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Electrical layout of the coupler

Pin assignment of the electrical coupler

So we have four wires, thus we need four contact pins on the electrical coupler? No, not that easy.
If we did that, we would not be able to change the direction of travel of the individual vehicles in our train. To make this possible, we must ensure that the same cables are always connected to each other, even when the vehicle is travelling "backwards".
Thus, the pin layout on the coupler has to be symetrical. The spring loaded pin on the left hand side needs to connect to the same wire, as the fixed pin on the right hand side.
This is exactly, how the VCC and GND wires will be connected to the coupler. The VCC wires will occupy the upper pair of contacts, while the GND wires will occupy the lower pair of contacts. This way, they can not be mixed up by changing the direction of individual vehicles in the train.

However, the C1 and C2 wires are different. In this special case, it actually fits quite well to have them switched around when changing the direction, the individual vehicle is traveling. Thus, they are not placed symetrical.
Instead, they do not change place on the other end of the vehicle. One wire is on the left hand side of the vehicle, and the other is on the right hand side of the vehicle.

Channel	Colour	Side	State when traveling forwards	State when traveling backwards
C1	blue	left	GND	VCC
C2	brown	right	VCC	GND

When changing the direction of an individual vehicle, the C1 pin of the changed vehicle will connect to the C2 pin of the other vehicles, and vice versa. When the train moves forwards, the power in the changed vehicle will suggest it going backwards. Which is exactly what we want, as the "backwards" of the backwards facing vehicle equals forwards of the forwards facing train.

The next two pictures show the principle implemented on a steam engine. The tender holds the battery as well as the bluetooth receiver.

In the first picture, the back of the engine is coupled to the front of the tender. Both of them face in the same direction. The battery in the tender powers the motor wires.
On the tender, the brown wire is connected to VCC, the blue wire is connected to GND.
On the engine, the brown wire is connected to VCC, the blue wire is connected to GND. The motor makes the engine drive forwards compared to the natural direction of the engine, which is also the same as the direction of the tender holding the battery.

Schematics of a tender engine where the tender and the engine face in the same direction

In the second picture, the front of the engine is coupled to the front of the tender. Both of them face in the opposite direction. The battery in the tender powers the motor wires.
On the tender, the brown wire is connected to VCC, the blue wire is connected to GND.
On the engine, the brown wire is connected to GND, the blue wire is connected to VCC. The motor makes the engine drive backwards compared to the natural direction of the engine, which is the forward as the direction of the tender holding the battery.

Schematics of a tender engine where the tender and the engine face in the opposite directions

As one can see, the train travels in the same direction both cases. It's the direction the car holding the receiver (and in this case battery as well) is facing, which is the tender in this case.
This alignment ensures all engines travel in the very same direction. It also ensures the train traveling in the direction, the remote control is dictating towards the receiver in the tender.

PF style flat ribbon cable

Instead of colourful wires, a flat ribbon cable shall work as well. These are well known in their use in power functions cabling. And they can be used for the couplers as well. Sadly, they don't offer the same level of opportunity to demonstrate the functions of the individual wires, so they don't fit well for demonstration purposes. They do look good though.

Coupler with a flat ribbon cable

Pin placement

Because one of the goals of the coupling is to be compatible with existing couplings, pogo pins come to mind. Male-female-contacts would collide with the existing couplings. Even whel protected by centering devices, they would have to be placed outside of the space, existing couplers occupy. Which results in a big coupler or the electrical coupler not being able to couple to the center of an existing coupler to the costs of holding force.
After deciding on using pogo-pins, I tried moving the contacts outside away from the center of the coupler while still keeping the coupler as small as possible and within the width of 2 studs. This comes with various benefits. The magnet inside of the coupler can be bigger.
Also when coupled with he old spinning magnet couplers, the contact pins are not touching the magnet. Instead they touch the magnet holder, which has an offset from the outer line of the magnet, resulting in the pogo pins not being fully compressed. Compressing the pogo pins costs force the couplers won't have holding the train. It's a small margin, but it's also without a downside. The almost same things holds true with the newer coupler design with the housed magnet. The edges of the coupler are bevelled, exactly where the pogo pins are applying their force. Because one of the goals of the coupling is to be compatible with existing couplings, pogo pins must be used. Male-female-contacts would collide with the existing couplings. In addition, the centering by the magnets is not precise enough for such small male-female-contacts.

Coupled togehter with known designs

Centering devices

These are one of, if not the mayor update on the design compared to the first version. The centering devices prevent short circuits but ensure the correct pin placement throughout. Without the centering devices, the C1/C2 pin row could connect to either the VCC or GND pin row, closing the C1 to C2 circuit without a electrical consumer, and thus overloading the IR receiver / similiar devices. I've grilled a couple of mould king 4.0 boxes like that. Preventing this issue was the number one goal of the new design. I've tested the new design for dozens of hours ensuring this will not happen again, and I do feel confident in this issue being solved with the new design.

The centering devices allow a certain amount of play between the couplers. They can pitch and yaw, but are restricted in rolling. This is important to compensate for non-perfect aligned track as well as transitioning between flat, uphill and downhill track.

Design of the centering devices allowing a certain amount of play

Allowed amount of play in between two couplers

The centering devices are not round, to both allow for a smaller footprint of 16 mm in total heigth as well as coupling together with the known designs. A small clip holds the centering devices in place.

Geometry of the centering devices seen from the front of the coupler

Housing

There are mayor improvements on the housing compared to the first version.
- centering devices exist
- instead of glueing in the pogo-pins, retaining clips hold the pogo-pins in place
- the housing can be printed on a FDM printer

The housing is 2 studs wide (16mm) and is as long as the original magnet couplers. The heigth of the coupler is a little lower than the heigth of the new original couplers. Instead of having a pin itself, the coupler uses a pin hole which reduces the precision needed on the print. Each housing consists of 6 parts in 4 different lots. There are the upper- and lower side of the housing, one of each. In plus, two identical centering devices as well as two identical retaining clips are needed to complete the assembly. All of these parts can be printed on a FDM printer. Recommended nozzle size is 0.2mm.

Drawing of the housing, dimensions in mm

Download the .STEP of the upper side of the housing.
Download the .STEP of the lower side of the housing.
Download the .STEP of the clip.
Download the .STEP of the centering device.
Download the .3mf of the whole coupler.
The "9V" and the lightning symbol can be painted as it is raised.