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A 35MHz AND 40MHz TRANSMITTER CIRCUIT

Commercial transmitters for use with model aircraft have a range of around a mile. This is provided by an RF section of usually three or four transistors with the use of plug-in crystals to change the frequency channel. The 35 MHz R/C band is now used for model aircraft in many countries, so the following circuit is shown with component values for the 35 MHz band. The basic transmit circuit is very simple and involves just the middle section of the diagram shown. This consists of a crystal oscillator stage T2 that runs at half the output frequency, followed by a frequency doubling stage T3.

All the circuitry to the right of T3 is simply there to filter harmonics and spurious signals. (and it does this particularly well by suppressing all but the 35 Meg signal by more than 56 dB! This is an excellent figure for EMC, ‘electro magnetic compatibility’ and non-contamination of the radio spectrum).

The circuitry to the left of T2 is there to give a small (0.75 KHz) ‘controlled’ shift the oscillator frequency, with each servo position pulse that comes from the transmitter coder circuitry. The resistor capacitor input to T1, slows the switching of T1, so that the shifting of the transmitter output frequency, with each pulse, is smooth. This keeps the transmitted signal ‘narrow band’. The output of this circuit has been independently tested by the ERA (Electrical Research Association) and is perfectly suitable for use on the 35 and 40 MHz bands.

It never ceases to amaze me that just two transistors and a few components together with a 9-volt battery can potentially transmit up to a mile radius! The circuit is based around ‘Futaba’ spec plug-in crystals (available from most model shops). All resistors are 0.25 Watt 5%. Capacitors are disk ceramic except C12 which is a 16v electrolytic. All coils are Toko 7mm type. It is important to note that the circuit will only work with genuine Micron or Futaba crystals.

(A Question about Centre Loaded aerials from Mike Oct 10/2019) Answer:- For many years micron used a standard length telescopic aerial for 27MHz and eventually 35MHz bands. Both aerials were true ‘Centre Loaded type ie, the loading coil was exactly at the middle of the telescopic aerial. This worked well allowing the loading coil (which is a very active RF transmitter) to add to the overall output and range of the Micron transmitter.

Later Micron transmitters (around 1978 on) used a MK2 version loaded aerial which was much shorter, only 67cm fully extended. Apart from not poking out fellow fliers eyes, these versions gave more RF output and range with less battery consumption. The RF section was limited to 75 milliamps which gave us out of site range. The coding circuitry current draw was minimal at just 3 milliamps. The advantage came with a 20 hours flying time using the standard 1800 MA/H eight cell nicad battery packs of the day. The coder circuitry is stable down to 6 volts, by which time both of the LEDs on the front of the transmitter would have sequentially stopped illuminating.

The technology for the change to the newer efficient aerial involved some empirical work using notes covered on page 14 of this website. The aerial appears to be a voltage converting circuit similar to a Pi-filter. The loading coil much lower down giving more RF voltage output from the top section of the aerial. The later aerial set up is shown with notes left. Instead of centre loaded it was positioned one third from the bottom socket of the transmitter.

A BARRIE ALLEN DESIGN POWERED GLIDER

The Sun electric motor was mentioned in ‘circuit notes2’. Well the guy who was in on testing this motor actually designed a model to go with it! It’s called ‘Whisper’(there are still some of these naturally talented people out there! Makes you sick!). The model however, turned out to be one of the most enjoyable electric planes I’ve ever flown and I still fly it whenever I can. It’s basically a V tail glider with wing area and overall dimensions that were dreamt up to suit the first examples we had of the Sun motor. The fuselage was made wide enough to just take two standard size servos side by side and deep enough (6 cm) below the wing to allow the 7 cell AA battery pack just foreword of the servos. The speed controller and receiver are positioned under a removable hatch just foreword of the wing. The Graupner 9x5 folder prop gives really good performance, producing thermal height two or three times using NH cells. Fuselage is of conventional balsa construction with foam wing. (built up wing could even be better). The motor was arranged to give a full 10 degrees downthrust to prevent too much ‘nose-up’ on full power. We found that very slow landings were achievable by applying up elevator, without the model dropping a wing. Think it’s the effect of the V tail cutting into the airflow when the tail is low, but I’m sure there are more qualified nuts out there who can explain this finding. Change the battery pack for a standard six cell sub-c pack and fit a Graupner speed 600 electric motor gives further performance. The servos need mounting further back to accommodate the larger battery pack. Note also that a standard six cell sub-c battery pack charges more easily from a portable 12 volt leisure battery supply.

IDEAS FROM NASA!

When a little web site like ours, popped up in the NASA Research Centre, USA, George Beeler kindly made some time to Email us about a simple check for a dead receiver!

Re: Simple first order-check on a dead receiver.

Dear Terry,
I happened on this quite by accident, but it is a very useful tool. If you have a relatively sensitive general coverage receiver, you never have to open a receiver to determine if the Local oscillator is non-functional.

Most good quality general coverage receivers (such as most amateurs and short wave listeners have) will be able to detect and pick up the un-modulated local oscillator.  So you either set your short-wave radio to the known oscillator frequency or offset the receive frequency by +/- 455 kHz to detect the local oscillator.  Since the oscillator is un-modulated, you will need to turn on the BFO (Beat Frequency Oscillator) in the receiver to be able to hear the squeal when you plug in or turn on the receiver.

So why doesn't a good frequency counter pick this up?  If it did, you would never get a good frequency indication, as it would be chasing every weak signal for many miles. So the threshold is set to some reasonable level to squelch the micro-Amp level signals.
This technique is great when you are checking old equipment, crash checking the receiver, and in handy checking for oscillator drift as the tone will change by the frequency change, which you can see on an oscilloscope.

I think it would be very useful for a group such as yours, who might not have thought about it, to have a good General coverage radio around.

Good to share, George B. Beeler.

Many thanks for that George. I wish the ‘Beeler check’ had been around some years ago when I was fixing R/C receivers for a living! Take care and do keep us in mind for the future!

A REASON FOR ‘SOFT UNCOMPRESSED FOAM RUBBER PACKAGING OF THE RECEIVER

Is that when used in a power model, some of the electronic components are ‘microphonic’. (probably not a dictionary word but means ‘act like a microphone’!). High vibration levels caused by poor packaging, induces an unwanted electronic ripple at the receive circuit output. This in turn can cause semi-erratic servo control on PPM systems and possible complete signal lock-out of PCM systems. The suspect ‘microphonic’ components in receivers were found to be the electro-mechanical items like coils and filters. Filters tended only to show a problem at certain resonant frequencies. NOW GO AND CAREFULLY PACK YOUR RECEIVER IN NICE SOFT, UNCOMPRESSED, FOAM RUBBER! (And don’t give a damn what the model shop people say!) 

AN EXPERIMENT

to prove the above point, can be done. (Sorry! The following info is aimed only at the real hardened electronic crackpots out there!). A test rig, can be built around a 120mm/150mm 8 OHM full range HI-FI speaker unit. A 2mm Ply disc of maximum diameter is dropped into the speaker cone, with epoxy around its edge. This gives a platform for the ‘test receiver’ to be strapped to, using hooks and elastic bands. I used a simple square wave ‘555’ astable oscillator with a PNP/NPN power drive to the speaker. The frequency of the drive unit was about 30 to 300 Hz. This reflected tick-over speed, to full power, of a typical Glow-Motor used for model flying. A 6v 0 6v nicad battery supply was used but this could be increased (if you wish the receiver to revert back to kit form within a few minutes!!) Flex wiring is needed for the receiver battery input and the oscilloscope output of the audio test point on the receiver. Warning! The resulting noise produced by the test rig will certainly wake the neighbours if not the dead!

If you’re a Crackpot like me then…..HAPPY RECEIVER DEMOLITION!

USE THE 459 MHz R/C BAND WITH YOUR YOUR FAVOURITE PPM TRANSMITTER!  Well Malcolm Perry has come up with a device that simply plugs into the Buddy-Box socket of your Tx and Hey Bingo! your transmitting on the UHF band! Malcolm has even developed a UHF receiver capable of up to eight channels!

