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Model Railway e l b a t n Tur By Les Kerr This Turntable is an excellent addition to just about any model railway layout. It allows you to turn a locomotive around at the end of a track and automatically reverses power to the rails, so they aren’t shorted out. The electronics are easy to build, while the other parts can be made with moderate machining skills. R ailway turntables have been around since 1830. Some early engines could only run in one direction, so there needed to be a way to turn them around. The solution was to lay rails on a bridge and then mount the whole assembly on a bearing. To reduce sag as the engine moved onto the ‘table’, four wheels on the bridge extremities transferred the weight to a circular rail that ran around the perimeter. Initially, the turntables were rotated by hand, but later on, they were motorised. I can remember as a young lad being fascinated that the driver and fireman could push a massive steam 42 Silicon Chip engine through 180° with little effort. In Australia, most major country towns on the railways had one. Still, once the steam era ended, they fell into decline as the diesel engines were double-ended; ie, they could be driven from both ends. As I run Peckett-style tank engines on my OO gauge model railway, I searched the internet for a suitably-­ sized turntable and found one at Swanage in the UK. I based my design on that, and you can see it operating in the video at siliconchip.au/Videos/ Model+Railway+Turntable If you have larger engines, the design can be used by increasing its Australia's electronics magazine dimensions. The only restriction is the maximum diameter that your lathe can turn. I am using a bipolar 200-step stepper motor to rotate the train deck. As you need to line up the moving rails with the stationary rails precisely, the motor is driven using eight micro steps. This means the motor moves through ⅛th of a step for each controller input pulse. To rotate it 180°, you need to pulse the motor 800 times, giving better accuracy than in single-step mode. The other challenge is that you need to provide power to the rotating rails and need to reverse the rail polarity once the turntable has rotated through siliconchip.com.au Fig.1: this diagram shows the order in which the major parts are assembled in the stack. The Spring Tension Spacer may not be required, or it might need to be thicker; that can be determined during final assembly. siliconchip.com.au Australia's electronics magazine March 2023  43 SPRING LOADED CONNECTOR PINS (ONE FOR EACH RAIL) ROTATING RAIL PLATFORM WHEEL ASS’Y RAILS WHEEL ASS’Y GIRDER INSULATOR SPRING TENSION SPACER STATIONARY RAIL PLATE CENTRING INSERT BOTTOM PCB (SEE BELOW) HEIGHT ADJUSTMENT SPACER STEPPER MOTOR (NEMA 17) TRAIN CONTROLLER NEGATIVE SUPPLY PCB HELD IN PLACE ON STATIONARY RAIL PLATE BY PINS THROUGH THESE HOLES TRAIN CONTROLLER POSITIVE SUPPLY BOTTOM PCB, VIEWED FROM THE PIN CONNECTION SIDE Fig.2: this ‘cutaway’ overview of the Turntable doesn’t include all the parts and details, but it shows how most of the parts go together. Fig.3: the Centring Insert fits inside the Housing and keeps the stepper motor and Turntable aligned. 44 Silicon Chip Australia's electronics magazine 180°. To achieve this, I used two goldplated spring-loaded pins (shown in Fig.2), one connected to each moving rail. The spring-loaded parts of the pin make contact with the tracks on the stationary gold-plated PCB below. Initially, the first pin is connected to the positive terminal of the controller and the second pin is connected to the negative pin. When the rails rotate through 180°, the connections are swapped. You will need a lathe and a milling machine to make the various parts. The rails must line up in both the vertical and horizontal planes, so it is essential that you use the dials (without backlash) on your milling machine to set the distance between holes and centre lines. Where possible, you should do all operations to the part in one session. To help align the rails in the vertical direction on the Turntable, I placed a grub screw near the end of each rail. By rotating the grub screws clockwise, I could jack up the rail and reduce the height by turning them in the opposite direction. Fig.1 shows the various parts that make up the Turntable. I will go through each one in detail. The materials needed are all shown in the parts list. #1 Centring