When I chose to equip such a wagon, I did not choose the easy way, as it seems to present no sufficiently discreet location where to put the circuits. But if I get there, I think I won’t worry about other wagons. I will use the REE lamps I have equipped with LEDs.
Above all, we must find where the lamps are attached on this type of wagon. After a long search on the Internet, I finally found an answer on the Loco Revue forum, given by the famous Gérard Huet, nicknamed SuperGG: it is in the red circle of the photo.
In fact, on the LSM model, at this location is a small outgrowth of 0.7 × 0.7 mm and a few tenths of a millimetre thick. Two questions arise immediately. How to hold the tail lamp there? And where to pass the wires?
For the first question, I think the best solution is to drill a ø 0.8 hole at the tail lamp base back, which will serve as a kind of mortise for the square tenon. This will not be sufficient to hold the lamp, but will be a centring point for gluing.
Picture SuperGG on forum.e-train.fr.
Note: I have lightened the part in question to make it more visible.
Obviously, it would have been better to make this drilling before installing the LED, because I may touch or even cut the wires.
For the second question, passing the wires along the edge of the gangway will be the most discreet, then under the chassis, in a groove that one would think made purposely. They will be glued slightly and then painted in aluminium grey.
The LEDs of the two lamps will be connected in series, and therefore the same current will run through them. Being in DCC, it would be enough to calculate the series resistance for a rectified voltage of approximately 15 V. A test on a breadboard makes me choose a 10 kΩ resistance, which will give a current less than 2 mA. But, for this time, I want to imitate the REE hopper circuit that regulates the current, and therefore the LED lighting, in a fairly wide voltage range. For this purpose, a constant current supply, obtained by means of a transistor, must be provided. See the diagram. This circuit can be used in analogue, for a voltage ranging from 6 V to more than 20 V if necessary. If the current is to be increased, the value of R2 may be reduced.
For a proper current pickup, the four wheels of the wagon should participate. Luckily, the axle boxes are provided with brass bearings. It will suffice to solder wires on them and to isolate half-axles by cutting them in the middle. I also need non-insulated wheels, which will be taken from another wagon of the same type. My usual method…
There I committed a blunder which made me waste a lot of time. I wanted to remove the brass bearings to ease soldering wires to them. Not easy to do without damage. I found a solution which consists in drilling a ø 0.3 hole in the central nut hollow of the axle box and then pushing the bearing from the outside with a short ø 0.3 piano string rod held in a chuck. The force must be quite large. All the bearings were pushed out willy-nilly, but I lost one… So, I had to make new bearings in brass sheet.
In fact, I should have dared a much faster method that I tested later on an LSM USI coach. It consists of milling a small space into the plastic against the brass bearing, putting a little flux in it, placing the end of the tin-bonded wire and soldering with a drop of tin, so quickly that the plastic cannot melt. It seems a bit risky, but thanks to the flux, it goes very well. Fast and no risk of loss. But of course, careful washing mandatory! I forgot to mention that this method is greatly facilitated by the fact that the axle-holder part is removable.
Now that everything is ready, the last question is to place the PCB as discreetly as possible. Behind the buffer beam? Not possible, because of the elongated drawbar. I do not want to remove it, as REE did with its hopper, because it would prevent manoeuvring the wagon. Under the chassis? Not good: it would be quite visible and the braking equipment would have to be sacrificed. The last possibility is under the axle holder. Yes: by levelling the ribs, and taking into account the presence of the axle, it is possible to install the circuit there. There, it will be the most discreet, hidden in large part by the wheels. See the etching mask.
Hover the picture to zoom. Caption: