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Constructional project Points Controller for Model Railways is is the sets of points, so th My layout has five with to up me t and label I ca control box lid layou control them. Project by Les Kerr Adding points to a model railway layout makes it a lot more fun and more realistic, too. This Controller lets you monitor and switch up to eight sets of points from a single control box with easy wiring; it could even be expanded to handle more than eight. We will also show how to make LED-based signals to go with each set of points. P oints (also known as “railroad switches”) are used where a single set of train tracks splits into two. If the points are facing one way, the train passes onto one set of tracks, while if they are facing the other way, it moves over to the others. For example, two sets of points could be used at either end of two parallel pairs of tracks to allow trains going in either direction to use either set of tracks. Points can also enable a train to move from the main tracks into a siding, or back out. Real railways have many points, especially in and around stations, so you should ideally have a few in a realistic model railway layout. So, how do you control them? Practical Electronics | February | 2025 This design minimises the number of wires needed between the control unit and each set of points by using serial data. That way, you only need a few wires running around the layout, from the Controller to the first set of points, then between pairs of points, rather than the ‘spaghetti’ required if each set of points had its own set of wires. The lead photo shows my control box that supports five sets of points in my layout, while Photo 1 shows the actual layout from above. The layout has two loops, each with a siding, plus a station at the centre. Two of the sets of points allow trains to move from one loop to the other or back, while the other three allow trains to move between one of the loops and the sidings/station. There are two LEDs and a toggle switch on the control box for each set of points. The green LEDs show the current direction of the points, while the toggle switch allows that to be changed. The most common way to change the points on a model railway layout is to use a points motor. The insides of a typical one are shown in Fig.1. If the motor is at position X and we apply 18V to the electromagnet windings between points A & B, the magnetic field attracts the iron arm, moving the sliding bar to the right (position Y). If we then apply 18V to the winding 37 Constructional project Fig.1: the basic configuration of a points motor. Depending on which side of the electromagnet is activated, the lever moves the points to one side or the other. between B & C, the points change back to their original position. The windings produce a strong magnetic field and are made of heavy-gauge wire, having a typical resistance of 4W. If we had a constant 18V across them, we would have a steady current of 4.5A, which would soon burn out the coil. So we need a means of applying the current for no more than 200ms. The second concern is the power supply's ability to deliver that much instantaneous power and current. That can be done using a circuit like the one shown in Fig.2. One end of the electromagnet coil is connected to the Mosfet drain while the other end connects to a 2200μF capacitor that is charged to 18V via a 47W resistor. The Mosfet acts like a switch that Fig.2: this basic circuit can switch a set of points in one direction. The Mosfet is pulsed to deliver enough current to switch it over, but not for so long that the coil burns out. Another Mosfet and diode is needed to provide switching in both directions. is off when the gate voltage is 0V. If the gate voltage is brought to +5V for 200ms, the capacitor discharges most of its energy into the electromagnet, producing a strong magnet field and a loud click as the points change. When the Mosfet switches off, the capacitor charges to approximately 18V in about 400ms, preparing it for the next pulse. A second Mosfet (not shown) is connected to the other end of the coil to switch the points back. They can share a single capacitor that’s connected to the centre tap. As mentioned earlier, this design's serial loop means you only need four wires from the control box for all the points. These are +18V, +5V, serial data and 0V. I ran a four-core alarm cable around my layout. Scope 1 shows this in action (see page 43). The cyan trace is the Mosfet gate voltage, which is high for 200ms, while the yellow trace is the voltage between the Mosfet drain and ground. You can see how the capacitor recharges over a second or so following the points motor activation. Block diagram Fig.3 shows how the modules are connected. One Receiver PCB is used for each set of points, with a single ‘Transmitter’ controlling up to eight sets (it transmits over a wire, not wirelessly). Each Receiver PCB has outputs to connect to the points motor and operate the associated signal (see Photo 2). Each Receiver is given a unique address (0-7) with the combination of three jumpers. An additional Fig.3: this system configuration keeps the wiring in the layout simple, as the Receiver modules can be mounted next to the points motors. The wiring between the Transmitter and Receivers can be daisychained or connected in any other way that provides the required four-wire bus. 38 Practical Electronics | February | 2025 Model Railway Points Controller Transmitter can be used if you need more than eight sets of points. The Transmitter is housed in the control box, with power for all the modules provided by a 12V AC 1A plugpack. If you have 12V AC available from a different source, you could use that instead. The complete system comprises the PCBs mentioned above, the points and motors, signals, control box wiring and layout wiring. Circuit details The circuit of the Transmitter (control box) is shown in Fig.4. Up to eight switches and sets of LEDs are wired to microcontroller IC1. Eight of its digital input pins (RA0, RC0-RC4, RA4 & RA5) are used to sense the positions of the points control switches. Each input has a 10kW pull-up, so either the switch pulls that input to GND or the resistor pulls it up to +5V. The same switch poles light one of the two connected green LEDs by pulling one of the cathodes to GND. The anodes are connected to a common 680W resistor to +5V. IC1 constantly checks the states of the eight switches and delivers a continuous serial stream at its RC5 digital output. That is fed to the eight ‘Receivers’ via a 1kW resistor, so they know which state the points need to be in. The 1kW series resistor protects the microprocessor from damage if the serial line is accidentally shorted to ground. For the power supply, the incoming 12V AC is applied to a bridge rectifier with a 2200μF smoothing capacitor to get around 18V DC. This depends on the transformer regulation and can range between 16V and 18V DC; 16V is sufficient to operate the points motor. That voltage is fed to the points motors and the input of linear regulator REG1, which produces the 5V DC supply for IC1 and the microcontrollers in up to eight connected Receivers. The 1000μF capacitor smooths out any ripple that makes its way through the 7805 regulator, while the 100nF capacitors reduce high-frequency transients from the supply and ensure stability in the linear regulator. Fig.5 shows the circuit of one Receiver. The serial data from the Transmitter goes to the RC0 digital input of IC2, which is powered by the 5V rail produced by the Transmitter. It Practical Electronics | February | 2025 Photo 1 (above): a view of my layout from above. You can see how it corresponds to the diagram and controls shown in the lead photo. Photo 2 (left): a close-up of one of the signals I designed to accompany the points. They can be made using a lathe and a few bits of metal you can get from hobby shops. 39 Constructional project Fig.4: the Transmitter circuit consists of a microcontroller, IC8, connected to up to eight toggle switches and eight pairs of LEDs. It encodes the switch positions into a serial stream at its pin 5 digital output that’s fed to the Receivers so they can actuate the points appropriately. Fig.5: microcontroller IC2 in the Receiver decodes the serial stream and, based on its identity set by jumpers JP1-JP3, extracts the appropriate command signals and drives Mosfets Q1 & Q2 to control the points motor. It also updates the state of the signal/semaphore when the points change. 