Insert Photo 1 shows the Centring Insert (Fig.3) fitted into the Housing, made from a piece of 65mm diameter aluminium round bar. Its purpose is to hold the stepper motor axis precisely in line with the axis of the base, ie, on-centre. The critical dimension is the 22mm hole through its centre, which must match the size of the locating boss on the top of the stepper motor assembly. In boring the hole, when I was just below the 22mm diameter, I made 1/1000th of an inch (25-micron) passes until the stepper motor just slid into place. To do this, mount the bar in a three-jaw lathe chuck so that at least 8mm protrudes. Face the end and reduce the outer diameter to 64mm. Drill a hole 8mm deep in the centre using a centre drill, followed by a 5mm diameter drill. Transfer the chuck to the milling machine. Using a centre finder, locate the centre of the 5mm hole. Drill the eight holes, tapping the outer four for M3 and countersinking the inner four holes. Return to the lathe and use a boring tool to enlarge the 5mm hole siliconchip.com.au Photo 1: the Timber Housing (base) with the Centring Insert and stepper motor already inside it. Photo 2: the timber Housing in the process of being turned. Note how the raw timber has been cut into a roughly octagonal shape to make turning it a bit easier. to 22mm, as described above. Part off and face the other side to a thickness of 4mm #2 Timber Housing The critical dimensions of the Timber Housing are the diameter and depth of the 64mm hole into which the Centring Insert fits (see Fig.4). It should fit tightly, and the top surface should be a few thousandths of an inch (about 0.1mm) below the bottom of the 120mm diameter hole. Start with a 140 × 140 × 45mm pine off-cut. To save time, cut off the corners to make it roughly octagonal, with the inscribed circle having a diameter of about 140mm. Use six wood screws and washers to mount the timber central on the lathe face plate (see Photo 2). Turn the outside diameter to 135mm for a length of 35mm. Drill a hole in the centre 35mm deep using a centre drill, followed by a 13mm drill. Fit a boring tool and cut the 120mm diameter hole to 18.6mm deep (see Photo 3). Next, bore out the hole for the Centring Insert as described above. Use a 400-grit emery cloth to smooth the surfaces. Fit the Centring Insert (shown in Photo 1) and, using a 2.5mm diameter drill and the centring piece as a template, drill the four holes that hold it in place. Next, enlarge the four holes in the Housing to 3mm diameter. Align the x/y coordinates of the milling machine with the four mounting holes. Using a 3/8in or 10mm end mill, cut out the rectangular clearance hole for the stepper motor to a depth of 12mm. Check that the stepper motor clears the cutout. siliconchip.com.au Fig.4: the Timber Housing forms the base of the Turntable with the stepper motor inside. The stationary Rail Plate fits inside it and the Turntable part rides on that. This diagram is shown at 75% of actual size. All cutting diagrams will be available for download on the Silicon Chip website. Australia's electronics magazine March 2023  45 Photo 3: at this stage of the turning, the timber Housing is almost complete. Fig.5: the Height Adjustment Spacer fits between the Centring Insert and Stationary Rail Plate. Its purpose is to allow you to adjust the height of the top of the Rail Platform to match the height of the top edge of the Housing. Photo 4: this shows the semi-rectangular recess in the Housing underside, where the stepper motor is mounted. While there, drill the 3.5mm clearance hole for the Rail Plate mounting and the two ¼in (or 6.5mm) holes for the rail power exit holes for the wires. The latter two are elongated at an angle using a round file. This makes it easier to get the wires through in the assembly process. Returning to the lathe, the next step is to machine the rear of the Housing, so it is the correct depth. Use a three-jaw chuck fitted with reverse jaws to hold the machined side against the chuck face, so it runs true. To prevent the timber from splitting when the jaws are expanded, fit a pipe clamp around the perimeter of the Housing (see Photo 4). Use a wood saw to reduce the thickness to about 30mm. Face the end to the finished thickness and clean up all the holes. Using four M3 × 10mm screws and shake-proof washers, fit the Centring Insert into the Housing. Attach the stepper motor using four M3 × 6mm countersunk head screws – refer to Fig.1. #3 Height Adjustment Spacer Photo 5: the Rail Plate inside the Timber Housing. Note that this was taken before all the holes were drilled. 