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RADIO CONTROL BY BOB YOUNG An F3b mixer module; Pt.2 In this final article on F3B sailplanes, we describe the circuit and construction of a mixer module to suit the encoder in the Silvertone Mk.22 transmitter. It provides a wide range of programmed functions using simple op amp stages. Last month, we covered the operation of the basic building blocks to be used in this F3B module. These comprised inverting and non-inverting mixers and end-point clamps to limit servo travel in certain configurations. If you are to fully understand how this module works, you will need to refer back to the circuit of the Mk.22 transmitter encoder which was presented in the March 1996 issue of SILICON CHIP. An 8-channel encoder, it has a column of 3-pin sockets of connections for the control sticks, auxiliary pots and toggle switches. Then there is a column of trimpots which are the ATV/dual rate set pots and another column of 3-pin sockets for the dual rate/normal/ATV programming pins. These functions can be added onto via the TB10 mix/expand socket on the encoder board and this mates with the TB18 mix/expand plug on the F3B mixer module. All control voltages from the transmitter front panel con­trols are available at TB10/TB18. Fig.1 shows the complete circuit diagram of the module. Note that there are four pairs of mixers: IC1a & IC1b (Aileron slave), IC1c & IC1d (Droop/ Crow), IC2a & IC2b (Flap/Elevator compensation) and finally IC2c & IC2d (“V” tail). IC3a & IC3b are the two end point clamps. 68  Silicon Chip Included on the circuit is a panel giving the recommended channel allocation for this module. TB11 is the patch cord plug for each channel and is numbered 1-8 from top to bottom. The pre-programming assumes this channel allocation is adhered to. As stated last month, the module essentially consists of matched pairs of mixers, one inverting and one non-inverting. Each mixer pair is fitted with 3-pin input and output plugs arranged in such a way that the pre-programmed functions can be activated by fitting micro-shunts. These input/output plugs may also be remotely switched or hard wired as the application de­mands. Alternatively, each op amp mixer may be used as a free mixer (non-programmed) by using a patch cord which is rotated 180 degrees to pick up the input and output pins, as illustrated last month. The pre-programmed lines have been drawn with heavy lines and they all begin and end at TB18 because we are drawing on a portion of the control voltages applied to the multiplexer inputs (4051) located on the main encoder PC board. We then modify them and reapply this modified control voltage back to the appro­priate multiplexer inputs. All of this takes place via TB18. The pre-programming on the module presented is as follows: a three-servo wing for flaps, slaved aileron servo, flap/elevator compensation, Droop/Crow and “V” tail. The four-servo wing setup (two flap servos) uses one of the free mixers, either on the module or the encoder via a patch cord. Last month I stated that in the Mk.22 F3B module, each pair of mixers share common input and output plugs and a consistent system has been adhered to in order to simplify programming. However, note that the Crow landing and “V”-tail mixers have four plugs that are cross-coupled. This deviation was called for in order to simplify the pre-programming and setup of servo direc­tions. In general, the lefthand trimpot is the inverting mixer gain control and the righthand trimpot is the non-inverting gain control. Input is always on the lefthand pair of 3-pin plugs and output on the right and the non-inverting input/output pair of pins is always closest to the row of pots. Clockwise rotation always increases servo travel. VR1 & VR2 are exceptions due to the nature of their operation. Aileron slave circuit The aileron slave circuit is straightforward. The aileron input is picked off at TB18 (pin 1) and fed to a suitable mixer via TB1. The output is then taken to TB18 (pin 10) via TB3. Fig.1: The F3B mixer module consists of a number of four pairs of inverting/ non-inverting op amp mixers together with a pair of end-point adjust circuits (IC3a, IC3b) to limit servo travel. December 1998  69 Fig.2: the double-sided PC board has surface mount components on both sides. The top view is at the top of the page, with the bottom view immediately above. The aim is to end up with two servos working in opposite directions from the same input signal. VR3 & VR6 are the servo travel ad­ justments and are used to set the travel of the slaved servo to match that of the master servo. Once the two travels are matched, both servos will track from the ATV control on the encoder PC board. As only one of this pair of mixers is used on the ailer­ons, there is always a free mixer in this pair. Flap/elevator compensation Flap/elevator compensation is also quite straightforward. The flap input is picked off at TB18 (pin 25) and fed into a mixer pair via TB7. Output is directed to the elevator input at TB18 (pin 2) via TB8. The usual arrangement here is to end up with elevators going down when the flaps are lowered. By replacing the