Why not UHF? By Malcolm Perry

The UK has for many years allocated a band in the UHF part of the radio spectrum for model control use. This part of the spectrum commonly known as the 459Mhz band is today hardly, if ever used for model control. Equipment has been manufactured in years gone by, by companies such as Reftec and Cotswold, but today there is, to the best of this writer's knowledge no manufacturer supplying model control equipment on UHF. When you consider that modellers often have to wait to use a frequency at a popular site, this is a great pity.

I have for several years experimented with systems using this band with various degrees of success. I thought that your readers might be interested in a recent attempt to develop a viable receiver using this band. In the past I have built complete systems - from scratch! - a laborious task if you consider all the mechanics as well as electronics that goes into a system!

The system I am going to describe makes use of the licence free (MPT) modules now available in this part of the spectrum. In fact the UHF band is now shared, so modellers do not have exclusive use of all the frequencies, which is a shame. The modules you can buy are in the lower part of the band, but this sharing is not all bad news. A characteristic of UHF is that it does not propagate far and to date I have not experienced any interference on the frequency I have chosen. The modules are designed to a specification, and the one chosen is a dual conversion superhet with a low noise amplifier front end. In fact the modules are complete and will with a little interfacing accept a TTL logic drive and at the receive end reproduce the same.

TRANSMITTER

My idea as you can see from the attached picture is to use the 'buddy box' trainer facility to take the output from a standard transmitter and interface it into the UHF TX module. Simple really and the advantage is that there is no modification to the transmitter that might invalidate its approval standard. The unit can be removed and the transmitter operated normally just by removing the DIN plug. In fact a simple adaptation of the interface means that when the UHF module is plugged in the normal internal

transmitter is disabled - an important additional safety feature.

RECEIVER

Perhaps the most difficult part of the exercise was to develop a practical receiver, given that today's modellers expect small size and lightweight as standard. You can tell from the attached picture that this receiver is reasonably small; its actual dimensions are Length 60mm, width 42mm, and depth 34mm. Weight 70grams. For this you get 8 channels, only 7 used with my transmitter combination and the exclusivity of the UHF band, It would be fair to add that the receiver is slightly more complicated, because I found that the recovered data from the UHF module was prone to noise bursts (glitching in modeller's terms) This was considerably reduced by the use of a Phase Locked Loop discriminator in place of the internal quadrature detector. The decoder board is fed by the receivers second IF frequency at 455kHz.

CONCLUSION

I think the answer to my question "Why not UHF" is pretty simple and it is not necessarily a technological one. The particular '459mhz' band is only available in the UK and the majority of model control manufactures are international companies - hence there is little incentive to manufacture equipment that could only be marketed in the UK. This maybe an over simplification, but at least part of the reason why we cannot buy equipment manufactured for this band.

Finally I would add a health warning - developing systems that perform safely requires careful design and analysis or the old adage that your "model might come crashing out of the sky" could become true! Still as I said at the start, it is a shame that more use is not made of this band or, as I noticed, one of your writers has already said we will loose it.

JOYSTICKS AND THINGS!

several Emails have arrived recently asking how to wire-in transmitter joysticks, that have a separate potentiometer for ‘in-flight trim’. The circuit shows one of the ways to do this and was actually used by Micron in many of their transmitter kits. The circuit used, terminates with a three-pin socket. If the socket is reversed when at the Tx coder, then the servo in the model works in the opposite direction (an added bonus!), giving what is called ‘servo-reverse’. Micron’s older ‘5044’ coder used this set-up without problem but the later ‘4017’ coder can give some problems owing to the restricted current drive capabilities of the standard Cmos 4017 chip. The trouble is, that available joysticks use a 5K main pot with an in-flight trim pot of also 5K. This results in a 2K5R load for the 4017 chip. Unfortunately this is borderline current output for the chip! A solution to this problem could be found by replacing the 5K ‘in-flight trimpot’ with a pot of higher value, say 22K, which would reduce the load on the 4017 chip. The HEF4017BT was found to give the best drive current some time ago but there may be 4017 devices with better drive capability now, which would alleviate the problem. However try this circuit first!