46 Silicon Chip The purpose of the Spacer (Fig.5) is to allow you to adjust the height of the Rail Platform relative to the top edge of the Housing. I made mine from a piece of scrap PCB material 1.6mm thick. The Spacer is to eliminate any variation in material thickness and machining tolerances. You may have to experiment with its thickness or make it out of several pieces to get the correct height. You won’t know this until after the final assembly. #4 Rail Plate The Rail Plate (Fig.6) is a slide fit Australia's electronics magazine into the Housing and, as the name suggests, it has a circular rail on its perimeter for the four support wheels to run on – see Photo 5. These take the weight of the locomotive as it moves onto the Platform. It has grooves cut into the underside for the rail power wires. It is made from a piece of ¼in (6.35mm) thick aluminium plate. To save machining time, I used a hacksaw to cut out a hexagonal piece inscribed on a circle of about 124mm diameter. To enable it to be mounted on the face plate of the lathe for machining, drill and tap four holes marked A as shown in Fig.6. Depending on your face plate size and shape, you may have to move the position of these holes. Drill a further 3mm hole in the centre to centre the workpiece. Mount it to the lathe using M4 machine screws and washers with a piece of Masonite between the faceplate and workpiece, so it runs true. Face the surface, then turn the outside diameter (approximately 120mm) so that it is a slide fit in the Housing. Enlarge the centre hole to 10mm and then use a boring tool to reduce the inside to a depth of 2.7mm and a diameter of 109.6mm. Change over to an RH tool and reduce the outside depth by 2.7mm so that you end up with a rail width of 1.2mm. Using emery cloth, slightly round the top edges of the rail and smooth the Rail Plate surface. The rear of the Rail Plate now has to be machined to size. Remove the plate from the face-plate and remount it so its rear is facing away from the chuck. As you are only facing the surface, it is not essential to set it running true. siliconchip.com.au Fig.6: the Rail Plate fits inside the Timber Housing and is the stationary part, with the Turntable assembly riding on it by the Wheel Assemblies. Holes A are temporary holes used to fix the job to the face plate when machining. They are 3.3mm diameter tapped to M4 and spaced 30° from horizontal on a 40mm radius from centre. Fig.7: the Locating Pins keep the Contact PCB stationary, locked to the Rail Plate while the Turntable rotates above it. Reduce the plate width so that the dimension between the bottom of the rail and the back of the plate is 3.3mm. Remove the job and transfer it to the milling machine to drill the holes and cut the grooves for the wires on the bottom. To centre the job, I turned a piece of scrap aluminium into a disc that was a slide fit in the 10mm hole in the centre of the Rail Plate. I drilled a 5mm hole in the centre of the disc. I then clamped the job down and using precision drilling, bored and tapped holes as shown in the drawing. I loosely clamped the job onto the base of the milling machine and, with a centre finder in the drill chuck, moved the job until the centre finder moved true. I then clamped the job down and, using precision drilling, bored out the holes. Finally, I used a 1/8in (3.2mm) diameter slot drill to cut the grooves for the wires. Now make and fit the Locating Pins siliconchip.com.au for the PCB – see Fig.7. Cut two 4.6mm lengths of 1.5mm diameter brass rod. Clean the ends up using the lathe, then use Loctite 620 to glue them in place into the Rail Plate, in the holes marked F. The last job is to fill the four 4mm holes that were used to mount the job in the machining process. I made a piece of threaded M4 aluminium rod and chopped it up into four 3.5mm lengths. I applied Loctite 620, fitted them in the holes and ground off the excess material. #5 Spring Tension Spacer This is a small washer made of 0.25mm card that is placed under the PCB (see Fig.2). This increases the height of the PCB and hence the tension in the contact. #6 Contact PCB This will be available as a gold-plated Australia's electronics magazine Fig.8: the gold-plated Contact PCB is responsible for transferring power from the stationary Housing to