micro-shunt on TB8 with a switch, the flap compensation may be switched in or out from the front panel. This switch may be combined with the Launch/Cruise/Crow switch and arranged so that elevator compensation is only activated with Crow. In this case we use a 4-pole ON-OFF-ON switch. Again, there is always a free mixer in this pair. “V” tail setup “V” tails can be devilishly difficult to program but not with the setup in 70  Silicon Chip this module. The essence of “V” tail mixing is cross-coupled inputs. In other words, the rudder channel is mixed into the elevator and the elevator is mixed into the rudder. Thus the elevator input is picked off at TB18 (pin 2), modified and applied to the rudder input at TB18 (pin 7). Likewise, the rudder input is picked off at TB18 (pin 3), modified and reapplied to the elevator input at TB18 (pin 6). The cross-coupled wiring on the four input/output plugs is to provide servo reversing if required. Thus each input is modi­fied by a non-inverting or inverting mixer as dictated by the placement of the micro-shunts on TB14, TB15, TB16 & TB17. The desired end result is usually to have both servos travelling in the same direction for elevator and in opposite directions for rudder. All four shunts must be placed on the same side of the connectors or all four moved across to reverse rotation. Instead of rudder and “V” tail mixing, we it could quite easily have Elevon mixing; “V” tail mixing and Elevon mixing are identical in structure. In the case of “V” tail mixing, rudder is mixed into elevator and in the case of elevons, ailerons are mixed into elevators. Thus the pre-programmed F3B module can be used as a delta mix (elevons) module simply by changing channel allocation on the encoder PC board, so that the aileron control lead plugs onto the rudder (channel four input, encoder PC board, TB9). Likewise, a simple two channel “V” tail glider such as the Stingray 2M would best be set up with the aileron stick as the primary steering control with the lead on channel four (encoder TB9). As soon as the four micro-shunts are placed on one side of TB14, TB15, TB16 & TB17, “V” tail mixing is available. To reverse the action, simply move all four micro-shunts to the other side of the connectors. Keep in mind here that the servo direction can still be reversed by rotating the lead on the encoder PC board, so there are many options. Despite the deviation in consistency of layout, both mixers are still available as free mixers by using the patch cord rotated by 180 degrees. Due to the cross-coupling, there will be two input and two output connections available. There is no free mixer in this pair in the pre-programmed mode. Droop/crow configuration The Droop Ailerons/Crow landing sub-module is a special case. Access to each mixer is on the centre pins of TB2, TB4, TB5 & TB6, contrary to the statement that the programming is always on the centre. The droop and crow mixer configuration is a tricky bit of work. Each surface of the ailerons works in the opposite P.C.B. Makers ! If you need: •  P.C.B. High Speed Drill •  P.C.B. Guillotine •  P.C.B. Material – Negative or Positive acting •  Light Box – Single or Double Sided – Large or Small •  Etch Tank – Bubble or Circulating – Large or Small •  U.V. Sensitive film for Negatives •  Electronic Components and This is the underside view of the completed F3B mixer module. Take care when mounting TB11 – see text. •  •  sense in that as one moves up the other goes down. Now to apply droop (both moving down simultaneously), one servo must be fed from a common point with a non-inverting input and the other with an inverting input. However in the case of the Crow landing configuration both servos must go up simultaneously, exactly the opposite to that of droop. In other words, the servo that was fed an inverting input now receives a non-inverting input and vice-versa. So why not save a pair of mixers by simply reversing the inputs from the Droop configuration? This is exactly what is done in the Droop/Crow circuit. If you now refer to the Fig.2, the component overlay for the PC board, you will notice that TB2 & TB4 are placed in the normal side-by-side arrangement and TB5 & TB6 are likewise. This allows the free mixer to be accessed with a patch cord from the centre of each pair of plugs. The cross coupling in this case is done with potentiometers VR1, VR2, with the mixing output coming from the wiper of each pot. R28 & R29 are simply zero ohm jump­ers. Switching is achieved by using an ON-OFF-ON double-pole switch wired to two standard servo plugs, one plugged onto the mixer outputs TB4 and one onto TB6; signal to the centre terminal in each case. Remember to keep the polarity of the plug the same on each mixer plug. The sense of operation of this switch may be reversed by rotating both plugs by 180°. This switch may be located on the front of the transmitter and becomes the Launch/Cruise/Crow master select switch. A micro-shunt placed on the input plugs TB2 & TB5 completes the programming of this sub-module. Thus when the centre-off switch is in the middle position, there is no mixing applied to the ailerons. When Launch mode is