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RADIO CONTROL BY BOB YOUNG Multi-channel radio control transmitter; Pt.5 This month we discuss the construction of the Mk.22 transmitter encoder PC board. This uses surface mount components throughout apart from the trimpots. If it had relied on conven­tional components it would have been a great deal larger. As a result of the experience gained from test flying of the Mk.22 system the transmitter has undergone a radical redesign from its original simple slide and plug concept for all modules. The original encoder layout proved difficult to program, so we have rotated the PC board through 90° and screwed it to the case as well. This results in all the programming pins being easily accessible from the rear, improved bonding to the earth for RF bypassing and a larger, less crowded PC board. There have also been some additions to the encoder as a result of customer requests. The meter has been changed to an expanded scale voltmeter as the original proved too insensitive. The meter now has zero suppression and reads from 8-10V. Fig.1 shows the circuit addition, involving trimpots VR16 and VR17, zener ZD1, and resistors R70 and R71. These have been included in the encoder PC board featured this month. TB7 also had to be extended from four to six pins. TB30 has been added at the request of a government depart­ ment. They required a hard wired system (no RF link). TB30 allows the receiver decoder to be coupled directly into the encoder with a two-wire patch cord via the existing socket on the decoder. You simply remove the receiver module, connect the patch cord between TB30 and the receiver decoder and presto, you have a hard-wired Fig.1: the zero suppression circuit for the meter which now reads from 8-10V for greater sensitivity. 60  Silicon Chip remote control system. This is also very handy for testing, as we shall see. The original circuit also did not allow for the dual con­trol (buddy box) connections, an oversight fixed by the addition of TB29. Finally and most importantly from my point of view, recent developments have indicated that a welded, seamless case is now economically possible, which will greatly improve the appearance of the finished product. AM vs FM debate The only other comment I receive on a regular basis is “why AM?”. To which I can only reply, “why FM?”. Although the following is a small diversion, I feel that I should deal with this furphy immediately. I have stated it before and will repeat it now: the FM thing is false advertising and largely a sales gimmick. Socalled FM radio control transmitters are not true FM; they are Narrow Band Frequency Shift Keying (NBFSK), with the emphasis on narrow band. Older sets use frequency shifts of as little as 400Hz. This places the signal down in the noise area and it is only recently that most imported sets have gone to a 1.5kHz shift, a small improvement. FM is supposed to offer a vast improvement in signal-to-noise ratio and of course it does when a bandwidth of 40kHz or more is used. NBFSK very definitely does not offer an improvement over AM, especially with systems running on 400Hz deviation. We flew very successfully for 30 years on AM and in the two years that have elapsed since the Mk.22 AM system began flying I have yet to receive a single receiver back due to in- June 1996  61 Fig.3: component layout for the underside of the encoder board. Fig.2: component layout for the topside of the encoder board. All surface mount components should be soldered to both sides of the board before installing the conventional (through hole) compon­ents. This is the topside of the finished encoder board. Note that some of the headers have micro shunts (shorting links) across them. terference. In fact I have only had two receivers back in those two years. One because a wing came off in flight and the crystal shattered (no other damage) and the other because the owner tried to use it with an FM transmitter which he believed to be AM. Yet to listen to the pundits, you Fig.4: this patch cord connects the servo test header, TB30, to the decoder (described in the April 1995 issue) for the final test. Fig.5: to test servos with the encoder and encoder, you will need a control stick. Wire it up to a three-pin socket as shown here. The pot wiper connects to the centre pin. Fig.6: if you do not have a control stick, this circuit can be used for testing servos with the encoder and decoder (see text). 62  Silicon Chip would believe it is no longer possible to operate an AM system. I still fly AM and feel no need to change, especially now that I have the Mk.22 transmit­ter with all its modern tricks. From a home construction point of view, AM is the best system to use because it is reliable and easier to service. It also requires less test equipment, is easier to align, the com­ponents are cheaper and, most important of all, the crystals are cheaper and readily available on the now deserted 29MHz band. Having come this far, I may as well go the full distance. I believe that most R/C manufacturers have lost the plot and are forcing the average sport flyer and hobbyist into buying expen­sive equipment they have little use for. Some of the latest gems being advertised include a transmitter with over one hundred model memories and others with rocker switch electrical trims, a very dangerous concept to my mind. At this point, it is appropriate to remind the reader that