40 Practical Electronics | February | 2025 Model Railway Points Controller Transmitter construction The Transmitter is built on a 74 × 47mm single-sided PCB coded 09101241 – see the overlay diagram, Fig.6. The power supply connections and four wires that go to the Receivers connect via the terminal blocks at the top of the PCB. In contrast, the offboard switches and LEDs are connected via the headers near the middle of the board. Photo 3 shows the assembled board. Fig.6: assembly of the Transmitter PCB is straightforward. The power supply inputs are at upper left, the four serial/power bus connections are at upper right, and the headers to connect up to eight toggle switches and indicator LEDs are in the middle. ► ► decodes the serial stream and ignores everything except the points position that matches its identity, 0-7, depending on the settings of jumpers JP1-JP3. Those jumpers connect to the RA5, RA4 & RC5 digital inputs of IC2. If a jumper is inserted, shorting the two header pins, it pulls the connected pin low. Otherwise, that pin is pulled high by a 10kW resistor. That means they are all at a high logic level unless a jumper shunt is added. Table 1 shows the jumper setting for each of the eight channels. When the desired points position changes, it brings one of the RC3 & RC4 digital outputs high for 200ms to drive the points motor as described earlier. It also updates the states of digital outputs RC1 & RC2 to light the appropriate LED in the signal, or change the state of the optional Semaphore with its signal input connected to SIG1 and its GND to 0V. Diodes D5 & D6 are provided because when Q1 or Q2 switches off, the magnetic field in the motor windings will collapse and cause a voltage spike at the drain of the Mosfet that was on. These diodes clamp the voltage, preventing damage to the Mosfets. The 100μF and 100nF supply bypass capacitors in each Receiver are necessary since the Transmitter that’s the source of the 5V rail could be some distance away, connected by relatively thin wires, so the supply needs local filtering. Fig.7: if using our commercially-produced Receiver PCBs, there’s no need to fit the four wire links shown here. Ensure the four bus terminals connect to the corresponding terminals on the Transmitter PCB. Start by fitting the resistors immediately on either side of IC1, followed by the IC socket with the notched end at the bottom. You can then solder the header pins, made from strips four or five pins long that can be snapped from longer headers. Follow with the capacitors, taking care with the orientation of the electrolytics (the longer lead is positive while the striped side of the can is negative). Don’t solder the PIC directly to the PCB, as there is no provision for in-­ circuit programming. Next, add the remaining resistors, which are mounted vertically, then dovetail the three terminal blocks and solder the whole lot at the top of the PCB, with the wire entries towards that edge. Solder in the 7805 voltage regulator and the 1N4004 diodes as per the layout diagram, taking care to match their orientations with what’s shown in those figures. If you have purchased the PIC16F1455 microcontroller 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 siliconchip.au/Shop/6/276 Check for dry joints and solder bridges and rectify them if you find any. You can then plug the header sockets onto the header pins, ready to solder the wires to the LEDs and switches. If you don’t have individual 4-pin & 5-pin strips, you can cut up longer strips with a hacksaw or side cutters. Receiver assembly The Receiver is built on a 56 × 45mm single-sided or double-sided PCB coded 09101242 – see the overlay diagram, Fig.7. The PCBs we supply will be double-sided, so they won’t need the four wire links. If you have single-sided boards (eg, you made them yourself), start by fitting the four wires shown in Fig.7. It Photos 3 & 4: the left-hand photo is the Transmitter PCB. Commercial PCBs will have silkscreened labelling. Note the headers for connecting the switches and LEDs; the extra pin is the 0V (GND) connection. The right-hand photo is the Receiver PCB. As commercially-made PCBs will have two layers, you won’t have to fit the links, saving some time. Practical Electronics | February | 2025 41 Constructional project is advisable to use solid-core insulated wire (‘Bell wire’). You can see from Photo 4 that I used tinned copper wire; if doing the same, be careful to route the wires so they can’t short against anything. The construction procedure is the same as for the Transmitter, although all the resistors are mounted vertically on this board. Watch the orientations of all diodes, Mosfets, electrolytic capacitors and the IC socket. Also check that the terminal block wire entries are facing the nearest edge of the board. You will see that I used pieces of socket strip for CON6 & CON7, although I have specified