the rotating Turntable above. The springloaded pins moving on its tracks reverse the polarity of the power to the rails as it passes through 90°. March 2023  47 Photo 6 (below): the Contact PCB used in the prototype is not gold-plated like the commercial version we’re making available, but it does the same job of transferring power to the rails. Photo 7 (right): this photo shows the Girder and Wheels attached to the Rail Platform along with the Insulator, springloaded pins and wires connecting the rails to those pins. You can also see where the Fence Posts are glued into holes along either side of the Rail Platform. PCB coded 09103232 (see Fig.8). It is held in place by the brass pins in the Rail Plate and the tension of the springloaded pins. #7 Girder This is made from a 118mm length of rectangular aluminium extrusion, 30 × 15 × 2mm (see Photo 7). You can purchase this from Bunnings in one-metre lengths. Take some time to locate the exact centre, then use a centre drill to drill a hole there, followed by the hole sizes shown in Fig.9. Precision drill all the holes on the top surface. Next, tap the two 1.4mm holes at the ends with 10BA threads. Note that two of the 2.3mm diameter holes are countersunk. The next step is to mill the sloping sides. To save milling time, use a hack saw to remove as much material as possible. Mount the 15mm sides between the jaws of the vice on the milling machine. Rotate the vice 3° and, using a long series end mill, cut the taper at one end until the desired thickness is reached. Repeat for the other end. Mill the thickness to the correct size. #8 Centring Bush As this part is a slide fit into the Girder, you should make it after the Girder is completed. Chuck a piece of 12mm diameter brass rod and reduce the outside diameter to 10mm for an 8mm length. For 1.9mm from the end, further reduce the outer diameter so that it is a slide fit into the 7.5mm hole in the centre of the Girder (see Fig.10). Using a centre drill, followed by a 5mm drill, bore a hole into the end for Fig.9: the Girder sits under the Rail Platform, strengthening it so that it doesn’t flex when the locomotive is driven onto the rails above. 48 Silicon Chip Australia's electronics magazine 8mm. Part off the piece to a finished length of 7.9mm. Transfer the part to the drill press, then drill and tap the hole for the 2.5 × 3mm grub screw. After that, fit the grub screw. The last operation is to glue the Bush into the Girder using Loctite 620 in the 7.5mm hole and drill the Allen key access hole for the grub screw. From the drawing, mark where the Allen key access hole should be. Insert the Bush in place and check that the tapped hole in its side lines up with the marked hole. When correct, drill the 1.8mm hole. When gluing it in place, make sure that the Bush is in the correct location by inserting an Allen key into the hole so that it fits into the grub screw and is at right angles to the side of the Girder. #9 Insulator This is made from a 38 × 25mm piece of blank PCB material (see Fig.11; FR4 fibreglass laminate). Locate the centre of the PCB and use precision drilling to drill the seven holes. Start each hole with a centre drill. The imperial drill size for the 3.97mm hole is 5/32in; the spring-loaded pins are a push-fit Fig.10: the Centring Bush ensures that the Rail Platform rotates evenly about its centre on the stepper motor shaft. siliconchip.com.au Photo 8: this shows how the Wheel Assemblies are mounted to the bottom of the Rail Platform. Ensure they’re angled correctly so the platform rotates smoothly about its centre. Fig.11: the Insulator prevents the pins carrying current to the train tracks from shorting onto the Rail Platform. into them (4mm is close enough if you don’t have a 5/32in drill). (1.6mm) thick sheet of aluminium (see Fig.13). Cut out a piece 50 × 118mm. Find the centre and inscribe a 59mm radius. Using a linisher, cut out the inscribed curved ends. Precision-drill all the holes, remembering that, except for the 2mm diameter holes, they must align with the Girder holes as shown in Fig.9. Countersink the six marked holes and clean off any burrs using emery cloth. #10 Wheel Assemblies The four wheels each consist of three parts: the wheel, the axle and the Housing (see Fig.12 & Photo 8). The wheels are made from ½in brass round bar stock. Face the end and turn the outside diameter to 7.9mm for 3mm. Use a centre drill followed by a 1mm drill to bore out the