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REMOTE CONTROL BY BOB YOUNG A 16-channel decoder for radio control This decoder is intended to be used with the 8-channel decoder published last month to give a total of 24 channels. It plugs into the 8-channel board and the piggyback AM receiver to give a very compact 24-channel receiver. The 16-channel expansion PC board is the third in the set for the Mk.22 receiver and in the following description will be termed PC board 3. The receiver board will be referred to as board 1 and the 8-channel decoder as board 2. To fit this expansion board to an existing 8-channel Mk.22 receiver, a new case bottom is required. The case lid remains the same although two additonal “U” shaped slots must be filed in the lid to accommodate the grommets for the first 8 channels which must now be on fly leads (see Fig.1). If the original 8-channel decoder has PC header pins for the servo connectors, these must be replaced with flyleads as there is no access to the PC pins once the third board is in place. The expansion port connector may also need to be fitted in addition to any missing servo connectors. The photograph of Fig.2 shows two prototype receivers, one 8-channel and the other a 24-channel. Note the lack of cover on the third PC board to highlight the similarity in receiver sizes. There is a surprisingly small difference in the size of the two receivers. The overall height of the 24-channel receiver is 39mm as against 29mm for the 8-channel receiver. The photo of Fig.3 shows the same 24-channel receiver with the cover removed. Note the location of the crystal and the 4-way header connector used to mate the receiver board to the 8-channel decod­er. The polarised servo connector access holes have not been punched in this case yet and the crystal access hole will not appear in the production 24-channel case. To show the progress in design over the years, the photo of Fig.4 shows an original 24-channel receiver supplied by Silver­ tone Electronics in about 1980. Note the large size and the original Mk.14 receiver module. NEW SLOTS PCB 1 PCB 2 PCB3 Circuit description If you wish to follow the circuit description, then I suggest you will also need to refer to page 71 of the April 1995 issue of SILICON CHIP. The basic circuit of Fig.5 consists simply of two additional shift registers (IC2 and IC3), with data, clock and enable being derived from PC board 2. The +4.8V and GND terminals are separate pads which are intended as the power input for the full 24-channel system with all 24 servos fitted. A “Y” lead made out of normal servo leads would not be at all adequate here, for the total current consump­tion of 24 servos could run up to 8A or more, if many servos were to switch simultaneously. The running current of a standard servo is about 100-150mA, so it is really only the start-up current that concerns us here. Fig.1: this exploded diagram shows how the three boards are assembled into the case. PCB1 is the receiver, PCB2 the 8-channel decoder & PCB3 the 16-channel decoder. These two inputs are connected directly to the servo power rails and power distribution to the 16 servos associated with this PC board is direct to the servos through TB11-26. TB27, the expansion port, provides May 1995  53 Fig.2: this photo shows two prototype receivers, one 8-channel & the other a 24-channel. Note the lack of cover on the third PC board to highlight the similarity in receiver sizes. the incoming data, clock and enable signal, as well as providing power to the receiver (PC board 1) and the 8-channel board (PC board 2). Resistors R19-R35 and capacitors C17-C32 form the noise filter networks (referred to last month) for the servo leads. R34 is a zero-ohm resistor and is used only as a jumper. C33 is a bypass capacitor for the power rail. In operation, the last channel (channel 8) on PC board 2 (IC1) is used as the data pulse for IC2 and is fed to pins 1 & 2 through jumper R34. The clock line is fed directly to pin 8 on each 74HC164 and the chip enable signal is again applied directly to pin 9 on both ICs. This voltage is derived from R13 & C13 on PC board 2. Thus, all three shift registers are running on commoned clock and enable lines, with the last output on each 74HC164 providing the data input (pins 1 & 2) for the following chip. Provided the signal conditions are correct on pins 1, 2, 8 and 9, the clock Fig.3: this photo shows the 24-channel receiver with the cover removed. pulses will be clocked through the shift reg­isters and an output pulse (high) will appear at each of the output pins, with a duration which is directly proportional to the control stick location. If all 24 channels are being trans­mitted, the sync pulse detector (R10, C10) on PC board 2 sets the data pins 1 & 2 on IC1 high after Fig.4: this photo clearly demonstrates the progress in design over the years. It shows an original 24-channel receiver supplied by Silvertone Electronics in about 1980. Note the large size & the original Mk.14 receiv­er module. 