polarised headers and matching plugs in the parts list. The advantage of the latter is that you can’t accidentally connect the points or signal backwards if you unplug and replug them later. While IC2 is the same type of chip as IC1 (a PIC16F1455), it is programmed differently, so make sure you get the right ones when purchasing preprogrammed chips. Similarly, if programming them yourself, use the HEX file ending in B for the Receiver chips and the -A file for the Transmitter chip. Check for dry joints and solder bridges, then refer to Table 1 to see which jumpers you need to plug into the headers for each Receiver based on its number. Photo 4 shows the jumper settings for points #5. Photo 5: a points motor connected to a set of points on a small section of track for testing. realism of the layout. Fig.8 shows how I made them. The mounting pole is made from a length of 3/32in (~2.38mm) square hollow brass tube. Cut it to size and clean up the ends using a file. The LED mounting plate is made from a piece of 0.05in thick by 0.5in wide (1.3 × 12.7mm) brass strip. Drill the 3mm diameter holes 6.5mm apart, then cut the plate to length. Use a linisher or file to round the ends to size and clean up the edges, then paint the plate matte black. For the base, place a piece of 20mm aluminium round rod into a three-jaw chuck so that 10mm protrudes. Face the end and turn it down to a 5mm diameter for a length of 3.5mm. Using a centre drill, followed by a 3mm Fig.8: here are the details of the parts used to make the optional signal to go with each set of points. You could use the Semaphore described in the April 2022 issue instead. drill, bore out the hole to a depth of 5mm. Part it off to a length of 4.5mm. Fit 3mm red and green LEDs into the LED mounting plate, noting the orientation shown on the drawing. Bend, cut and solder the leads as shown to create the LED assembly. They are soldered anode-to-cathode, in inverse parallel. The LED assembly is then soldered to the post. Clean, tin and flux the mating surfaces between the LED assembly and the post. Use a soldering iron to heat the assembly until you see solder coming out of the joint. File off any excess solder. Slide the base onto the post and lock it in place 25mm from the green LED lead using Loctite GO2 (or equivalent). To get power to the LEDs, take two 300mm lengths of thin hookup wire (red & black). You can strip these out of an old USB cable. Remove about 2mm of the insulation on both ends and tin the exposed wire. Clean and tin the bottom edge of the post, then place the red wire on top and solder it to the post. Thread the black wire up the centre of the post and connect it to the LEDs, as shown in Fig.8. Attach header pins to the other end of the red and black wires, and cover the wire connections with heatshrink tubing. Cover the LED assembly with masking tape and spray the rest with silver paint. Finally, test the signal by connecting a 680W resistor in series with the positive lead of a 5V DC power supply. Connect the other end of the 680W resistor to the signal red lead and the black lead to the supply's negative. The red LED should 42 Practical Electronics | February | 2025 Making the signals You don’t strictly need the signals, but they improve the appearance and Model Railway Points Controller Scope 1 (left): the Mosfet gate drive (cyan) and drain voltage (yellow) when driving one side of a points motor. After switching the points, the capacitor takes about 400ms to recover its charge. Scope 2 (right): if the Transmitter is operating correctly, the serial waveform from pin 5 of IC1 should look like this. glow. Reverse the connections, and the green LED will light. Mounting the signal If your layout is on a timber base, drill a 3mm hole at a suitable location near the entry to the points. Insert the signal wire end into the hole first, until the base is flush with the board. Glue it in place using Loctite GO2. My layout is on a polyurethane base, so I did the same but used a 2mm drill. I enlarged the hole to 3mm from the underside with about 24mm of the hole length remaining at 2mm. Wait till you have tested the PCBs before securing the signals in place. Preparing for testing the header on the Receiver. I soldered the wires to header pins to match the sockets I soldered to the board, and covered the solder joints with heatshrink tubing. Transmitter testing Check the orientation of the capacitors, diodes, and the voltage regulator, then apply 12V AC to the