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CARRIAGE UNCOUPLER for model railways By Les Kerr This mechanism automatically uncouples carriages from a locomotive or other carriages, adding realism to a model railway layout. It’s hidden under a section of track and activated by a switch after the locomotive is driven into position. It’s actuated by a servo motor with simple control electronics and a relatively straightforward mechanical system that you can make. L ocomotives and carriages can be coupled by simply pushing them together, but uncoupling them requires more work. This device uses a special section of track to automatically uncouple carriages, allowing you to reconfigure your model trains in a realistic manner. You can see it in operation in the video at siliconchip.au/link/abl8 Initially, I thought I could use a solenoid to raise a platform above the rails, thus lifting the coupling hooks on both the carriage and locomotive, allowing them to be pulled apart. I found a miniature solenoid that moved through 5mm when activated. On checking with a carriage, I found that I only needed the solenoid to move through 2.7mm to uncouple it. However, even with a 50% duty cycle, the 1.1A solenoid required 600mA continuously from the power supply, which seemed like a lot to lift a little platform. The other problem is the speed at which the solenoid operates. 68 Silicon Chip It would be travelling very fast when it hit the platform-lifting pin, raising the platform too rapidly and making a lot of noise when it hit its maximum height. This made me decide instead to use a miniature servo motor. Such a motor would only draw a few hundred milliamps at most, and a basic 8-pin microcontroller could easily control its speed and travel distance. Initially, I thought I could couple the servo arm that came with the servo to lift the platform raising pin, but I found that the servo would only have to move through a few degrees to achieve the required 2.7mm lift. A 1ms control pulse change will make a servo move through 90°, so we would have to change the pulse length by just tens of microseconds to get a change of just a few degrees. To control the speed of the motor, we feed it with increments of about 1/20th of the total pulse width until the required duration is reached. Unfortunately, these increments would only Australia's electronics magazine be one or maybe a few microseconds, which is difficult to achieve reliably. The solution was to use a cam attached to the servo shaft, which provides the 2.7mm lift when the servo rotates through 90°. The minimum and maximum lift values are set using two potentiometers. Figs.1(a) & (b) show the final arrangement of the metalwork in both the Platform up and down positions. A piece of single-length Hornby OO scale rail is attached to two L-shaped brackets by two 10BA screws. With the Platform down, the cam is rotated fully anti-clockwise to its minimum lift position. As the pins are firmly fixed to the Platform by Loctite, the springs and gravity pull the Platform down until it touches the sleepers, so the Platform is roughly level with the rails. Having three pins means that the Platform always remains parallel to the rail. The cam is attached to the servo motor shaft by the 8BA screw. siliconchip.com.au Fig.1: three views of the completed Uncoupler mechanism; (a) from the side in the down position, (b) in the up position, (c) from underneath. The Platform slides on three Pins, two held in Collars supported by Springs, and one in a Bush that the Cam acts on. When the servo motor rotates clockwise, the cam follows, putting pressure on the centre pin with the result that the Platform lifts and the springs compress, as shown in the Platform up drawing. Servo control The control circuit is shown in Fig.2. To rotate the servo motor through 90°, it is fed with continuous siliconchip.com.au 2ms-wide pulses at about 50Hz in the down position and 1ms pulses in the up position. These come from the GP0 digital output (pin 7) of microcontroller IC1. The exact pulse widths and thus, up and down positions, are set using trimpots VR1 & VR2. They are connected across the 5V supply with padder resistors to generate 2-3V (VR1) and 2.7-3.7V (VR2) at their wipers. That Australia's electronics magazine voltage is measured ratiometrically (so the exact voltage of the 5V supply doesn’t matter) using IC1’s internal 10-bit analog-­to-digital converter via the AN1 (pin 6) and AN3 (pin 3) inputs, respectively. The 10-bit ADC produces values from 0 to 1023 (210 − 1) for voltages of 0-5V. The software multiplies the value measured at AN1 by two for a delay in microseconds, so the range is July 2023  69 Fig.2: the control circuit is straightforward, with microcontroller IC1 generating 50Hz pulses to control the servo motor. The up/down switch, S1, selects which of trimpots VR1 & VR2 determine the pulse width and hence target servo rotation. The positions are usually set to vary by about 90°. 