54  Silicon Chip about 6ms and the count begins again from channel 1. If less than 24 channels are being transmitted, then the pulse output after the last transmitted channel will be the sync pause. For example, a 6-channel transmitter with an 8ms sync pause will generate a high pulse on the channel 7 output which will be 8ms wide. Channel 8 will be a repeat of channel 1, channel 9 a repeat of channel 2 and so on, until channel 14 which will again be the sync pause (8ms). The sequence will repeat again until channel 21 which is the next sync pause and from there to channel 24 which will be a repeat of channel 3. At this point, the sequence stops until the next sync pause resets data high on IC1 and the se­quence begins all over. As a result of this train of events, a 24-channel decoder can be tested with a 2-channel transmitter, in which case the sync pause will appear at every third channel output. As long as the output goes high for the correct amount of time, then you know the decoder is working. If less channels are installed in the receiver than in the transmitter, then the count will proceed in an orderly manner until the last clock pulse and then wait until the sync pause appears, which will reset the data on channel 1 high and the count begins again. In other words, all outputs after the last clock pulse will remain low. Thus, a 7-channel receiver with a 16-channel transmitter will only give outputs on the first seven channels. As you can see from the foregoing, there does not need to be any compatibility between the channel counts on the transmit­ter and receiver. Any receiver will work on any transmitter as long as both are AM. The only difficulty that may arise is that the sync pause in some commercial brands of transmitters may be shorter than the time constant on the sync separator (R10, C10 on PC board 2). In this case, reduce the value of C10 until the correct time constant is arrived at. The waveshapes at the various key points on PC board 3 are all repeats of the waveshapes pictured last month. Pins 1 & 2 on IC2 receive the output of channel 8 which corresponds to Fig.5 on page 72 of the April issue. Pins 1 & 2 on IC3 receive the output of channel 16, so it is similar again to the preceding oscilloscope trace. The May 1995  55 R34 0 14 O3 O2 O1 O0 6 5 4 3 R19 1k R20 1k R21 1k R22 1k C32 .001 C17 .001 C18 .001 C19 .001 C20 .001 C21 .001 C22 .001 R25 1k C23 .001 R26 1k R24 1k R23 1k SILVERTONE MK22 24 CHANNEL DECODER 7 10 IC2 O4 1 74HC164 11 O5 A 12 2 B O6 13 O7 9 MR Fig.5: the 16-channel expansion board circuit consists simply of two additional shift registers, IC2 and IC3, with data, clock and enable being derived from the 8-channel decoder described last month. EXPANSION TB27 8 CLK CHANNEL 16 TB11 CHANNEL 15 TB12 CHANNEL 14 TB13 CHANNEL 13 TB14 CHANNEL 12 TB15 CHANNEL 11 TB16 CHANNEL 10 TB17 CHANNEL 9 TB18 14 O2 O1 O0 5 4 3 7 6 O3 10 IC3 O4 1 74HC164 11 O5 A 12 2 B O6 13 O7 9 MR 8 CLK C33 47 R35 1k R33 1k R32 1k R31 1k R30 1k C26 .001 C31 .001 C30 .001 C29 .001 C28 .001 C27 .001 R29 1k C25 .001 R28 1k C24 .001 R27 1k CHANNEL 24 TB26 CHANNEL 23 TB25 CHANNEL 22 TB24 CHANNEL 21 TB23 CHANNEL 20 TB22 CHANNEL 19 TB21 CHANNEL 18 TB20 CHANNEL 17 TB19 GND +4.8V TB11 TB12 TB13 TB14 TB15 TB16 TB17 TB18 TB27 J1 1 C32 IC3 74HC164 IC2 74HC164 R34 R26 C23 C21 C22 R25 R23 R24 C20 C19 R22 R21 C18 C17 R19 R20 J5 TB26 1 R35 TB25 J2 C31 R33 C29 C30 R32 C28 R31 R30 C27 R29 C26 C25 R28 C24 R27 C33 +4.8V GND TB19 TB20 TB21 TB22 TB23 TB24 Fig.6: both sides of the PC board are shown here. Note that C33, a tantalum capacitor, is quite large & must be laid on its side, as shown in Fig.7. clock pin (pin 8) on both IC2 and IC3 should correspond to Fig.3 from last month. The enable pin (pin 9) on each IC will again have a DC voltage with a shallow ripple. This voltage should be in the order of +4.5V. One final point on the servos themselves: servos designed for modelling applications are usually designed around a 14-20ms repetition (frame) rate and the pulse stretching capacitor in the servo is chosen accordingly. With all 24 channels being transmitted, the frame rate will be somewhat longer. The worst case will be with all channels at extreme width, which gives a frame rate of (24 x 2) + 8ms = 56ms. Some servos may begin to slow down at this frame rate and the pulse stretching capacitor must be increased to compensate. Assembly Begin by setting up the polarity of the servo rails. As delivered, the PC board is set up as centre-pin positive, which suits such sets as Futaba, JR and Hitech. To reverse the polarity of the system, simply cut the thin tracks connecting the compon­ ent supply rails to the power rails as shown in the component wiring diagram of Fig.6. This shows both sides of the PC board. There are small pads situated alongside the power rails for this purpose. Reconnect the supply rails to the power rails using 10A fuse wire or a thin component leg. Note that one pad is located on the top layer and the other on the bottom layer. Remember here that the same must be done to the 8-channel decoder PC board to keep 56  Silicon Chip the whole system compatible. Be sure to mark the finished receiver clearly, positive or negative centre-pin, when you have finished the receiver. Begin the assembly by tinning a single pad for each surface mount component as usual and mount all of the resistors and capacitors on the busy side of the board. When this is complete, turn the PC board over and place the two ICs and the three 1206 packages on the reverse side. Next, mount the required number of servo connectors in the appropriate locations. Once again, these are mounted from the busy side of the PC board with the plastic on the busy side and the long section of the servo pins going through the PC board. Solder the pins on the IC side of the PC board and snip them off flush. Now remove the plastic from the pins and you have a set of pins the correct length for servo connectors. You can build this PC board with only eight Receiver & Decoder Kit Availability Receiver PC board (double-sided with plated-through holes) ..........