screw terminals as shown in Fig.6 (the two at upper left). Use a DVM to check that you have +5V and between 16-18V referenced to 0V on the terminal blocks. With the DVM black lead connected to pin 14 and the red lead to pin 1 of IC1’s socket, check that you measure +5V DC. Remove power and plug in the PIC16F1455, being careful to avoid folding its legs. Reconnect the supply and, if you have an oscilloscope, check to see that serial data is being sent out from the serial screw terminal, as shown in Scope 2. Otherwise, you can use a frequency counter to check for activity. The next step is to connect the Transmitter to a Receiver but, before doing so, recheck the Receiver board to verify that the diodes, Mosfets, capacitors and IC2 are correctly orientated. Connect the points assembly, Transmitter and Receiver as shown in Fig.9. Set the jumper links for points 1 (see Table 1). Apply 12V AC to the Transmitter, and you should see the green signal LED light and the points motor switch to the left. Short pin 13 of IC1 to ground (pin 14 is ground); the red signal LED should light, and the points motor should switch to the right. Switch off the power and change the jumper settings to #2. Switching the power on will again cause the signal Before testing the Transmitter and Receiver PCBs, make a temporary set of points with a points motor attached, as shown in Photo 5. I mounted it on a scrap piece of 30mm polystyrene. Firstly, mount the points using 0.78 × 25mm pins. Using the points operation lever, move the points in the direction shown in the photo. Take a points motor and orientate it with its actuator down. Place the hole in the actuator directly over the pin in the point’s operation lever and pin the motor in place. Switch the points manually, checking that the point motor's actuator moves smoothly in and out. Prepare the wires on the points motor to connect to a Receiver PCB. If using the specified polarised headers, that means crimping and/or soldering them into the header plug pins, then pushing those pins into the moulded plastic block in the correct order to mate with Fig.9: the wiring for the first set of points. It’s the same for the other seven sets of points, except that the three jumper settings change (see Table 1 below). Practical Electronics | February | 2025 43 Constructional project Fig.10: the suggested positions for the PCB mounting holes, power input socket and serial bus cable in the control box. green LED to glow and the points motor to go to the left. This time, short pin 10 of IC1 to ground; the red signal LED will glow, and the points motor will move to the right. Repeat for the remaining point channels, referring to Table 1 and Fig.4. When finished, set each Receiver to a different ID, referring to Table 1, and use a small label or marker pin to write the IDs you’ve assigned on the Receiver PCBs. Finishing the control box You will now need to create a suit44 able label for the control box. I did this on the computer, scaled it to size to fit the control box lid and printed it onto silver sticky decal paper. Remove the backing sheet and carefully fit the label to the box, avoiding any air bubbles under the surface. As every layout is different, I haven’t made a drawing of the drilling details of the lid. However, Fig.10 shows the drilling details for the base and sides of the box. Drill out the holes for the green LEDs and switches, then fit them to the case. To connect the 12V AC plugpack, you need to drill a hole in the back of the box for the barrel connector, plus another for the four-wire serial cable exit. The Transmitter PCB is mounted on the bottom of the box using M2.5 screws and nuts. Fig.11 shows the wiring for the first set of points, which connects to 0V, P1 and LP1. The other channels follow the same scheme; eg, for the second set of points, the wires connect to 0V, P2 and LP2. These connections can be made by soldering the wire to the socket pin, covering the solder joint with a short Practical Electronics | February | 2025 Model Railway Points Controller Parts List – Model Railway Points Controller Transmitter_________________________________________________________________________________ 1 single-sided PCB coded 09101241, 74 × 47mm 1 flanged ABS plastic enclosure, 171 × 121 × 55mm [Gainta G313MF] 1 14-pin DIL IC socket (for IC1) 1-8 SPDT or DPDT toggle switches (S1-S8) (one per set of points) 3 2-way mini terminal blocks, 5/5.08mm pitch (CON1-CON3) 1 panel-mount barrel socket to