hole for the axle. Part off for a length of 2mm. Repeat for the other three wheels. For the axles, cut off four 7mm lengths of 1mm diameter brass rod. Clean up the ends in the lathe. The wheel housings are a bit more complicated. As I had to make four of these to the same accurate size, I first milled out a 70mm length of 5 × 7.5mm rectangular aluminium bar. I then mounted it in the vice with the 7.5mm side horizontal and then, using a 3/32in (2.4mm) slitting saw mounted in the chuck, cut the wheel slot 6.7mm deep. Next, I drilled the hole for the axle using a centre drill followed by a 1mm drill. I rotated the job so that the 5mm side was horizontal, then drilled and tapped the 1.8mm hole with an 8BA thread. The distance between this hole and the axle hole should be precisely 6.4mm. Cut off to length and create the 2.5mm radius using a linisher. Repeat for the other three housings. Fit the wheels and axles and, using a dob of Loctite Extreme Glue Gel (available from Bunnings), lock the axles in place. #11 Rail Platform The Platform is made from a 1/16in #12 Fence First, you need to cut 14 Posts 17.8mm in length from hollow 1/16in (1.6mm) square rod, as shown in Fig.14. Once cut, clean any burrs from the ends. Next, use the drilling machine at high speed to drill the holes for the wires to go through (see Fig.15). Again, clean off any burrs. Fig.12: these pieces make up the Wheel Assemblies that allow the rotating Rail Platform to ride on the Rail Plate. Fig.13: the Rail Platform is the rotating part of the Turntable that the train tracks are mounted to. Fences are fitted on either side to make it look realistic. siliconchip.com.au Australia's electronics magazine March 2023  49 Fig.14: these Posts are the vertical parts of the fences on either side of the train tracks. To make the rails for the Fence, cut four 100mm lengths of 0.5mm diameter brass rod. Insert the Posts into the Rail Plate and thread the 0.5mm rails through the Posts on both sides. Use Loctite Extreme Glue Gel to set the Posts and wires. #13 Rails/Tracks The locomotive rails are made from a length of R600 Hornby rail. Reduce the rail length by removing an equal amount from each end so the final length, as shown in the drawing, is 117mm. Clean up the cuts with emery cloth. Check that the existing 1.4mm holes are 90.4mm apart, then enlarge them to 1.8mm. As mentioned earlier, four 2.5mm grub screws should be inserted in the ends of the rail sleepers to adjust their final height. So drill four 2mm diameter holes in the sleepers, as shown in Fig.16, and tap them 2.5mm. Finally, to enable electrical contact Photo 9: The painted top side of the Rail Platform with the rails and Fences attached. Everything is painted matte black except for the rails, so that power can be transferred to the model locomotive. to be made to the rails, carefully remove the plastic shown in red in Fig.16. #14 Painting I sprayed the Rail Platform with two coats of black Rust-oleum Ultra Matte (available from Bunnings). At the same time, I sprayed the heads of six of the 8BA × ¼in screws and the sides of the Girder. I sprayed the top and inside edge of the timber Housing with rust-­coloured paint. Mask the edge of the Rail Plate and spray its top with a couple of coats of Riviera Grey Dulux Duramax Chalky Finish. When dry, use emery cloth Fig.15: here’s how the Fences are mounted on either side of the Rail Platform. Fig.16: the rails/tracks come pre-made but you need to make some modifications. After cutting them to length, some holes need to be added, others enlarged and a couple of pieces of the plastic insulation cut away so the springloaded pins can make contact with the conductive tracks. 50 Silicon Chip Australia's electronics magazine to remove the paint from the top of the rail. #15 Control electronics The chosen stepper motor is a bipolar type rated at 1A per phase and 200 steps. As mentioned earlier, we need to operate it in 1/8th step mode to achieve sufficient accuracy. The Allegro A3967 IC is ideal for the task and provides additional inputs to reverse the motor, regulate the motor drive current and has a 5V DC regulated output to power the driver microprocessor. When I went to purchase the A3967 IC, I found it much cheaper to buy it mounted on a small module named “Easy Driver stepper motor driver”. This also has the advantage that you don’t have to solder surface-mount components. The circuit diagram, Fig.17, shows that the module