$11.50 Basic receiver kit: all parts except crystal .........................................$45.00 Built & tested AM receiver less crystal .............................................$59.00 Decoder PC board (double-sided with plated-through holes) ..........$11.50 8-channel decoder kit: all parts less servo pins or connec­tors .........$32.00 Built & tested 8-channel decoder but less servo plugs ....................$45.00 Expansion kit: all components to build the 16-channel decoder ......$42.00 Built & tested 16-channel decoder less servo connectors ...............$55.00 8-channel receiver case (includes labels) ........................................$11.50 16-channel receiver case (includes labels) ......................................$19.50 Machine wound RF coils ....................................................................$2.95 Machine wound IF coils ......................................................................$2.95 Crystals (AM) per pair ......................................................................$17.95 Servo header pins (each) ...................................................................$0.12 Futaba Ext lead ..................................................................................$3.40 J.R. Ext lead .......................................................................................$3.40 Sanwa Ext lead ..................................................................................$3.40 Notes: (1). When ordering crystals, do not forget to specify frequency. (2). All orders should add $3.00 for postage and packing. Payments may be made by cheque, money order, Bankcard, Visa Card or Mas­tercard. Send all orders to Silvertone Electronics, PO Box 580, Riverwood, NSW 2210. Phone (02) 533 3517. Fig.7: this photo shows the completed 16-channel expansion PC board from the servo connector side. Note the tantalum capacitor (C33) which must be laid over on its side. Fig.8 (right): this view shows the completed 24-channel receiver with the receiver deck removed. The receiver plugs into the 8-channel decoder board & this in turn plugs into the 16-channel decoder which sits in the bottom of the case. channels, in which case delete IC3 and all its associated components. Next, mount the 5-pin expansion socket which mounts on the opposite side of the PC board to the servo pins. Finally, mount the 47µF capacitor C33. This component must lay flat against the PC board, so bend its legs at right angles as close as possible to the capacitor body, taking care not to go too close to the enamel in order to avoid breaking the seal on the legs. Be careful to ensure the correct polarity before bend­ing. This completes the component assembly. Unfortunately, I had to use four jumpers on this PC board because of the lack of space. These must now be added. Use the wire-wrap wire supplied for J1 and J2 and the 5A hook-up wire for the two power rail jumpers J3 and J4. There are pads provided for J1 and J2 but J3 and J4 are soldered direct to the power rails. Finally, solder the two lengths of 8A hook-up wire into the power input pads, red for positive and black for negative (ground). Do not forget to reverse the order of these wires in the power input pads if Sanwa, KO or other centre-pin negative servos are to be used. Make sure here that the appropriate changes have already been made to the supply rails as described above. Fit the 8A connector provided, red closest to the triangular side of the housing and using the pins in this housing. That completes the assembly of the PC board. Testing Using a pre-tested receiver and 8-channel decoder, plug the 16-channel PC board into the expansion socket with the assembly out of the case. Great care must be exercised here to ensure the PC boards do not touch. A piece of cardboard inserted between the two will prevent shorts. Better still, make up an extension lead to separate the two PC boards so that you can work a lot more easily on both sides of the board. Switch on the transmitter and receiver and check the waveshapes at pins 1, 2, 8 and 9 on each IC and compare them with the oscilloscope photos in last month’s issue. Next, check each servo output and compare them with Fig.5 (last month). All 16 outputs should be more or less identical if a 24-channel transmitter is used. If a lesser number of channels is transmit­ted, see the above circuit description of where the sync pause will appear at the servo outputs. Do not plug a servo into this channel, as it will drive the servo hard against the gear stops and damage the servo gears. If you are using a transmitter with less channels than the receiver, be sure to cover these outputs to prevent accidentally plugging a servo into these channels. If all channels are working, it remains only to clip the three PC boards into the case bottom and the unit is complete. Debugging follows the same general pattern as that described last month. This PC board is very simple and few problems should be encountered if care is taken during assembly. The photo of Fig.7 shows the completed 16-channel expansion PC board from the servo connector side. The photo of Fig.8 gives an excellent view of the completed 24-channel receiver with the receiver deck removed. The original case lid is used but two additional slots must be filed into the case side to accommodate the grommets for the servo leads on PC board 2, which come out of the case side – see Fig 1. Congratulations, you now have a SC working 24-channel receiv­er. May 1995  57