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RADIO CONTROL BY BOB YOUNG An in-line mixer for radio control receivers This month, we will look at a simple 2-channel in-line mixer for use with R/C systems that are not equipped with mixers in the transmitters. This can be used to control two servos together when complex models are involved. This mixer was to have been the ultimate “simple job” – take a through-hole design that has been in produc­tion for 20 years and convert it to surface mount components, greatly reducing the size in the process. No electronic redesign, no black magic RF or other issues to get underfoot, just relay the PC board. Pretty simple, right? . . . WRONG! The original unit was designed for Silvertone in the golden years before tariff reductions cut the heart out of the business. This mixer was developed by Bob Lawrence, a very clever engineer and the man who designed the last television set pro­ duced in Australia. Bob consulted on many jobs for me in those days, even though the concepts and challenges I presented him with used to drive him to his limits. The only thing that kept Bob Lawrence coming back for more was the fact that the jobs we gave him were so interesting. Now I look back and shudder and wonder what possessed me to undertake some of the jobs I became involved with. There were 32-channel robotic puppets, radio-controlled fullsize motor vehicles, R/C machines six stories high, 80-tonne R/C shot blast trolleys and RPVs, to name just a few. Years later, I lost touch with Bob and have not seen him to this day. By now you are all asking what on earth has all this to do with this column? The fact is that when the first prototype SMD mixer refused to work I found myself wishing that Bob La­ w­rence was still around. It is a very clever little circuit and quite tricky to service. That night I went home and who should be on the TV (Good Medicine) telling the story about the great new breath test for Heliobacta Bacillus (the bug often associated with ulcers)? . . . none other than Bob Lawrence (Bob, if you read this I would like to hear from you). Reversed inputs Fig.1(a): mixed elevators/flaps are used for aerobatics or as compensation for trim shift. Fig.1(b) shows elevator trim compensation for the pitch changes that takes place when the takeoff or landing flaps are selected. 78  Silicon Chip As it turned out, I did not need Bob’s help for I discov­ e red after hours of hair-tearing effort that the Protel schematic library component for the 3900 op amp has the input pins re­versed. This meant that the PC board was wrong and that I had no hope of making that mixer work. I also checked the new Protel for Windows (Advanced PCB) and found that the error was in that library as well. I have enormous confidence in the Protel Auto­trax system and never Fig.2: the circuit takes in separate input channels and converts them into two separate composite signals, Common out and Comple­mentary out. Mixing is set by trimpot RV2. once questioned the schematic or PC board. It was only after I had exhausted all other avenues that I had to look further. I might add that this is the first time that Protel has ever let me down. The second prototype worked perfectly once I had corrected the schematic library and located the solder bridge I had created across two of the pot pins (even the experts do it). So much for the so-called “simple job”. Mixing concepts For those not familiar with the concept of mixing as ap­plied to R/C transmitters, see the article entitled “The myster­ies Of Mixing” in the December 1995 issue and the October 1996 issue which featured the “Multi-Channel Radio Control Transmit­ter; Pt.8”. These articles give a full and detailed explanation of the intricacies of electronic mixing of flight controls. Whilst these articles deal mainly with mixing in transmitters, the principles still apply to add-on mixers for receivers. Briefly, mixing is the coupling of controls so that moving one control results in one or more servos operating simultaneous­ ly in ratios and directions preset by the operator. Common applications include elevons for tailless aircraft, collective and tail-rotor pitch for helicopters, and coupled aileron/rudder and flaps with elevator compensation on fixed wing aircraft. Less common are twin screw boats and tracked vehicles which incorporate speed and steering by the common/differential use of throttle. As you can see, mixing is a very important feature, making models simpler to operate and the modern R/C transmitter reflects this with all sorts of mixing features built in. Unfor­tunately, such features usually come with a built-in high price tag as well. However, owners of older transmitters without mixing may fit an in-line mixer in the model itself and this will work almost as well. I say almost because usually two functions are the maximum available in an in-line mixer. Transmitters with electronic mixing usually allow multiple point mixing but as most applications use 2-point mixing this is not a serious disadvan­tage. Setting up an in-line mixer can be a tricky business, espe­cially with transmitters without servo reversing so I should repeat some of the October 1996 article dealing with setting up for delta mix. Before proceeding any further, there is a very important point to bear in mind when setting up mixing functions. Each mixer input has an additive effect on servo throw and this must be taken into account when setting mix ratios. Failure to observe this may result