0-2.046ms with 1.023ms at the midpoint. Similarly, the reading from pin 3 of IC1 is multiplied by 3 for a range of 0-3.069ms for VR2, with the midpoint giving about 1.535ms and about 2ms at its 2/3rds position. The 10μF capacitors from these two pins to ground stop any supply noise or ripple from affecting the ADC readings. When up/down switch S1 is in the down position, digital input GP2 (pin 5) of IC1 is low, so VR2 is used to determine the servo motor pulse lengths, resulting in it turning anti-clockwise. With S1 up, it changes to shorter pulses based on VR1, causing the motor to rotate clockwise. The 100nF capacitor from pin 5 to +5V protects the input from stray RF, while the 5.6kW pull-down resistor ensures GP2 is always high or low. The Fig.3: like the circuit, the PCB is pretty simple. The three + pads are for 5V power in/out, S1 is the switch common, SIG is the servo motor’s control signal, and the two 0V pads are grounds. The only polarised components that can be inserted incorrectly are IC1 and the three electrolytic capacitors. 70 Silicon Chip 100μF and 100nF capacitors across the supply stabilise the supply voltage for IC1. PCB assembly Assembly of the control PCB, shown in Fig.3, is straightforward. The PCB is coded 09105231 and measures 34 × 48mm, and the assembled PCB is shown in Photo 1. Pin headers are used to connect the wires to the board. Start by fitting the header pins, the 8-pin IC socket and the capacitors. The IC socket makes it easier to remove the microprocessor and re-program it later if necessary. Take care to orientate the socket and the electrolytic capacitors correctly. Now add the resistors, which are mounted vertically. Don’t fit the PIC12F617 microprocessor yet. 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, the files can be downloaded from the Silicon Chip website, but you will need a suitable programmer and socket adaptor. Wiring it up Using hookup wire, connect the up/ down switch, power pack and servo as shown in Fig.4. Check that the +5V lead of the power pack connects to the PCB positive terminal and the 0V lead goes to the 0V point on the PCB. The red wire from the servo should connect to the +5V terminal of the PCB and the brown wire to the 0V terminal on the PCB. Finally, the orange wire from the servo should connect to the servo output on the PCB (“SIG”). Testing the electronics Photo 1: the PCB is a single-sided design. This photo shows it fully assembled and wired up via singlepin headers. Before powering it up, check that the 100μF and 10μF capacitors are orientated correctly and inspect the rear of the PCB for dry joints or solder bridges between pads or tracks. Rectify if necessary. Next, power up the 5V supply and connect the positive lead of a digital voltmeter to pin 1 of the IC socket and the negative lead to pin 8. If you get a reading of +5V, you can proceed. If you read -5V, either the IC socket or the 5V supply is reversed. Remove power and plug in the PIC12F617 microprocessor as shown in Fig.3, with its pin 1 end over the socket notch. Set trimpots VR1 and VR2 to their mid positions. If you have Australia's electronics magazine siliconchip.com.au Fig.5: the Cam is a metal ellipse with one side cut flat and a couple of holes drilled. It’s made from a cylindrical piece of aluminium cut to 3mm thick and then ground into this shape. Fig.4: the wiring is straightforward, as shown here. Consider how long you need the wires to be, especially from the control board to the servo. Most servos come with relatively short wires, so they will probably need to be extended. an oscilloscope, connect it between the servo connection and ground, and set the vertical deflection to 10V and the timebase to 500μs. Switch the up/down switch to up (closed) and apply power. The servo motor should rotate clockwise to its maximum position, and the oscilloscope should display a positive-going 5V pulse of about 1ms width. Rotate VR1, and you should see the servo motor move and the 1ms pulse width change. Rotate VR1 back and both the servo motor and pulse widths should return to their original positions. Leave VR1 in its mid position. Now change the up/down switch to the down position, and the servo should rotate about 90° anti-­clockwise, with the pulse width increasing to about 2ms. This time, adjust VR2 and the pulse width and servo motor should change position. Leave VR2 in its mid position. In one of my previous projects that used the same servo