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Model Railway Carriage Lights BY LES KERR It’s nice to have carriage lights on a model train, to add to the realism. These model train carriage lights (designed for OO-gauge) are batterypowered and can be switched on and off with an external magnet. I t might seem trivial to add lights to a model railway carriage, but there are a few considerations that make it a bit more difficult than that. One important factor is that the battery must be small, so the circuit must avoid discharging it when the lights are off. Also, you need a way of switching the lights on or off easily. This little circuit powers five white LEDs and only draws a couple of microamps when off, and just 8mA when on. The low off-current puts a negligible load on the battery. The low 8mA operating current means that you can use two AAA batteries (cells, really) in series giving 3V this will power the circuit for about 100 hours. If you lack the space for that setup, you can use a single 3.7V Li-ion 800mAh battery such as the Jaycar SB2300. The carriage size determines which batteries can be used. The circuit stops working when the battery falls below 2V. Until recently, lights in model railway carriages were powered from the track. This is because small incandescent lamps required a relatively large current, so they couldn’t be battery powered. To obtain the power, the carriage needed to have metal wheels 60 Silicon Chip with some form of voltage pickup attached to them, and they had to be insulated from each other. Today, most carriage wheels are made of plastic, so they need substantial modification to pick up power from the track. Also, it isn’t that easy to make a reliable pick up. Now that efficient white LEDs are available, it is practical to power them from a small battery inside the carriage. The problem then becomes how to switch the lights on and off. My simple solution is to mount a normally-open reed switch inside the carriage, either under the roof or on the floor. When a magnet is placed near the reed switch, its contacts close, signalling the circuit to toggle the lights on or off. You can see a video of the prototype’s operation at siliconchip.com. au/Videos/Carriage+Lights With this arrangement, you can add a magnet on the tracks just outside a tunnel so that when the train approaches, it switches the lights on. Another magnet placed near the tunnel exit switches off the lights when the train leaves the tunnel. If you want to use the train at night, you can mount the reed switch under the roof so that Australia’s electronics magazine you can manually switch the lights on and off by waving a magnet across it. Circuit description Fig.1 shows the full circuit diagram. The LT1932 IC2 constant-current DC-to-DC LED driver provides a fixed current that drives the series LED lights from the battery. It is about 70% efficient and will work down to a battery voltage of 2V. It has a shutdown input that, when taken low, switches off the LEDs and reduces its current draw to less than 1μA. I have specified high-intensity white LEDs which give adequate light when driven with 1mA. The 70% efficiency figure given above is for a 10mA LED current. To reduce this to the 1mA required without unduly affecting the efficiency, the shutdown pin is fed with a 10% duty cycle (1-to-9 markspace ratio) PWM waveform. The driver oscillates at 1.2MHz and uses inductor L1, schottky diode D1 and a 1μF ceramic capacitor to step up the battery voltage to the 15V or so needed by the LED string. To protect IC2 in case the LEDs are accidentally disconnected, 24V zener diode ZD1 clamps the maximum output voltage. The peak current through the LEDs is siliconchip.com.au Fig.1: the Carriage Lights circuit is based on a 6-pin LT1932 (IC2) constant-current switchmode (boost) LED driver and an 8-bit, 8-pin microcontroller (IC1). The role of IC1 is twofold: it monitors the contact closure of reed switch S1 to switch the lights on and off, and when the lights are on, it drives the SHDN pin of IC2 with a 10% duty cycle square wave, reducing the LED current consumption without impacting the efficiency of the driver circuit. set to about 10mA by the 2.2kW resistor from IC2’s Rset pin to ground. An inexpensive PIC12F617 8-bit microcontroller is used to generate the PWM waveform to drive the pin 5 SHDN input of IC2. When reed switch S1 closes, it takes the GP2 digital input (pin 5) of IC1 high. The 10kW pulldown resistor and 100nF capacitor help to debounce the switch contacts. This signals the microcontroller to come out of sleep mode and provide the switching waveform to IC2, turning on the lights. If S1 is operated again, IC1 goes back into sleep mode, and its GP0 output at pin 7 goes low, switching off the lights. In sleep mode, IC1 draws about 1μA from the battery. If you add to this the <1μA of IC2 in shutdown mode, you get a total current drain of less