the Mk.22 is designed to expand with the user’s requirements, starting with a simple two or 4-channel system and adding as you know and grow. In other words, it is an attempt to provide the modern concepts that users feel are desirable for their applica­tions, combined with a return to the simple and more user friend­ly systems of the pre-microprocessor era. Construction The component layout diagrams for both sides of the PC board are shown in Fig.2 & Fig.3. For those not familiar with surface mount assembly, I suggest reading the article “Working With Surface Mount Components”, as featured in the January 1995 issue of SILICON CHIP. You will need a pair of magnifying specta­cles, a fine-tipped soldering iron and a pair of tweezers with very fine tips. The diagrams of Fig.2 & Fig.3 depict the full component count for a complete 8-channel system with all the trimmings. If you intend to build a simpler version then photocopy the assembly drawings and white-out all of the components you do not need. In fact it is a good idea to do this anyway and then mark off each component as you mount it. Begin by tinning one pad at each of the surface mount com­ponent position, as set out in the above article. Now is a good time to establish which components are to be mounted by only tinning those pads. The surface mount assembly is very straightforward. In fact, the whole assembly is quite straightforward; there is just a lot of it. I usually empty all of the components of one type into a small tray and beginning at the top left hand corner, mount all of the components of one type down through the PC board. When all of the surface mount components are mounted on the topside of the board, turn it over and mount all of the SM com­ponents on the reverse side. Once complete we are ready for the conventional components. The header pins come first and there is no height restric­tion to limit which way they are mounted. You can mount the whole header with the black plastic base included or you can invert the pins (long side through the PC board) as we did in the transmit­ter module, and remove the black plastic base when finished. This gives a much Underneath the assembled encoder board, showing all the surface mount components. You can use this photo as a crosscheck with the component diagram of Fig.2. This view shows how the configuration module, to be discussed in future article, fits on the header pins for the mix expansion socket TB30. neater looking finished item. This is the way the commercial units are assembled. If you do not mount the header pins first then you will not be able to remove the black base. The header pins supplied in the kit are in strips of 40 pins and must be cut into the required number of pins for each terminal block. If you have no intention of expanding beyond eight chan­nels, the header pins TB11, TB12, TB13 and TB14 can be deleted altogether. There is a short on the PC board which automatically programs the PC board to eight channels. If you do intend to go beyond eight channels then this short must be cut and the header pins installed. Do not forget also that all the components not placed during the original build can be easily added later. This close-up view shows the micro shunts fitted to pin pairs 4-11 on TB30. TB29, the dual control (buddy box) header, also has a short across it on the PC board. If you intend to install this feature, install TB29 and cut the track between the pins. Adding this feature will be described in a later column so for the moment leave this track uncut. TB30, the servo test header, must be fitted since it allows you to set up the entire system without the transmitter and receiver modules installed. TB29 is a polarised 2-pin connector so be sure it is mounted exactly as shown on the overlay. This is the only connector not made out of header pin strip. TB10, the mix expand port, is a June 1996  63 of the solder connections are complete. It is very easy to miss soldering one end of a surface mount component or to short out two pins on an IC. Testing This scope photo shows the staircase waveform at pin 1 of IC3a (upper trace) and the pulse waveform from pin 7 of IC1b (lower trace). special case and must be mounted with the plastic base left in place and the long side of the pins uppermost (short side through the PC board). The black plastic base provides the clearance height to keep the configuration module above the surface mount components. This header pin set carries the configuration modules which are used during setup and provides the mix points for the on-board mix­ers. In order to provide access for the configuration inputs, the tracks are broken between each of the pin pairs 4-11 (refer back to the encoder circuit on pages 56 & 57 of the March 1996 issue). For normal operation, shorting links (micro-shunts) must be placed across these pin pairs for circuit continuity. If you do not intend using mixing, then TB10 can be left out and hard wired shorting links wired across the pin pairs 4-11. Again, if you don’t want mixing, all of the components associated with TB27 and TB28, including the headers themselves can also be omitted. The only other item of note in the header pin department is the clipping of pin two on the power and expansion terminal blocks to provide polarisation. This is essential as these con­nectors carry the DC power to the encoder board and the 24-channel expansion board. When we come to wiring the transmitter looms, then we will talk about fitting jumpers to the header sockets. Now complete the assembly by mounting the remaining conven­tional components. Zener diode (ZD1) in the meter circuit is best left standing a little proud of the PC board to keep it well clear of the SM components. Now go back and check your work, taking particular care to ensure that all Kit Availability Kits for the Mk.22 encoder module are available in several differ­ent forms, as follows: Fully assembled module........................................................................