motor, one user complained that the motor continually rotated. On investigation, we found that you can purchase a servo that is the same size but designed for 360° rotation. You need to use the 180° type in this project. Making the mechanical parts The next job is to make the Cam, shown in Fig.5. Chuck a piece of 25.4mm (one inch) aluminium round bar stock with about 5mm protruding from the chuck. Face the end and bore siliconchip.com.au Photo 2: print, cut out and glue the Cam shape guide onto the metal disc as a guide for grinding it to the required oval shape. Parts List – Model Railway Carriage Uncoupler 1 5V DC 1A plugpack 1 180° 9G servo motor [DF9GMS; Core Electronics SER0006] 1 Hornby R600 or R601 rail section 1 single-sided PCB coded 09105231, 48 × 34mm 2 2kW top-adjust mini trimpots (VR1, VR2) 1 PIC12F617-I/P 8-bit microcontroller programmed with 0910523A.HEX, DIP-8 (IC1) 1 8-pin DIL IC socket (for IC1) 1 SPDT toggle switch (S1) [Jaycar ST0335] 1 7-pin snappable header, 2.54mm pitch or 7 PC stakes Capacitors 1 100μF 16V radial electrolytic 2 10μF 16V radial electrolytic 2 100nF 50V ceramic Resistors (all 1/4W 1% axial) 1 10kW 2 5.6kW 2 3.9kW 1 2.7kW Hardware 1 aluminium plate, 60 × 10 × 2.5mm 1 205mm length of 40 × 25 × 1.6mm aluminium unequal angle [Bunnings 1138199] 1 20mm length of 25mm or 1in diameter aluminium round bar stock 1 70mm length of 3/32in [2.4mm] brass round bar stock [K&S Metals] 1 30mm length of 20mm diameter aluminium round bar stock 1 40mm length of 10mm diameter aluminium round bar stock 1 can of Rust-oleum Ultra Matte black spray paint Fasteners 2 10BA x 1/4in or 3/8in hex head machine screws [EJ Winter] 3 8BA x 3/8in, 12mm or 1/2in hex head machine screws [EJ Winter] 2 M3 × 6mm panhead machine screws 2 M2.5 × 8mm panhead machine screws 4 M2.5 hex nuts Wire 1 4m length of 0.315mm diam. nichrome resistance wire [Jaycar WW4040] various lengths and colours of light-duty hookup wire Australia's electronics magazine July 2023  71 Photo 3: after grinding, the Cam has had a flat cut in its side, a hole drilled in the middle and a tapped hole in the centre of the flat side. Fig.6: a 7mm hole needs to be made in the middle of the rail for the mechanism to project through, and the existing mounting holes need to be enlarged, as shown here. a 4mm deep hole using a centre drill followed by a 4.9mm drill. Reduce the outside diameter to 25.2mm and part off a 3mm section. You can download a 1:1 drawing of the Cam outline as a PDF from the Silicon Chip website. Print this at actual size and cut around the circumference using scissors. Glue this to the 3mm section using a suitable glue (such as Tarzan’s Grip), so it is symmetrically placed, as shown in Photo 2. Transfer the 3mm section to the linisher and carefully grind out the shape of the Cam on one side. Next, use a hacksaw to remove the lower section and clean up the edge with a file or an end mill in the milling machine. Then transfer the job to the milling machine and drill and tap the hole for an 8BA screw. The tapping drill size for 8BA is 1.8mm. Use emery cloth to clean up the remaining edges and the 4.9mm hole. The result is shown in Photo 3. Hornby rail modification The required modifications are shown in Fig.6 and Photo 4. You can use either the Hornby R600 single rail or the Hornby R601 double rail; the difference is the spacing of the 1.4mm holes. For the R600, it is 90.4mm, and for the R601, it is 76.8mm. Enlarge the two existing 1.4mm holes to 2mm in diameter. To allow clearance for the 7mm diameter end of the Bush, parts of the two middle sleepers have to be removed. Use a small half-round file to do this. Two Springs The two Springs to make are shown in Fig.7 and are visible in Photo 5. Use a piece of 8mm diameter rod as a former and close-wind two turns of 28 B&S (0.33mm diameter) nichrome wire at one end, followed by three turns spaced 3.2mm apart in the middle and finally, two turns at the end. Trim off the excess wire. Mounting Bracket The Mounting Bracket, shown in Fig.8 and Photo 6, is made from a 25 × 40 × 1.6mm aluminium L-shaped extrusion. Cut the extrusion to 100mm long and clean up the ends with a file, or do the whole operation in the milling machine fitted with a slot drill. Use a hacksaw to remove the rectangular sections at the ends of the 40mm side of the extrusion. Photo 4: the Hornby rail after modification. As well as making the central hole, the two preexisting attachment holes have been enlarged. 