suit plugpack (CON4) 3 4-pin headers 1 5-pin header 3 4-pin female header sockets 1 5-pin female header socket 4 M2.5 × 10mm panhead machine screws 8 M2.5 hex nuts 1 long four-core wire (to connect the Transmitter to all Receivers) various lengths and colours of hookup wire various lengths of heatshrink tubing 1 12V AC 1A plugpack Semiconductors 1 PIC16F1455-I/P micro programmed with 0910124A.HEX, DIP-14 (IC1) 1 7805 5V 1A linear regulator, TO-220 (REG1) 2-16 3mm green LEDs (LED1-16; two per set of points) 4 1N4004 400V 1A diodes (D1-D4) Capacitors 1 2200μF 25V low-ESR radial electrolytic 1 1000μF 16V low-ESR radial electrolytic (5mm lead pitch) 2 100nF 50V ceramic Resistors (all 1/4W 1% axial) 1 1kW 8 680W 9 10kW Receiver (per set of points, 1-8 per Transmitter)____________________________________________ 1 single-sided or double-sided PCB coded 09101242, 56 × 45mm 1 set of points 1 PECO PL-11 points motor 1 14-pin DIL IC socket (for IC2) 2 2-way mini terminal blocks, 5/5.08mm pitch (CON5) 1 2-pin polarised header with matching plug and pins (CON6) 1 3-pin polarised header with matching plug and pins (CON7) 3 2-pin headers (JP1-JP3) 0-3 jumper shunts (JP1-JP3; number required depends on Receiver ID) various lengths and colours of hookup wire various lengths of heatshrink tubing Semiconductors 1 PIC16F1455-I/P micro programmed with 0910124B.HEX, DIP-14 (IC2) 2 IRL540N, MTP3055VL or IPP80N06S4L-07 N-channel logic-level Mosfet or similar, TO-220 (Q1, Q2) 2 1N4004 400V 1A diodes (D5, D6) Capacitors 1 2200μF 25V low-ESR radial electrolytic 1 100μF 16V low-ESR radial electrolytic (2-2.54mm lead pitch) 1 100nF 50V ceramic Resistors (all 1/4W 1% axial) 1 4.7kW 1 680W 2 220W 1 47W 3 10kW Signal (per optional signal)_________________________________________________________________ 1 50mm length of 3/32in (~2.38mm) square hollow brass tube 1 20mm length of 0.025in thick, 0.5in wide brass strip 1 20mm length of 20mm diameter solid aluminium rod 1 3mm green LED (LED17) 1 3mm red LED (LED18) Practical Electronics | February | 2025 45 Constructional project www.poscope.com/epe - USB - Ethernet - Web server - Modbus - CNC (Mach3/4) - IO - PWM - Encoders - LCD - Analog inputs - Compact PLC - up to 256 - up to 32 microsteps microsteps - 50 V / 6 A - 30 V / 2.5 A - USB configuration - Isolated PoScope Mega1+ PoScope Mega50 Fig.11 (above): this shows some of the wiring for the Transmitter PCB inside the control box. Additional switches and LEDs are wired similarly but to terminals with higher numbers (P2/L2, P3/L3 etc). Fig.12 (right): you will need to figure out where to position the switches and LEDs to suit your layout, but in general, this shows how they should operate. If yours does the opposite, reverse the switch or the wiring that is going to it. length of heatshrink tubing and using a hot air gun to shrink it. The 12V AC comes in via its attached plug and the socket that screws into the 8mm hole on the rear of the box. The connector must then be wired to the 12V AC screw terminals on the PCB. Use four-way alarm cable or similar to make the connections between the Transmitter and the Receivers, as shown in Figs.3, 9 & 11. The cable exits the control box through the 6mm hole. The Receiver PCBs can be mounted underneath the layout. Final testing - up to 50MS/s - resolution up to 12bit - Lowest power consumption - Smallest and lightest - 7 in 1: Oscilloscope, FFT, X/Y, Recorder, Logic Analyzer, Protocol decoder, Signal generator 46 With all the points’ switches in the up position, the green LEDs on the control box should indicate which way the points are switched, as shown diagramatically in Fig.12. Each signal should be green. Changing a switch to the lower position should cause the associated set of points to change and the corresponding signal to go red. This should be reflected on the associated control box LED. Due to the number of combinations of points types, motor positions and signals, you may find this isn’t the case. If the problem is with the points, try swapping the points motor's red and black wires at the Receiver PCB. If the problem is with the signal, that can be rectified by swapping the red and black wires from the signal where they connect to the associated PE Receiver PCB. Table 1 – Receiver jumper settings # A B C 1 Jumper Jumper Jumper 2 Open Jumper Jumper 3 Jumper Open Jumper 4 Open Open Jumper 5 Jumper Jumper Open 6 Open Jumper Open 7 Jumper Open Open 8 Open Open Open Practical Electronics | February | 2025