than 2μA, which is a negligible load on the battery. Construction There are components on both sides of the PCB, so there are two overlay diagrams, Figs.2 & 3. The Carriage Lights controller is built on a 28 x 16mm PCB coded 09109211. It has been deliberately kept small to fit inside a typical OO-gauge carriage. Since I etched mine myself, it is a single-sided design, although you can get the double-sided version from Silicon Chip, which avoids the need to fit a wire link. To enable this PCB to be kept small, most of the components are SMDs. This is a good project if you’re siliconchip.com.au interested in improving your SMD soldering skills since it has a few different types and sizes of components, but nothing especially difficult. Perhaps unsurprisingly, the surface mount components go on the copper side of the board, while the throughhole components are inserted from the opposite side. The SOIC-package PIC12F617 micro will need to be programmed at some point. The easiest way is to purchase a pre-programmed PIC, although it is possible to program it in-circuit; see the panel below if you plan to do it that way. Use a flux pen or syringe of flux paste to coat IC1’s leads and its associated pads. Hold IC1 in place (eg, using tweezers) with the correct orientation and use your soldering iron to tack solder one lead into place, then check that it is positioned correctly (it’s also a good idea to re-check its orientation). If so, solder the remaining leads. Clean off the flux residue and inspect the leads under magnification to ensure that all the solder joints have formed correctly. If you are not sure about any of them, add more flux and apply heat (and possibly more solder) to reflow the joint. If you have bridged any pins, use more flux and some solder wick to remove the excess solder. Here is an example of how you can ► mount the project into a carriage. Note the clear plastic insulation under the battery and PCB. Australia’s electronics magazine November 2021  61 Parts List – Carriage Lights Figs.2 & 3: the top and bottom side PCB overlay diagrams (shown enlarged). To save space and allow the board to use single-sided copper, all the SMDs are on one side and the through-hole parts on the other. The orange wire link does not need to be installed if a double-sided PCB is used (eg, from our Online Shop). Watch the orientations of the ICs, diodes and electrolytic capacitor during assembly. Now that you’ve done that successfully, move on to IC2, which is slightly trickier as its pins are smaller and closer together. As its body is also quite small, you might have trouble seeing the pin 1 indicator. You will need to make sure you’ve found that (eg, using a magnifier) as it must be placed with the correct orientation. Use the same basic procedure to solder it as IC1, but keep in mind that it’s very difficult to avoid bridging the pins with solder. If you have flux paste, once the part has been tacked down, you can drag-solder the three pins on the opposite side and then the three pins on the other side. Still, it’s also acceptable to just solder them individually without worrying too much about creating bridges. After all, it’s pretty easy to remove any bridges that have formed with solder wick, as long as you add a bit of flux to make the process go smoothly, and avoid heating the wick any more than necessary to prevent damage to the PCB. Once again, clean the flux residue away from IC2 and scrutinise its solder joints, then go back and fix any that do not appear to have formed correctly, or are still bridged. Now use a similar procedure to fit all the remaining SMDs, except for the 6.8μH inductor. The only remaining SMD where polarity is important is schottky diode D1; its cathode stripe should be visible on the top of 62 Silicon Chip 1 single-sided or double-sided PCB coded 09109211, 28 x 16mm 1 6.8μH 200mA inductor, SMD 2.0x1.6mm up to 2.5x2.0mm, 200mA+ <0.5W DCR [RS Cat 879-0742 or Taiyo Yuden LB2016T6R8M] 1 miniature single-pole normally-open (SP-NO) reed switch (S1) [RS Cat 3622518 or Jaycar SM1002] 1 magnet suitable for use with a reed switch [RS Cat 118-7108] 1 3V battery pack [eg, 2 x AAA pack or 1 x 3.7V 800mAh Li-ion, Jaycar SB2300] various lengths and colours of light-duty hookup wire Semiconductors 1 PIC12F617-I/SN or PIC12F617T-I/SN 8-bit microcontroller programmed with 0910921A.HEX (IC1) 1 LT1932ES6#TRMPBF LED driver, SOT-23-6 (IC2) [RS Cat 7618504] 1 1N4749 24V 1W zener diode, through-hole (ZD1) [Jaycar ZR1424] 1 SS14 40V 1A schottky diode, DO-214AC (D1) [RS Cat 6387915] 5 high-intensity 3mm or 5mm white LEDs (LED1-LED5) Capacitors 1 100μF 6.3V radial electrolytic capacitor [RS Cat 390176] 1 1μF 50V multi-layer ceramic through-hole capacitor [Jaycar RC5499] 2 100nF 50V X7R SMD ceramic capacitor, M2012/0805 size [RS Cat 135-9033] Resistors (all SMD 1% 1/8W M2012/0805 size) 2 10kW 1 2.2kW the body, and this must be located as