has four outputs to connect the two windings of the stepper motor. Two other inputs, MS1 and MS2, determine the number of steps per positive going pulse on the step input according to Table 1. As we want 1/8th steps, we leave those terminals unconnected and allow the internal pull-up resistors to keep them high. An enable input turns the driver on when low and off when it is high. Finally, if you ground the direction input, the motor will turn in the opposite direction. To turn the motor through 360° with the Full Step setting, we need to apply 200 pulses. In our case, we only want it to rotate through 180°, but as we are using it in the 1/8th step setting, we will need to apply 800 pulses on the step input. siliconchip.com.au Fig.17: most of the circuitry in the control module is within the Easy Driver stepper motor driving module (yellow shaded box). IC1 sends it signals when pushbutton S1 is pressed to rotate the platform by 180°. The pulse width and the delay between each pulse determine the Turntable rotation time. We are using a PIC12F675 microcontroller to generate the pulses. Its GP2 input (pin 5) is set to interrupt the microprocessor when it goes low, ie, when you press pushbutton S1. The 100nF capacitor from that pin to ground eliminates any contact bounce. The interrupt routine causes digital output GP1 (pin 6) to go low, enabling the motor, and produces 800 positive siliconchip.com.au pulses from the GP0 digital output (pin 7) that step the motor through 180°. At the end of the routine, GP1 goes high again, disabling the stepper motor. #16 PCB assembly The circuit is built on a 56 × 51mm PCB coded 09103231 that the Easy Driver module is mounted on, shown in Fig.18. Header pins are used to make the wire connections to the power supply, pushbutton and stepper motor. Start by fitting the male header pins Australia's electronics magazine (not the ones for the Easy Driver), the 8-pin IC socket, and the capacitors. Of the capacitors, only the 100µF type is polarised; its longer lead must Table 1 – steps per input pulse MS1 MS2 Resolution low low Full Step (two-phase) high low Half step low high Quarter step high high Eighth step March 2023  51 be soldered to the right-hand pad labelled “+”. The IC socket should also be installed the right way around, with its notch to the left. The reason for the IC socket is so that, if we wish to change the program, we can remove the microcontroller and reprogram it. Now add the resistors; they are mounted vertically. The wire link can be made from a leftover resistor wire off-cut; however, if you purchase the PCB from Silicon Chip, it will be a double-sided board, so the wire link is not needed. Fit the PIC12F675 microprocessor in the socket. If you have purchased this from the Silicon Chip Online Shop, it will already have the firmware loaded. If you wish to do this yourself, you can download the files from the Silicon Chip website. To enable the Easy Driver module to be removed, it is socketed. Cut apart the socket strip into five pairs of pins, one three-pin strip and one four-pin strip and solder them to the positions that will be under the Easy Driver. Insert matching male header pins into the sockets, drop the Easy Driver module on top, ensuring all the pins go into its pads, then solder them in place. #17 wiring & testing Check the PCB for solder bridges and dry joints, then wire up the 12V DC socket, stepper motor and the normally-­open pushbutton switch, as shown in Fig.18. The other connections are not used. Before switching on the power, double-­check the power supply polarity connections. The Easy Driver and PIC could be destroyed if they are the wrong way around. Temporarily remove IC1 from its socket. Switch on the power, and the LED on the Easy Driver module should glow. Use a voltmeter to check that you have 5V between pins 1 & 8 of IC1. If it’s OK, switch off the power, wait for the capacitors to discharge, then plug IC1 back into its socket. Next, set the current limit by powering it back up and adjusting the trimpot on the Easy Driver so that there is +4.2V between TP1 and ground. Then press the pushbutton, and you should see the stepper motor shaft rotated through 180°. Press it again, and the shaft should return to its original position. The Easy Driver board has two pairs of shorting pads on it. The first, if closed, reduces the output voltage to 3.3V, while the second enables the 5V output. When supplied, the