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REMOTE CONTROL BY BOB YOUNG Building a complete remote control system for models This month, we begin what will be a series of articles on the design & construction of a complete R/C system for models. In its simplest form, it will be a 4-channel transmitter & re­ceiver, while the most complex version will cater for up to 24 channels. Over the past 20 years, R/C systems for models have come a long way and in that time there has been nothing published in Australia on the design and construction of these systems, with the exception of my own article in “Electronics Australia” in 1966 (or CONTROL PANEL 12 thereabouts). This unit was a state-ofthe-art single channel relay receiver featuring such advanced concepts as a super-regen valve front end driving transistorised (gasp) audio and relay driver stages. I received enquires for that kit for over 10 years so this CH24 MODEL 12 CONTROL PANEL 3 CH6 CH5 MODEL 3 CONTROL PANEL 2 CH4 MODULATOR TRANSMITTER CH3 MODEL 2 CONTROL PANEL 1 CH2 CH1 MODEL 1 CLOCK Fig.1: up to 24 channels could be controlled via this proposed transmitter system. It could be applied to model aircraft & possibly enable formation flying, with each operator having loose control for trim & one master operator controlling the formation. It could also be applied to a large model railway layout. 84  Silicon Chip one should take us all into the 21st cen­tury. From the outset, I must stress that the following system is intended for those who want a reliable, simple-tobuild system which will use over the counter components. If you are looking for a fully computerised system then look elsewhere, for you will not find it in this series. The design as presented will be a modular system featuring a 24-channel transmitter, made up of 3 x 8 channel encoder mod­ules and a plug-in transmitter module which will be available in both AM and FM versions. All channels may be switched, propor­tional or a mixture of both. The versatility of this system is so great that it will be impossible for me to present the full system in all its forms. Instead, suggestions will be made along the way, to lead the reader towards construction of the system that best suits his or her own requirements. The basic system presented and thus available in kit form will consist of a 2-stick, 4-channel Tx case, an 8-channel encod­er with mixing and servo reversing, and an RF module (either AM or FM). The choice and layout of the mechanical arrangement of the last four channels will be left to the reader to decide. These may be slide controls (proportional) or switched as for retracts, dropping bombs, waving pilots, turning on and off devices such as tape decks, internal lighting, etc. The circuits and PC boards presented will at all times show the way to the full 24-channel system so that readers may then construct their own mechanical layouts to suit their own parame­ters. A mechanical layout for These views show the top side & underside of the AM receiver module which will be described in detail next month. Most of the components, apart from the coils, ceramic resonator & crystal, are surface mount devices which have the virtue of being able to withstand very high levels of vibration & impact shock. a full 24-channel system will not be presented, although photographs of some 16 and 24-channel transmitters will be shown. The receiver is a three-PC board affair with PC board 1 for the receiver, PC board 2 for the first 8-channel decoder, and PC board 3 for the 16-channel add-on decoder to take the system to 24 channels. All of the above will be housed in a robust alumini­um case measuring approximately 43 x 33 x 35mm. The photos show one of the three prototype AM receiver boards currently being test flown. The construction article for this receiver will appear next month, followed by the 8-channel decoder and then the 16-channel add-on. FM or AM? The receiver also comes in an AM or FM version, so you can see that we have covered all possibilities from a cheap 2-channel AM system to an all-singing, all-dancing 24-channel FM system for those who love spending money. Now before we proceed any further I must stop to explain a few things to the hardheads who by now will have collapsed on the floor laughing. “24 channels! Who is he kidding? How do you control 24 channels with two thumbs? Perhaps he is planning to sell these things to Octopi, HO, HO, HO”. “And AM? He has set the movement back 20 years!” Over the years, I have built and installed literally hun­dreds of oddball R/C installations for all kinds of uses – from the R/C boat pond in Coney Island, Luna Park to real time acting robots in Hollywood, USA. All of these installations had one thing in common – they all used 24 channels or more. Now there are two factors which played an important part