72 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.7: the Springs are wound from nichrome wire on a 8mm diameter cylindrical former. Photo 5 (right): the assembled Uncoupler with the Platform in the upper position. The Cam is rotated so that its long axis is pressing on the central Pin. Use a 1/8-inch or 3mm end mill in the chuck of a milling machine to clean up the cuts to size. For smooth operation of the Platform, the location of the 3/32in holes in the Platform should match the corresponding 2.5mm and 5mm diameter holes in the Mounting Bracket (fitted with the Bush) exactly. If they don’t line up, the Platform will jam in operation. If you haven’t any means of precision drilling, I suggest you clamp the Platform and the Bracket together, then drill the 3/32in holes through both (instructions for making the Platform are below). The end holes in the Bracket can then be enlarged to 2.5mm and the centre hole to 5mm in diameter. On the same centre line, drill and tap the 10BA holes. The drill tapping size for 10BA is 1.4mm. If you are using the single R600 rail, you need the holes marked F1, or if using the R601 double rail, you need the holes marked F2. Turn the Bracket over onto the 40mm side and use a centre drill followed by a 2.5mm drill to make the two holes in the centre of the slots. If you have a milling machine, use a 2.5mm slot drill to elongate the holes. If you don’t have one, use needle files to perform the same operation. Use a hacksaw and chain drilling to make the 23 × 24mm rectangular notch. If you don’t have a milling machine, smooth the sides with a series of files. I did this using a milling machine fitted with a 2.5mm slot drill. Next, using a 2.5mm drill and an M3 tap, make the holes for the cover bracket connecting screws. Bush This should be made after the Mounting Bracket as it must be a tight fit in it. The details are in Fig.9; you can see it inserted in the Bracket in Photo 6. Chuck a piece of 10mm diameter aluminium round bar stock with 12mm protruding from the chuck. Face the end and, using a centre drill, then a 2.4mm drill, bore a 12mm-deep hole. Reduce the outside diameter to 7mm Fig.8: the Mounting Bracket is cut from a length of aluminium angle stock. siliconchip.com.au Australia's electronics magazine for a depth of 11mm. Further reduce the outside diameter to just over 5mm for 7.5mm, then reduce it further by small amounts until it is a slide fit in the 5mm hole in the Bracket. Part it off to a length of 10mm. Finally, insert the Bush into the Bracket and lock it in place using Loctite 620. Be sure to clean out any excess Loctite from the centre of the Bush and any remaining on the Bracket. Leave the piece for 24 hours to let the Loctite set, then mask the holes in the Bush with tape and give the Bracket several light coats of Rust-oleum Ultra Matte black spray paint. Two Collars The Collar details are in Fig.10 and they are visible in Photo 5. Chuck a piece of 10mm diameter aluminium round bar stock with 6mm protruding. Face the end and, using a centre drill followed by a 2.4mm drill, bore a hole 4mm deep. Part off a 3mm section. Using the mill, drill the 1.8mm hole for the 8BA screw and tap Photo 6: the Mounting Bracket is on the right, with the Platform attached to it via the three Pins, while the Cover Plate is on the left. The Cover Plate mounts on the back of the Mounting Bracket. The Bush is the part around the central sliding pin. July 2023  73 Fig.9: the Bush fits in a hole in the Mounting Bracket and guides the central Pin that the Cam acts on. Fig.10: The collars keep the spring in place when they are under tension. for 8BA. Finally, clean up the 2.4mm diameter hole. aluminium extrusion and is shown in Photo 6. It hides the servo motor that sits next to the Platform. Cut the extrusion to 100mm long and clean up the ends with a file, or do the whole operation in a milling machine fitted with a slot drill. Use a hacksaw to remove the rectangular sections at the ends of the 40mm side of the extrusion, and reduce the width from 40mm to 15mm. With a 1/8in or 3mm end mill in the chuck of a milling machine, clean up the cuts to size, then drill the two 3.5mm diameter mounting holes. Photo 6 also shows two small holes on the 20mm side of the Cover Plate. My layout is made from polystyrene foam, so I insert small pins through these holes to lock the assembly down. If you want to do something like that, drill the holes in similar locations. Finally, apply several light coats of Rust-oleum Ultra Matte black spray paint. Three Pins The details are shown in Fig.11 and the three Pins are visible in Photo 6. Chuck a piece of 3/32in (2.38mm) diameter brass rod with 20mm protruding. Face the end and use fine emery cloth to clean up any burrs from the end, and polish the circumference for 20mm. Part off an 18.6mm length. For the two outer Pins, leave the burrs on the parted-off end, as these will prevent the Pin from going all the way through the