$159.00 Encoder kit.............................................................................................$110.00 Encoder PC board...................................................................................$29.50 Post and packing of the above kits is $3.00. Payment may be made by Bank­­ card, cheque or money order payable to Silvertone Electronics. Send orders to Silvertone Electronics, PO Box 580, Riverwood, NSW 2210. Phone (02) 533 3517. 64  Silicon Chip Once you are satisfied that all is well, load the micro shunts onto pins 4-11 of TB10 (mix expand) and onto the NORMAL side of TB1, TB3, etc. Set all potentiometers to the midpoint, including VR2. This pot has been changed to a 10-turn trimpot on the production PC board (same value) to improve the accuracy and stability of the neutral adjustment. Switch your multimeter to a low “ohms” range and check between pins 3 and 6 of TB7 to ensure there isn’t a dead short across the board. Hook up a 10V supply to pins 3 and 6 of TB7 and check the voltages at the following points: input to the voltage regulator REG1, +10V; output of REG1, +5V; junction of the voltage refer­ences R22/R23 and R58/R61, 2.5V; cathode of ZD1, +7.5V and finally, the centre terminal of VR2, +1.5V. With 10V applied, run along the four output pins of IC3 (pins 1, 7, 8, 14) and check with an oscilloscope to see that pulses are present. The waveform at pin 1 should be as in the scope photo accompanying this article. Now go to pin 1 on power connector TB7 and you should have a negative going pulse of about 10V peak-to-peak with a pulse width of approximately 1.5ms. There should be nine nega­tivegoing spikes. Congratulations, you now have a working basic encoder modu­le. All that remains for this month is to make up a patch cord to connect the servo test header, TB30, to the decoder (described in the April 1995 issue) for the final test. This patch cord is quite simple, consisting of a ground and signal connection – see Fig.4. Take care to get the polarity correct on the 2-pin connector for TB30. The output of TB30 is a positive-going pulse in order to match the receiver output. The 3-pin plug for the decoder is a bit of a problem as this socket is non-polarised, so paint dots on the mated connector to ensure correct alignment during later use. The ground connec­tion is the pin closest to the receiver crystal. The socket we are discussing here is the black plastic socket used to mate the receiver to SILICON CHIP SOFTWARE 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. 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Plug the patch cord onto TB30 and into the decoder socket. Plug in a receiver battery and one or more servos or better still, a pulse width meter. If you do not have a pulse width meter, then a servo set to 1.5ms neutral (most modern servos) will be quite adequate. Remove any connections you may have to the encoder except the power lead and patch cord. Switch on the power to both the encoder and decoder and the servos should all take up the same position. Adjust VR2 to bring the servos to neutral (1.5ms) and you now have an aligned encoder. If you do not have a Mk.22 receiver then you may want to hook up the encoder to the transmitter module. Simply connect ground, 10V and signal on the two boards, set the programming shunt on TB3 in the RF module to the AM position and you should have a modulated RF signal adequate in strength to drive the receiver at close range. Carry out the above adjustment and you are all set for the programming which will be described when we deal with system alignment. Finally, for those who just cannot contain themselves and must see a servo move from an input, if you have an old control stick, just wire up a 3-pin socket as shown in Fig.5. If you do not have a control stick then wire up a 5kΩ linear pot as shown in Fig.6. The two 4.7kΩ resistors simulate the mechanical stops in the control sticks. Set the programming shunts supplied to the NORMAL position on the input programming headers TB1, TB3, etc. Plug the pot into the channel 1 input and the servo into channel 1 on the receiver/decoder. Rotating the pot or moving the stick will result in servo movement. To reverse the direction of travel, simply rotate the pot connector through 180°. When satisfied that all is working and the novelty has worn off reversing the servo, check each channel input. A word of warning here. When reversing the servo, keep in mind that the pulse width must be set at precisely 1.5ms (servo in exact neutral) or else any error in position will be multi­plied by a factor of two when you reverse the servo. In other words, if a servo is at one end of its travel (for example the throttle), then it will fly to the other end as soon as you re­verse its travel. Next month, we will discuss construction of the transmitter case. SC June 1996  65