shown in Fig.2. As the inductor has only solder pads on the underside at either end, it’s harder to solder it in place compared to the rest of the components. To enable you to do this, I made the PCB lands for the inductor larger than the component body, so there is room to get your soldering iron tip in to heat the lands. Coat both the lands and inductor pads with flux and lightly tin the pads with solder. Place the inductor on the PCB and apply heat from your soldering iron to the land on one side until you see the solder melt. Do the same for the other side. Depending on how much solder you applied to the pads initially, you might want to feed a bit of extra solder into the sides while heating them. Now make a final check of the SMD components to verify there are no solder bridges or shorts, and if there are, fix them up with a bit of flux paste and the solder wick. Turn the board over and solder in the wire link (if you are using a single-sided board), zener diode, 1μF ceramic capacitor and the 100μF electrolytic, making sure the diode and electrolytic capacitor are the right way around. These components are all shown in the underside overlay diagram, Fig.3. Wiring it up While Fig.3 shows the basic external wiring connections, there are more Programming IC1 in-circuit To program the micro in-circuit, you will need to solder wires to the +3V and GND battery pads (see Fig.3), as well as the PCB pads provided to connect to the PCLK pad (pin 6 of IC1), PDAT pad (pin 7 of IC1; the pad is also used for the wire link) and the MCLR pad (pin 4 of IC1). As IC1 is mounted over the PCLK pad, solder this wire first and use as little solder as possible. Cut the part of the wire that projects from the solder joint as short as possible so that when you solder IC1, it isn’t lifted above its pads. With those wires in place and IC1 soldered to the board correctly, connect the soldered wires to your programmer. For the PICkit series, the triangle indicates pin 1, and the connections are MCLR to pin 1, +3V to pin 2, GND to pin 3, PDAT to pin 4 and PCLK to pin 5. With those connections made, load up the programming software, open the HEX file, “carriage lights (0910921A).HEX” and upload it to the chip. If you are using a pre-programmed PIC12F617, it isn’t necessary to solder these extra wires to the board. Australia’s electronics magazine siliconchip.com.au Here are both sides of the finished project shown greatly enlarged for clarity. While you can definitely etch the single-sided board yourself given the right supplies, we will be selling a doublesided version for convenience. That time of year is nearly here... CHRISTMAS Spice up your festive season with eight LED decorations! Tiny LED Xmas Tree 54 x 41mm PCB SC5181 – $2.50 Tiny LED Cap 55 x 57mm PCB SC5687 – $3.00 details shown in the wiring diagram, Fig.4. So that you can work out the length of the board connecting wires, you need to decide how and where the components fit into the carriage. In my case, I glued the 5mm white LEDs and the reed switch to the underside of the carriage roof and taped the battery and PCB to the floor of the carriage. If you have a smaller carriage, you might prefer to use 3mm LEDs. Another solution for the LED mounting is to solder them to a thin strip of Veroboard which can be attached to the underside of the roof. If the floor of your carriage is black like mine, you can improve the radiated light by covering it with aluminium foil to reflect the light back up. If you do this, insulate the PCB with tape so that none of the tracks short out on the foil. Once you have decided on the layout, cut the wires to length and solder them to the PCB. Testing Connect up the reed switch and LEDs, and before connecting the battery, have a last look at the board for any faults or dry joints. Make sure that you connect the battery the right way around, as the circuit will be destroyed if you don’t. If you now place the magnet parallel to the reed switch a few millimetres away, the LEDs should light. Remove the magnet, then put it back where you had it, and the LEDs should SC extinguish. Tiny LED Stocking 41 x 83mm PCB SC5688 – $3.00 Tiny LED Reindeer 91 x 98mm PCB SC5689 – $3.00 Tiny LED Bauble 52.5 x 45.5mm SC5690 – $3.00 Tiny LED Sleigh 80 x 92mm PCB SC5691 – $3.00 Tiny LED Star 57 x 54mm PCB SC5692 – $3.00 Tiny LED Cane 84 x 60mm PCB SC5693 – $3.00 Fig.4: there are just three items to wire to the board; the power supply (in this case, a 3V battery pack using two AAA cells), the reed switch and the string of white LEDs. You can use just about any type of white LED as long as the voltage required to power the string is in the range of about 5-20V. Various LED mounting arrangements are possible, too – whatever suits your carriage(s). siliconchip.com.au Australia’s electronics magazine We also sell a kit containing all required components for just $14 per board ➟ SC5579 November 2021  63