Norcim rc electronics page 6……
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.
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
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.
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.
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
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.