A POSSIBLE CIRCUIT FOR VARIABLE SERVO THROW INCLUDING SERVO REVERSE. The single OP-AMP shown provides variable servo throw and servo reverse via the single 100K trimpot in th e back of the transmitter. An LMC661CN contains four OP-AMPS so this single IC could offer independent variable servo reverse settings for all four primary joystick controls. The joystick pot provides the input voltage to the OP-AMP. The output voltage of the OP-AMP however moves exactly opposite to the input. i.e  If the joystick wiper in the diagram slides upwards (more positive), then the OP-AMP output will go downwards (more negative) by the same amount. The output voltage of the circuit is taken from the trimpot at the top of the schematic. With its wiper ‘centre to left’ would produce zero to full normal servo rotation. With the wiper position ‘centre to right’ would produce zero to full reversed servo rotation.

GOT ONE OF THE THOUSANDS OF PPM RADIO CONTROL SYSTEMS OUT THERE?

Then read on again, because this Multi Servo Failsafe unit from Alan Pratt will transform the safety of your system and add to your flying, sailing, or driving confidence! The picture shows one of the DIY prototypes that I managed to snaffle for testing. With only five components, it can be assembled in around fifteen minutes!  (Wow, have things changed since I was a lad!).

The commercial possibilities of a Surface Mount version of this unit, in my opinion, are big by modelling standards. It is estimated that 10s of thousands of eligible model radio control systems exist in the UK alone. Production investment, with only five components, would be relatively low and the unit can be customer pre-set to suit almost any make of R/C system and any model aircraft, boat, or land vehicle.

The unit simply plugs in-line from the receiver to the servos. Under heavy radio interference or loss of signal, it will automatically drive the servos to a position, pre-set by the pilot! This is magic for the average flyer and a must for quarter scale. Initial bench testing has also revealed other technical benefits of the AntiFerence program that Alan has developed for use in his microcomputer device. I am convinced that this is not the last we are going to hear of this exciting development! Me?…well I’m off to fly with the snaffled unit installed and just itching to switch the transmitter off at 500 feet!

HAD AN EMAIL

from Mohamed Shiraz Kaleel and well worth a mention! Mohamed pointed me in the direction of a real interesting group web site. These people actually build their own electric flight motors from CDROM drive motors! The site is extremely detailed and professional. A massive amount of technical info is available there showing how to convert CDROM motors into powerful brushless type flight motors capable from slow flight to 2kW! There is even details of the necessary electronic drive unit including, even a kit of parts! Well worth a visit if you are a mechanical and electronic nut, wishing to see the cutting edge of brushless technology…. http://www.yahoogroups.com/group/lrk-torquemax/

IN 1950 (WHEN BEER WAS TUPENCE A PINT!)

There was just one transmitting frequency for model control! At the flying field you simply waited for the existing model in the air to ‘fly away’ then it was your turn to entertain the masses of kids that would materialise from absolutely everywhere! It was probably as far back as this, that the ‘initial seeds’ were sown, that slowly germinated into what is now called the ‘Radio Control Council’ of the UK. This is a body of representatives, experts and Gurus from all walks of the radio control model world. They descend upon the Boardroom at Bletchley Park MOD establishment around twice a year and with coffee and sandwiches, discuss the future of ‘model radio control’. Liaison with the government ‘Radio Agency’ is continual and on occasions Radio Agency officials attend these meetings.

One of the staggering achievements of the RCC is that UK modellers now have over 120 frequencies available for model control!

Other technical inputs of the RCC include the introduction of FM radio in 1978, Narrow Band 10KHz FM radio 1980, A new 35 MHz band for models 1981, Development of transmitter ‘Type Approval’ testing procedure, A special surface model (boats/cars) band on 40 MHz, (leaving the 35 MHz band for aircraft only), European Harmonisation of model frequencies and specs, Discussion of receiver ‘Type Approval’, R/C System EMC testing……..and the list goes on!…… (Not a lot of people know that!!!)

Thanks for reading.