first link is usually open but the second is closed. If you aren’t getting 5V out of it, check that both are set correctly. The circuit diagram and documentation for the Easy Driver are available at www.schmalzhaus.com/EasyDriver/ #18 mechanical assembly Attach the Insulator with the spring-loaded pins to the underside of the Girder using two unpainted 8BA x ¼in screws and nuts. Next, place the Rail Platform on top of the Girder. Use two painted 8BA × ¼in screws and nuts to join them together. Solder 50mm lengths of hook-up wire to the bottom of each rail (where you removed the plastic), ensuring that the wire insulation goes all the way up to the solder joints. Place the rail over the Rail Platform assembly and insert the wires through the holes marked “D” in the Rail Platform and the Girder. Fit the two 10BA × 3/8in screws, but leave them finger-tight at this stage. Solder the wires to the spring-loaded pins, leaving slack, as shown in Photo 7. Attach the Wheel Assemblies to the Rail Platform using the four remaining painted 8BA screws. The stepper motor, Centring Bush and Housing can now all be assembled as in Photo 1. Fit the Spacer over the stepper motor shaft, followed by the Rail Plate. Use 16mm M3 screws plus extra washers to fit the Rail Plate so that the end of the screws are flush with the plate. The next task is to get power to the Contact PCB. Cut two 300mm lengths of good-quality hook-up wire of different colours and strip away about 3mm of insulation from one end of each wire. Tin the ends and insert the wires into the board from the component side and solder them in place. Use as little solder as possible, as we don’t want any solder on the springloaded contact pins tracks on the PCB. Hold the PCB with the copper side up and feed the wires through the grooves and holes until they exit from the bottom of the Housing. Using a marking pen, place a mark on the edge of the Rail Plate that can be seen from the top, as shown in Fig.6. This mark Fig.18: both the PCB assembly and wiring are straightforward, as shown here. IC1 and the Easy Driver module are both socketed to make replacement and reprogramming (of IC1) simpler. 52 Silicon Chip Australia's electronics magazine siliconchip.com.au will be used later in positioning the Turntable in the final layout. The PCB should now be flat on the Rail Plate and held in place by the two Locating Pins. Strip the ends of the wires and, using an ohmmeter, check that there aren’t any shorts to the Rail Plate. Loosen the grub screw in the Rail Plate assembly and slide it over the stepper motor shaft. Loosen the screws on the Wheel Assembly to adjust their angles so that the wheels align with the track on the Rail Plate. Re-tighten the screws. Push it all the way down until the wheels make contact with the Rail Plate and note how much the springloaded contact pins compress. Ideally, this should be about 1mm. If it is less than 1mm, this can be increased by adding the Spring Tension Spacer, a small washer about 20mm in diameter with an 8mm hole in the centre made from 0.25mm card. It is placed under the Contact PCB. Next, check the level of the bottom of the rails in relation to the Housing side. If it is too low, you can adjust the height by increasing the thickness of the Height Adjustment Spacer. If all is well, tighten the 2.5mm grub screw in the Bush. You should now be able to rotate the Rail Plate assembly freely using your fingers. Connect an ohmmeter to one rail and the other end to one of the wires protruding from the base. Now rotate the Rail Plate assembly; depending on its position, it will either be a short circuit or open-circuit. Do the same for the other rail. #19 homing the stepper motor When you apply power to the circuit (with S1 not pressed), the motor windings receive power for a short time, causing the stepper (rail bridge assembly) to lock in one position. If you rotate the rail bridge less than 7.2° in either direction, on switching the power off and on, the rail bridge will return to the original position. There are 50 positions 7.2° apart at which the motor will lock in place. We need to pick one of these for the point at which the Turntable track and the train entry tracks align. This way, the bridge and entry tracks will be aligned when you switch the power on. #20 final set-up My layout is built on polyurethane siliconchip.com.au Parts List – Model Railway Turntable 1 12V DC 500mA+ plugpack 1 17HS08-1004S 