in the servo being driven into its internal mechanical end stops with attendant gear damage. Therefore, be sure to check the final servo travel with the full extremes of mixing applied, as servo travel varies with the brand of servos used and the transmitters used. An illustration To illustrate the point being made in the above warning, let us examine the mixing process for a delta aircraft featuring elevons (delta mix). Such an aircraft uses two control surfaces, one on each wing and each control surface performs two functions, aileron and elevator control; hence the name elevon. The diagram of Fig.1 shows the control sequence in detail. To bank such an aircraft, one control surface goes up and the other goes down, thereby imparting a rollJuly 1997  79 Fig.3: these diagrams show the component layout on the top and bottom of the PC board. Install all the surface mount components first then mount the trimpots and other component on the top of the board. Fig.4: the full size etching patterns for the PC board. ing motion to the aircraft. For pitch control, both control surfaces go up to raise the nose and down to lower the nose. Complications arise when one wants to bank and climb simul­ taneously. If full throw on the aileron servo gives the desired rate of roll what happens when we then apply full up elevator to impart a climbing motion to the aircraft? If we are turning left then some UP mixed into the right elevon (which is DOWN in a left roll) is easily accommodated. However, there is no more travel available in the left servo which is already full UP. To apply an additional pulse width variation will only drive the servo hard into the end stops and possibly strip the gears. Therefore, the controls must be mech­anically arranged so that 50% differential servo travel (one UP, one DOWN) gives the maximum rate of roll and 50% common servo travel (Both UP or DOWN) gives the maximum pitch angle. Then we may apply full pitch and roll commands simultane­ously. Oddly enough, at this point only one servo actually moves and it goes to full travel. The two commands on the opposing servo cancel each other out and 80  Silicon Chip the servo remains in neutral. Elevon controls are very complex controls to set up correctly, especially when you start to consider the reflex and unequal differential angles which must be taken into account for the correct aerodynamic conditions required by tailless aircraft. Circuit description The full circuit is shown in Fig.2. Briefly, the circuit takes in two separate input channels and converts them into two separate composite signals. The primary input is defined as the common input and it must come first in the input channel trans­ mission order. For example, if we are mixing for elevons (ailer­on/elevator) and the transmitter channel order is Aileron, Chan­nel 1 and Elevator, Channel 2, then the common input is plugged into the Aileron Channel. It is for this reason that the common input lead must be clearly identified on the finished mixer. A short piece of heatshrink tubing shrunk onto the lead just behind the servo plug does the trick nicely. It is a good idea to similarly mark the common output as an aid to testing. The operation of the common input is fairly straightfor­ward. IC1c is the input buffer/inverter and it drives, IC2b, another buffer inverter which feeds the mix ratio trimpot RV2. Following RV2, the two resistors R5 & R6 form a splitter network and feed the two pulse converter op amps IC3a & IC3b. The two identical op amp pulse converters consist of IC3a, IC3c & IC1a for the Complementary section and IC3b, IC3d & IC1b for the Common section. IC1a & IC1b are used as buffer/inverters to provide the desired positive-going output pulse. Differential input channel The operation of the differential input channel input is a little more tricky. The buffer/inverter IC2a drives a monostable oscillator consisting of NAND gates IC2c & IC2d. The mon­ ostable pulse width at pin 4 of IC2c is set to twice the neutral pulse width used by the R/C system the mixer is fitted to. As most modern R/C systems use a 1.5ms neutral pulse, RV1 is therefore set for a monostable pulse of 3ms. This 3ms pulse is used to generate the complement of the differential channel in IC2d using the gating action of the 4011. Thus if the differential channel moves to 2ms then the complement is 1ms. Likewise, if the differential channel moves to 1ms then the complement is 2ms. At neutral both input pulses are set to 1.5ms, therefore the complement is 1.5ms. The common control pulse and the 3ms pulse are added in RV2 to produce a composite with a variable ratio but constant sum. Diode D3 gates out the control pulse part of the 3ms pulse so that the sum of the common pulse plus the complement of the differential pulse is applied to the pulse converters to produce the complementary output pairs. The final composite outputs are a true mix of both input channels. Thus the differential channel adds to the common output channel and subtracts from the complementary output channel in a ratio again set by the mix ratio pot RV2. As a corollary, the common input adds or subtracts from both outputs in equal amounts, again in a ratio set by RV2. The range of the mix is set by R8 & R12. As the operation of the mixer becomes non-linear beyond 80-20%, I suggest using 75-25%. This is more than adequate for the real world. Where To Buy A Kit Of Parts The inline mixer module is available as follows: SOUND EASY V2,BOXCAD V2 BY BODZIO SOFTWARE Comprehensive s/design software available distributed by WAR AUDIO Fully assembled module complete with servo leads ........................