Here, I will use Viessmann couplings which are in fact Profi Fleischmann modified by addition of two contacts. The interest is twofold: they remain compatible with the Profi, and they allow the reversal of a car without changing the polarity, because the contacts are superimposed, not juxtaposed.
These couplings are originally equipped with extra-flexible wires, but a little large to pass freely through the slot of the brass piece located above the drawbar. So, I change them for my usual 0.5 mm outer diameter wires. Attention: the de-soldering then the soldering of the wires requires to extract the contacts from the coupling, under penalty of melting it.
Here is a view showing the connection of the 70 mm long wires on the strip. They are slightly glued to the strip with CA, leaving a lot of clearance. The blue, positive, is connected to the upper contact of the coupling. The green is connected to the lower contact.
Note: the holder visible on this picture was finally not left in place, because it could interfere with the body repositioning.
The BD car is first tested alone:
At this moment, the lantern circuit has not yet been painted black…
Then the whole train:
But after just half an hour of operation, the train starts flashing while running, and then completely switches off. Diagnosis: fouling of strip / wheel contacts. So I have to modify the assembly so that the contact is made on the inner sides of the flanges, not on the tyres.
I make small parts from a brass strip 0.5 mm thick and 3 mm wide. The lower part is folded back on itself so as to obtain a thickness of 1 mm which will hold it in force in the housing of the bogie. The important dimension is the horizontal value of 3 mm. Indeed, if this dimension were larger, the part would conflict with the pivot supports.
The bronze strip is soldered on the short tongue; the latter must then be shortened to the maximum. The connection wire will be connected on the long tongue after installation. Soldering time must be as short as possible!
This time, the operation is correct. However, there are two minor disadvantages: first, it is necessary to remove the bogie rotational stops (arrows), into which the strips would bump. Then, the friction is a little stronger than before. But, again, for a short train, this is not a real trouble.
In total, this installation required three days of work (not full time!), including the study, the taking of dimensions, drawing and etching circuits, assembly and testing… and taking photos !