1A 16Ncm stepper motor [eBay] 1 gold-plated Contact PCB coded 09103232, 29 × 29mm 1 assembled control module (see Fig.18) 1 chassis-mounting DC barrel socket (to suit plugpack) various lengths and colours of medium-duty hook-up wire Fasteners 2 M3 × 16mm Phillips panhead machine screws 4 M3 × 10mm Phillips head machine screws 4 M3 × 6mm countersunk head machine screws 6 M3 shakeproof washers 5 M2.5 × 3mm grub screws 8 8BA × ¼in countersunk screws [E & J Winter ] 4 8BA nuts [E & J Winter ] 2 10BA × ⅜in hex head bolts [E & J Winter ] Other hardware 2 Mill Max 0861015208214110 spring-loaded contacts [element14 2751176] 1 70 × 70mm × 1.6mm piece of copper-laminated FR4 (unetched clad PCB) 1 25 × 28mm × 1.6mm blank FR4 laminate (unclad PCB) 1 Hornby R600 rail [K&S Metals] 1 round aluminium bar, 65mm diameter, 15mm long 1 140mm × 140mm × 45mm piece of pine 1 125mm × 125mm × 6.35mm aluminium plate 1 120mm length of 30mm × 15mm × 2mm hollow rectangular extruded aluminium tube [Bunnings 1130544] 1 30mm length of ½in diameter brass round bar 1 35mm length of 1mm diameter brass round bar [K&S Metals] 1 10mm length of 1.5mm diameter brass round bar [K&S Metals] 1 70mm length of 12mm × 12mm square aluminium bar 1 300mm length of hollow 1/16in square brass bar [K&S Metals] 1 400mm length of 0.5mm diameter brass round bar [K&S Metals] 1 20 × 20mm piece of 0.25mm-thick card 1 small container of Loctite Extreme Glue No Drip Gel [Bunnings 0273717] 1 small container of Loctite 620 retaining compound [AIMS Industrial A0116625] 1 spray can of Rust-oleum Ultra Matte black paint [Bunnings 0197886] 1 spray can of Duramax Rust Effect Spray Paint or similar [Bunnings 0195384] 1 spray can of Dulux Duramax Chalky Finish Riviera Grey paint [Bunnings 1400964]  or another specialised fastener supplier Control module parts 1 single-sided or double-sided PCB coded 09103231, 56 × 51mm 1 Easy Driver stepper motor driver [Core Electronics ROB-12779] 1 PIC12F675-I/P 8-bit microcontroller programmed with 0910323A.HEX, DIP-8 (IC1) 1 8-pin DIL IC socket 1 SPST miniature pushbutton switch [Jaycar SP0710] 1 40-pin header, 2.54mm pitch [Jaycar HM3212] 1 40-pin female header, 2.54mm pitch [Jaycar HM3230] 1 100μF 16V radial electrolytic capacitor 2 100nF 50V ceramic, MKT or multi-layer ceramic capacitors 2 10kW 1% ¼W axial resistors 3 6.8kW 1% ¼W axial resistors Australia's electronics magazine March 2023  53 Fig.19: you need to ensure the tracks are aligned vertically and horizontally between the fixed and rotating sections before using the Turntable and that the wiring polarity is correct, so there is no voltage between the co-linear track sections. sheets, so all I had to do was cut a hole the same diameter as the Housing for the Turntable to fit in. The same would apply to layouts built of other materials. The centre of the hole should lie on the projection of the centre line of the entry track at a distance of 59.6mm from the end of the entry track. Fit the Turntable so that the top of the Housing is flush with the surface of the layout and the middle of the external track entry is roughly in line with the mark you previously placed on the Rail Plate. Switch the power on and off to find the homing position of the Turntable track. Once found, rotate the Turntable so the entry track lines up with the Turntable track. Switch the power on and press the rotate push button. If all is well, the other end of the Turntable track should align with the entry track. If not, the Turntable track isn’t aligned exactly in the centre of rotation. You can correct this by elongating the 1.8mm holes for the 10BA screws, allowing you to slightly move the position of the Turntable track on the rail bridge. The last job before tightening the 10BA screws is to adjust the height of the ends of the Turntable track to match those of the entry track. We now need to connect the train controller power to the Turntable track. If it is the wrong way around, the power supply will be shorted out when the engine wheels hit the Turntable track. With no engine on the track, connect the Turntable to the power supply so that there isn’t any voltage between the connecting tracks, as shown in Fig.19. #21 operation Switch the power on and use your train controller to shunt the engine slowly onto the Turntable. Press the rotate button, and when the table stops rotating, back the engine out. SC 54 Silicon Chip Australia's electronics magazine siliconchip.com.au