selected, one mixer is connected to one end of VR2 and the other to one end of VR1. When Crow is selected, the order is reversed and the mixer connected to VR1 is connected to the other end of VR2 and vice-versa. Thus VR1 & VR2 are balance pots which set the ratio of Crow to Droop signal applied to each aileron (approximately 80:20 - 20:80). VR4 & VR5 set the overall gain of the mixers (servo throw) and are also used to set the balance for each servo trav­el. Once the servo throws are equal, VR1 & VR2 distribute it to the servos in the correct proportions. The usual arrangement is have more Crow movement than Droop. It soon becomes apparent that by removing the micro-shunts on the inputs of this mixer pair (TB2, TB5), the pre-programmed coupling with the flaps is removed and any other suitable source of control voltage may be substituted for the flap input. This voltage could come from switched pots, auxiliary levers or pots etc. As stated previously, only the imagination and level of understanding of the operator limit the Mk.22 system. The same is usually true of the really smart computer systems, so Equipment for TAFEs, Colleges and Schools FREE ADVICE ON ANY OF OUR PRODUCTS FROM DEDICATED PEOPLE WITH HANDS-ON EXPERIENCE Prompt and Economical Delivery KALEX 40 Wallis Ave E. Ivanhoe 3079 Ph (03) 9497 3422 FAX (03) 9499 2381 •  ALL MAJOR CREDIT CARDS ACCEPTED December 1998  71 This photo shows the top view of the assembled F3B mixer module. It plugs into the encoder board for the Silvertone Mk.22 transmitter. Note that this final version differs slightly from the prototype shown last month. take the trouble to fully understand your system. End point clamps Using the flap lever to activate the Droop/Crow function in a half-rail encoder introduces some complications as unwanted mixing will be applied at the top end of the flap travel. To overcome this, we use an end-point clamp to set the neutral point (servo end travel) at the half-rail voltage. Thus if one of the auxiliary potent­iometers is plugged onto TB9 or TB13, the auxili­ ary pot on the transmitter front panel can be used to set the flap position. In operation, the servo follows the flap lever until the end-point is reached and movement ceases. Now this provides a very useful function in that wing camber is now directly controllable from the front panel in flight via the auxiliary pot. R23, R24 and R26, R27 are limit resistors and restrict the amount of camber variation available. The larger the values of the resistors, the smaller the camber change angle becomes. Thus camber can be set to suit the condi­ tions of the day or during trimming of the model before transfer­ring the values into set pots. Diodes D1, D2 and D3, D4 reverse the end-point. By placing the micro-shunt on the appropriate half of TB10 or TB12, high end or low-endpoint adjustment is available. Out 1 and Out 2 are the patch plugs for the end-point clamps and are sim72  Silicon Chip ply single header pins or may be hard wired into the circuit. These can go to any pin on TB11. If you are using them for aileron differential, one must go to each aileron input. To use the F3B module on a Mk.22 transmitter, simply remove the existing eight micro-shunts from TB10 on the encoder PC board and plug in the module. Connect the appropriate switches and pots to the 3-pin plugs and you are ready to set servo directions. Place the micro-shunts on one half of the 3-pin plugs and switch on. To reverse the servo, simply move both micro-shunts to the other side of the 3-pin connectors. Adjust the servo throws and you are ready to fly; all very simple. Assembly Assembly is quite simple. As it is all in surface mount, it might pay to read “Working with Surface Mount Components”, as featured in the January 1995 issue of SILICON CHIP, before you start. Begin by mounting all the ICs, then do all of the smaller surface mount components, remembering that there are SMDs on both sides of the PC board. Next, mount the large connector TB18 on the side away from the ICs, followed by TB11. TB11 is a little tricky in that it protrudes an equal distance either side of the PC board. Be sure to use the long header pins provided for this connector. TB11 is the patch cord input and provides two inputs for each channel by virtue of the fact that there is sufficient length either side of the PC board to plug on a patch cord. Next mount the header pins with the pins on the IC side of the board. Finally, mount the trimpots. Assembly is completed by either wiring the end-point clamp(s) permanently into the appropriate channel(s) or making a small single pin patch cord for each channel. Do not forget the single header pin in each of the end-point out pads. Acknowledgment: I would like to thank Dean Herbert of Microherb Electronics for his assistance with the end-point clamp. Bob Young is the principal of Silvertone Electronics. Phone: (02) 9533 3517. Web-site: www.silvertone.com.au Kit Availability The F3B mixer module is priced as follows: Fully assembled module ........ $99.50 Complete kit with PC board ... $75.00 Double-sided PC board ......... $19.50 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.