mounting hole in the Platform when assembled. However, while the centre Pin is in the lathe, use a file and emery cloth to round the end that will make contact with the Cam. Two Spacers As shown in Fig.12, mount an M2.5 nut in the lathe chuck and use a drill to enlarge the hole to 2.5mm in diameter. Platform The details are shown in Fig.13 and the Platform is visible in Photo 6. It is made from a piece of 2.5mm-thick aluminium plate. Cut the plate to size using a hacksaw and file the sides smooth, or mill the plate out using a milling machine fitted with a slot drill. Cut the end chamfers with a file or use a milling machine. Precision-drill the three 3/32in (2.38mm) diameter holes, if you didn’t already do it when making the Mounting Bracket. Cover Plate The Cover Plate (Fig.14) is made from a 25 × 40 × 1.6mm L-shaped This shows the size and shape of the specified servo motor. 74 Silicon Chip Marker Post So that you know where to stop the train, a small Marker Post is mounted beside the rail opposite the centre of the Uncoupler Platform. In operation, you drive the train up to the Marker where you want to split it. When the Platform raises, the coupling hooks of the carriages next to the Marker are lifted, and when the train moves forward, the two carriages are split apart. The Marker Post consists of three parts: top, support & post (see Fig.15). For the top, chuck a piece of 10mm diameter aluminium rod with about 10mm protruding. Face the end and reduce the outside diameter to 8mm for a depth of 6mm. Cut the 0.4mm recess using a 1/4in, 6mm or 6.5mm slot drill. Part off a 3.5mm length, mount the other side in the chuck and cut the other recess using the same slot drill. Next, mount the piece in the milling machine vice and drill the 2.1mm hole for the post. For the support, chuck a piece of 20mm diameter aluminium rod with about 8mm protruding. Face the end Australia's electronics magazine Fig.11: the Pins slide up and down in the Bush and Collars, with the central Pin being acted on directly by the Cam that’s rotated by the stepper motor. and reduce the outside diameter to 4mm for 3.5mm, then to a diameter of 12mm for 3mm. Using a centre drill, followed by a 2.1mm drill, bore out the end hole for 6mm and part off a length of 3.5mm. For the post, cut a piece of 1/16in (1.58mm) square hollow brass to a length of 61mm and clean up the ends. Using Loctite 620, assemble the parts as shown in the drawing. Leave it for 24 hours, then apply two light coats of Rust-oleum Ultra Matte black spray paint. Mechanical assembly Refer back to Figs.1(a)-(c) as a guide during the final assembly. Photos 5 & 6 should also help. To start, join the Cover Plate to the Mounting Bracket using two M3 × 6mm panhead machine screws, forming a ‘T’ shape. Next, attach the modified Hornby rail to the Mounting Bracket using two ½in or 13mm long 10BA screws. Slide the centre Pin into the Platform with the round end going in first. If the Pin is too tight, slightly reduce its diameter by returning it to the lathe and polishing its outside with emery cloth. Use Loctite 620 to lock the pin in place. Be sure to clean off any Loctite, as the last thing we want is to lock the Platform into the Bracket. Do the same for the outer Pins, only this time, they must be fitted with the parted-off end last. Again, make sure to remove any excess Loctite. Leave the assembly for 24 hours to allow the Loctite to set fully, cover the Pins with masking tape, and apply several light coats of matte black spray paint. When the paint is dry, remove the masking tape and clean off any remaining glue from the Pins. If all is well, the Platform should slide into the Mounting Bracket under its own weight when the Bracket is horizontal. The Collars and Springs can now be fitted. The Collars are held in place by two 8BA screws. Before fitting the screws, if they are 1/2in long (12.7mm), reduce their length by 2mm, to around 10mm. siliconchip.com.au Fig.12: these Spacers are made from M2.5 hex nuts and are used for mounting the servo motor in the correct position. An alternative way to stop the Collars from coming off is to use a soldering iron to apply a small amount of solder onto the ends of the outer Pins. If you need them to come off later, remove the solder with your soldering iron and solder wick. Use two M2.5 × 8mm panhead screws and the Spacers to mount the servo motor loosely, as shown in Fig 1. Set trim potentiometers VR1 and VR2 to their mid positions. With the switch in the down position (switch open), apply 5V to the PCB. The servo motor should now be in its fully anti-clockwise position. Attach the Cam as shown in Fig.1(a), with the Platform in its lowest position, then tighten its retaining