in making such installations viable: (1) the operator had more than two hands(!); and (2) some of these installations had a very elabo­rate tape deck control which allowed us to prefabricate a tape by programming four channels at a time. Thus on the first pass, channels 1-4 were programmed, then channels 5-8 and so on. In this way, a full 24-channel tape could be assembled very easily by one man. The film robots used this system. Computers have long ago rendered this system obsolete but, at the time I was in Holly­wood, we led the world in this type of system. I was voted an honorary puppet master by the camera crews, many of whom had worked with the Star Wars robots and had learned to hate them with a passion. That was before they stuck little men inside them. But that is another story. Getting back to the more than two hands business, some of the funniest scenes in my memory of my Hollywood days is when the director would announce a sudden change to the scene which of course rendered the pre-programmed tape completely useless. We would then need up to 10 people to get their hands onto the transmitter at once, so that we could ad-lib the controls. You should try it some time – very cosy, especially with those Holly­wood starlets. As people with more than two hands are hard to find in Australia, and keeping in mind the above experience, we must make it possible to get as many hands around the transmitter as possi­ ble if there is no tape control. Preferably this should be done in comfort and this can be done quite simply by breaking the control panel into smaller sections. By plugging six 4-channel control boxes into the master transmitter, we could have six people controlling a 24-channel robot in complete comfort if not very economically. A more practical application would be to plug 12 2-channel control boxes into the master transmitter. We can now, for example, control 12 model cars very economically, both financially and from a spectrum point of view, from the one transmitter. This was how the Luna Park installation was set up, only the control boxes were huge, fitted as they were with what looked like Mississippi paddle steamer steering wheels and engine con­trol pedestals. The boats were all fitted with 24-channel receiv­ers and to code a boat to any one control station, we simply plugged the two servos into the appropriate channels. Thus, boat number nine used channels 17 and 18. Let me tell you, keeping RF out of the encoder with half a mile of cabling running around the room was my biggest headache. Keeping water out of the boats was their biggest headache. Corrosion was the bane of their lives and eventually led to the demise of the system. Multiple applications Thus, you can see that this system is not designed solely for model aircraft but for the person who has a situation in which radio control will help solve their control problems. The uses are myriad and include the control of multiple model trains on a single layout, multi-channel robots, commercial R/C car tracks and a host of other applications not named. As stated previously, the versatility of the system is staggering and limited only by the operator’s imagination. As an extreme example, one very interesting concept which arises from having 24 channels is the possibility of accurately controlling up to six aircraft in formation from a single trans­mitter. Formation flying has long been a dream of R/C pilots but the difficulties are formidable. The main problem is depth per­ception but there are many more, not the least being the coordi­nation called for when six people attempt to get their timing into sync – not all that important on a slow moving robot but life and death stuff at 200km/h. Using this system, it will be possible to plug six 4-channel transmitters into December 1994  85 the master transmitter. From there, with what amounts to an elaborate dual control system, each pilot hands over control to the master pilot who then proceeds to fly all six aircraft at once. By now the hardheads, who hopefully sobered up and picked themselves up off the floor during the previous explanation, will be back there doubled up in hysterics. “All six models flown by one pilot! The man has left the planet and now resides in cloud cuckoo land!” Allow me to complete the explanation. I did say with what amounts to a very elaborate dual control system. However this system has one major difference. By injecting the control inputs through the mixer, some control would be retained by each pilot, sufficient to allow each pilot to trim his aircraft to keep it in formation, in spite of small differences in speed, wind gusts, turning radius of the model, etc. Thus, whilst the master pilot initiates all manoeuvres, each pilot is still in control, working to keep his model in perfect formation. At any time, control