of Model Radio Control aircraft however, began before the ‘Digital’ era..

Click here for the ‘Model Airplane News’ website for more historic info.



Ed Thompson, Technical Editor of RCM magazine in 1966, pioneered the R/C digital revolution with a magazine series. His work covered in great detail the complete construction of a digital radio control system. Thousands of his radio control systems called ‘Digitrio’ were assembled by modellers world-wide. The exceptional mechanical and electronic detail of the series provided many technical seeds for the manufacturers of the time.








Ed Thompson’s transmitter circuit is shown > Both the transmit section and the three channel coder is shown. Later versions became available with more servo channels. Amplitude Modulation is provided by switching the crystal oscillator Q1 on and off producing the digital ‘off’ pulses of around a quarter of a millisecond each. L2 provides RF drive to the power amplifier stage. As with CB, a centre loaded antenna improved range.

During the mid sixties, transistors capable of running at 27Mhz and producing reasonable transmitter range were difficult to find. Cleverly, Ed used two readily available switching transistors in Parallel to share the output and the heat produced during the process.

The coder used a unijunction transistor to generate the frame rate and the string of ‘half shots’ produced the individual servo channel timings of one to two milliseconds each.

This type of coder circuit was to stay with many model radio control manufacturers for the next decade.

The innovative use of two parallel transistors in the output stages of model transmitters quickly changed to a single larger RF transistor as better UHF transistors became available.







< The Digitrio receiver circuit is shown left. The transmitter radio frequency (27.00MHz) signal arrives at the base of Q1 from the flex aerial. At the same time another radio frequency (26.545MHz) signal also arrives at the base of Q1 from the on board crystal oscillator Q4. The two signals at slightly different frequencies cause a third ‘ripple’ frequency to occur as the two frequencies go in and out of phase. The difference of the two incoming frequencies is 0.455Mhz and this is the frequency of the ‘ripple’. The coil T1 is tuned to resonate at 0.455MHz and passes this frequency on to the following amplifier transistor stages. The two initial frequencies (from the transmitter and the receiver crystal) pass through T1 with little resistance and are lost forever. The following amplifier transistors also have tuned coils which only accept a 0.455MHz signal. This gives tremendous rejection to signals from other transmitters that are on even slightly different frequencies.

Some of the amplified signal is rectified by D1 and fed back to the input of Q2. This adjusts the overall sensitivity of the receiver to cater for the varying distance of the receiver from the transmitter as flying takes place.

Q6 and Q7 simply ‘squares up’ the received pulses for good triggering of the decoder circuit. The decoder circuitry (not covered here) separates the incoming pulses from the received transmitter, providing individual outputs for each servo.






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 UK ‘Type Approved’ model control transmitter for use on the 35MHz model aircraft band or the 40MHz surface vehicle band. (model boats and cars etc). One of the norcim readers, Pete, has been transforming the above coder circuit into a practical PCB form. Wow! I have a picture of the result so far and it is so compact that it would fit into any R/C transmitter that’s developed a terminal fault. These un-repairable Txs are often available at club ‘bring and buy’ events or via local model shops. They go for peanuts but remember, they have a moulded case, two good stick units, often nicads and Tx output meter and a telescopic aerial. All you need for an R/C electronics project! Simply snip out all the wiring and defunct PCBs and you have an excellent basis for a home grown system.

BoardsSAWith 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 (Battery negative). It may be possible to get the SLM electronic sticks to work with the circuit by returning the control pot wiring to a centre tap of the 9v6 transmitter battery. (i.e. +4v8). An update on this will be available soon.




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 components of this R/C receiver ‘decoder’ circuit will set you back less than £1.00 Stirling! (little more than one Euro!). The circuit responds to negative going 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. RX DECODE3The 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).








encExistence 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:-




It uses a ‘floating input stage’. This type of circuit is superior to the simple decoder circuit shown decabove 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:-


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 UK we now have 36 channels in the band 34.945 to 35.305 MHz with the first channel centre frequency at 35.950 MHz, channel 55, with the centre of channel 90 at 35.300 MHz. Frequency shift keying, commonly known as FM, is the modulation mode.


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






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 UK and the EU……….The Image Frequency of the 35 MHz flying band is 13.5 MHz. For most of the dual conversion R/C receivers out there this is bang in the middle of the very active ‘Broadcast Band’. Transmissions on this band include music and speech that are thousands of times stronger than our simple battery operated R/C transmitters. So unfortunately what the DC receiver technology gained on the swings….it lost here in the UK and EU on the roundabouts. With thousands of plug-in Xtals produced by the manufactures for Dual Conversion receivers and receiver circuitry to suit, it would be very difficult to change course. However, at least one manufacturer sussed this situation before production and delivery to the UK and EU. They produced their receiver Xtals to give an Image Frequency above the 35 MHz (around 48.5 MHz). At this frequency there is much….much less activity. Very unlikely they will be bothered with ‘image frequency’ interference. This was a major foresight!





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.