screw. The Platform and centre Pin should be fully down. Slide the servo motor until the middle Pin just touches the Cam and tighten the 2.5mm screws holding the servo motor. Change the switch to the up position, and the servo motor and Cam should rotate clockwise, lifting the Pin and Platform assembly. Set the height of the top of the Platform above the rail to 2.7mm by adjusting VR1. Change the switch to the down position, and the Platform should move down until it is flush with the rail sleepers. Its height can be trimmed with VR2. Fig.13: the Platform sits inside the rails (above the sleepers) and is moved up and down by the servo motor and Cam acting on the central Pin. Fig.14: the Cover Plate mounts opposite the Cam, so there is a continuous rectangle of painted metal under the rails, except where the Pins pass through to lift the Platform, hiding the mechanism. Fig.15: the Marker Post is placed next to the rail in line with the centre of the Platform, so you know where to stop the locomotive before activating the Uncoupler. After gluing it together, I suggest you paint it matte black like mine. Layout assembly I decided that the best place to fit the Uncoupler was one rail length before the end of a siding. This way, I could back a train into it and uncouple one or two carriages, then the rest of the train could leave the siding. Later, the train could return, recouple the carriages and remove them from the siding. My train layout is made from 50mm-thick polyurethane sheets that sit on a 15mm-thick timber board. I cut out some foam to enable the Uncoupler to fit flat with the surface, as shown in Photo 7. I then drilled a 7mm diameter hole in the upper right-hand corner to let the servo motor wires go through the timber. You can make a similar cut-out if your rails are mounted on timber. siliconchip.com.au Photo 7: this shows the hole I cut into the polyurethane foam on my layout to make room for the Uncoupler to sit below its surface. Note how the servo wires pass through a hole in the timber base. Australia's electronics magazine July 2023  75 An alternative circuit without a microcontroller Some builders are put off projects because they use microcontrollers that require programming. Usually, the design with a micro uses fewer components and hence is cheaper to build, but in this case, it is marginal. This alternative circuit (Fig.16) uses two inexpensive CMOS 4047 monostable/astable ICs. IC1 is wired as an astable that produces a symmetrical square wave output from its pin 10. The frequency is set by the 120kW resistor and the 39nF capacitor by the formula f = 1 ÷ (4.4 × R × C), which gives approximately 49Hz, close enough to the required 50Hz. IC2 is wired as a monostable that is triggered on every positive-going edge fed to its input pin 8. In this case, it is triggered every 20ms. The output is a positive-going pulse with a period set by the 33nF capacitor and the resistance between pin 2 and pin 3, according to the formula t = 2.48 × R × C. As explained in the main article, we need this to be about 2ms at the low position and close to 1ms at the upper position. These timings are adjusted by 10kW potentiometer VR2 and 5kW potentiometer VR1, respectively. The up/down switch selects which is active. If you do the sums, you will see that the 10kW potentiometer with 18kW series resistor enables the period to be changed from approximately 1.5ms to 2.3ms, and the 5kW potentiometer with 10kW series resistor gives a range of approximately 0.8ms to 1.2ms. Fig.16: if you don’t want to use a microcontroller, you could build this circuit using logic chips instead. It does much the same job, although I haven’t designed a PCB to host it. I mounted the electronics together with the up/down switch, then used light-gauge three-core cable (similar to the wires attached to the servo motor) to connect the PCB to the motor and the 5V power supply. I then covered the wire junctions at the servo end with heatshrink tubing. Photo 8 shows the Uncoupler installed. Using it Photo 8: the Uncoupler sitting under a rail section on my layout. Note how the Cover Plate hides the servo motor beneath and the way the Marker Post is positioned in line with the centre of the Platform. Back the train down over the Uncoupler, line up the carriage junction you wish to uncouple with the Marker Post and stop the train. Throw the Uncoupler switch into the up position and drive the train forward. The carriages should now be uncoupled, and you can return the Uncoupler switch to the down position. To reconnect the carriages, back the train slowly into the stationary carriage, and it will automatically hook up. SC Australia's electronics magazine siliconchip.com.au 76 Silicon Chip