could be taken back by any one pilot, thus allowing complete safety at all times. It is an interesting concept and I will be curious to see if anyone takes up the challenge. Hot potato So now we come to the hot potato. Why present an AM system at all? Everybody knows that FM is better than AM so why do it? I have dealt with this subject at length before so I will just recap what I said previously. FM undoubtedly is much better than AM in audio transmission, especially when the full 50-70kHz shift is used. This results in an excellent signalto-noise ratio with the results we all expect. What everybody does not seem to realise is that model FM systems do not use FM. They use NBFSK (narrow band frequency shift keying), with the emphasis on narrow band. Most model systems shift the carrier by only 400500Hz, a paltry figure which results in signal-to-noise ratios no better than AM, or in most cases worse. From a home constructor’s point of view, NBFSK also presents serious difficulties with regard to setting up the transmitter and viewing the modulation. This calls for specialised instru­ ments which few home constructors have access to. The situation with AM, on the other hand, calls for very few instruments, the most elaborate being a CRO if one is available. The modulation on a 29MHz transmitter is clearly visible, even on a cheap 10MHz oscilloscope. However the most serious problem with FM in regard to the concepts presented in this series is the cost of crystals. Here we are talking about a single transmitter using up to 12 SILICON CHIP FLOPPY INDEX WITH FILE VIEWER Now available: the complete index to all SILICON CHIP articles since the first issue in November 1987. The Floppy Index comes with a handy file viewer that lets you look at the index line by line or page by page for quick browsing, or you can use the search function. All commands are listed on the screen, so you’ll always know what to do next. Notes & Errata also now available: this file lets you quickly check out the Notes & Errata (if any) for all articles published in SILICON CHIP. Not an index but a complete copy of all Notes & Errata text (diagrams not included). The file viewer is included in the price, so that you can quickly locate the item of interest. The Floppy Index and Notes & Errata files are supplied in ASCII format on a 3.5-inch or 5.25-inch floppy disc to suit PC-compatible computers. Note: the File Viewer requires MSDOS 3.3 or above. Price $7.00 + $3 p&p. Send your order to: Silicon Chip Publications, PO Box 139, Collaroy 2097; or phone (02) 979 5644 & quote your credit card number; or fax the details to (02) 979 6503. Please specify 3.5-inch or 5.25-inch disc. 86  Silicon Chip receiv­ers in some installations. The difference in the price of AM and FM crystals is great ($17 per pair for AM versus $49 per pair for FM – most model shops will not sell you one crystal). Multiply that price difference by 12 and you can spend hundreds of unne­cessary dollars on one installation. I say unnecessary because AM will perform equally as well as NBFSK in 99 out of 100 applications, even in model aircraft, despite what the pundits will try to tell you. What annoys me in this argument is that people come to me all the time asking does AM still work, so great is the anti-AM propaganda. We flew for more than 20 years on AM systems and very successfully I might add. I am still flying with AM and feel no need to go to NBFSK. Where NBFSK does outperform AM is in two areas. One is on very crowded model fields where the maximum utilisation of the frequencies available is required and 10kHz band spacing is the order of the day. Second, the AGC time constants must be very carefully set in AM model aircraft receivers to avoid glitches due to rapidly fluctuating AGC levels. On the first count, most applications of the system to be presented do not call for narrow band spacing. Quite the contrary in fact, because here I am proposing a single transmitter to control 12 models – no frequency clutter here. On the second count, model trains do not roar past the transmitter at 200km/h, so the AGC time constants do not present much of a worry. Also the AM receiver to be presented has an excellent AGC system and is free of this problem. So to reiterate, unless you love spending money unnecessar­ily or are forced to go to NBFSK for your application, use AM. There are also some interesting applications which arise from the system to be presented. The modular receivers lend themselves to all sorts of applications. The system can be tuned over the range from 27-50MHz with suitable coil and capacitor changes, allowing use in such applications as garage door openers, etc. Next month, I will present the circuit description of the receiver followed the month after by a detailed procedure on how to build it. See you SC then.