$69.50 Complete kit with PC board and servo leads....................................$49.50 PC board only ..................................................................................$11.50 When ordering, purchasers should nominate the R/C system they are using. Postage & packing for the above kits is $3.00. Payment may be made by Bankcard, cheque or money order to Silvertone Elec­tronics. Send orders to Silvertone Electronics, PO Box 580, Riverwood, NSW 2210. Phone/fax (02) 9533 3517. Inside the suggested range the mixer holds neutral to within 5% over a temperature range of 10°C to 50°C and a voltage range from +4V to +5.2V. RV3 is a balance control to set the neutral on the second servo. The neutral on the first servo may be set by the comple­mentary pot RV1. Diode D1 performs a dual function. Firstly, it protects against re­ verse polarity. Secondly and more importantly, it drops the rail voltage to +4.2V. This is an important point for compatibil­ity with imported sets, particularly with some of the newer sets that have output pulse voltages under 3V. CMOS chips need the input to exceed one half rail voltage for reliable switching. Resistor R4 and capacitor C6 provide a supply decoupling network for IC1 and IC2. IC1f in the 40106 hex inverter is unused. Board construction Construction is very straightforward, with surface mount components used for maximum reliability and minimum size. If you have not worked with surface mount before then once again I would suggest reading “Working With Surface Mount Components” in the January 1995 issue of SILICON CHIP. Install the surface mount components first and the through-hole components next. Fit the servo leads, remembering to slip the short piece of heatshrink tubing over the common input and output leads before soldering them into the PC board. Testing First, set up the transmitter trims to neutral and plug two servos into the channels to be mixed to ensure that both servos are on neutral. Set all mixer control pots to mid range and plug the input leads into the receiver and the servos into the mixer servo sockets. Adjust trimpot RV1 to neutralise the servo in the common output. Set the neutral on the servo in the complementary output using RV3. Now move the transmitter sticks first in one axis and then the other, checking to ensure that both servos travel in approx­ imately equal amounts on each axis. Moving the common axis stick will result in both servos moving in the same direction. Moving the differential axis stick will result in the servos moving in opposite directions. This of course assumes that both servos rotate in the same direction without the mixer. It may be neces­sary to reverse one servo to get the correct direction of rota­tion on both outputs. Now wind the mix ratio pot RV2 fully anticlockwise. One stick axis should barely move the servos, whilst the other should give almost full travel in both servos. If this checks out, wind RV2 fully clockwise and ascertain that the opposite is true. The full travel axis should now be reduced to almost zero travel whilst the reduced travel axis should now deliver almost full range. One small warning here. If the trims are not on exact neu­tral the servos will appear to move off neutral as the ratio of mix is increased. This is deceiving for what is actually happen­ing is that the servo movement is increasing and moving the servo away from its original position. This is most noticeable if the ratio of mix is changed when the throttle channel is one of the mixed channels and the throttle stick is at one end of its range. That’s it – you are now in business. SC Add the case and go and fly. Windows interface.SVGA. Box modelling , 7 type enclosures, 10 alignments for box optimizer, Box time response, Room placement, Import Clio, Lms, Imp, Mlissa etc, Crossover modelling , Optimizing , D’APPOLITO modelling and much more. BOX CAD includes complex impedence and electrical modelling and more. $350.00 upgrades from $60.00. Clio Professional electro-acoustic measurement system $1650.00 Frequency Response • Electrical & Acoustical Phase • FFT Analysis • THD • Anechoic Transfer Function • MLS Analysis • Impulse Response • ETC • Waterfall • Impedance THD+Noise 0.015% • T/S Parameters • 1/3 Octave RTA • Signal Generator / Level Meter • Oscilloscope • SPL • dBV • Volt Amplitude • LC Meter • 16-Bit D/A • Freq. Range 1Hz22kHz ±1dB • Freq. Accuracy > 0.01% • WAR AUDIO U203/396 Scarb Bch Rd Osborne Park W.A. 6017 Ph 09-2425538 F 09-4452579 ACUTTON, AXON, FOCAL, RAVEN LUMINOUS, NEW, CABASSE, Coming Next Month* 600 watt power amplifier So you haven’t been impressed by amplifier modules delivering 350 watts into 4Ω loads? Well, you’re not alone because we have had many requests for bigger amplifiers. Now, after a lot of R&D work, we have come up with the goods. This new design will deliv­er 600 watts into a 4Ω load. It’s a big brute, it won’t be cheap but it’s got the power. TENS unit for pain relief TENS stands for Transcutaneous Electrical Neural Stimulation and is widely used by physiotherapists for relief of chronic pain. This compact design offers variable pulse width, pulse rate (frequency) and variable voltage output. It is battery operated for safety. On sale 30th July Australia-wide *Note: the preparation of these articles is well advanced but circumstances may change the final content. July 1997  81