Electrical coupler for LEGO trains - Coupler design

Menu

Electrical layout of the coupler

So we have four wires, thus we need four contact pins on the electrical coupler? No, not that easy.
If we'd do that, we could not change the direction of individual vehicles in our train. To be able to do that, we need to make sure, that the same wires always connect to each other, even when the vehicle is driving "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.

A tender from below. The direction of the tender is marked as well as the front and the back facing couplers. The cabling can be seen. The tender from below

Pin assignment of the electrical coupler facing forwards Pin assignment of the electrical coupler facing forwards

Pin assignment of the electrical coupler facing backwards Pin assignment of the electrical coupler facing backwards

Pin placement

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.
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 the 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 chamfered, exactly where the pogo pins are applying their force.

The elctrical coupler coupled with the old Lego coupler The elctrical coupler coupled with the new Lego coupler Coupled togehter with known designs

Housing

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.

Drawing of the housing Drawing of the housing, dimensions in mm

STL download link.
The "9V" and the lightning symbol can be painted as it is raised.
A future improvement on the mounting of the individual pogo pins will likely be made. But in a fully compatible way.

Parts list

What Amount per coupler Comment How to get it?
Housing 2 So far I only printed them with resin. 3D print it yourself, the download link can be found above in the section "Housing".
Magnet 6x4 round 1 I use neodyn magnets for my couplers. Sold online on various sites
Pogo Pin Mill-Max 0962-0-15-20-75-14-11-0 3 This pogo pin fits in the housing. In case you want to use a different pogo pin, you might need to adjust the housing for it. Sold online on various sites
Wires, around AWG 23 or 0,25mm2, colours red, blue, brown and black you deicde I recommend flexible silicone wires for this application. You can also use smaller wires, but mind the intended amps you want to transfer here. You can also use different colours of course. Sold online on various sites
Copper adhesive tape around 3cm Sold online on various sites
Crimp tooling just as tool Either you use the proper DMC, or... you simply just use any plier available Sold online on various sites
Instant adhesive a couple of drops Sold online on various sites
Isolation tape or shrink tubing around 3cm Sold online on various sites
Technic, Pin 1/2 without Friction Ridges, Item No: 4274 1 I use these to mount the coupler to the train. You can use what ever you want to use to mount the coupler. Your brick collection. In case you don't have one, it's sold online on various sites
Plate, Modified 2 x 3 with Hole, Item No: 3176 1 I use these to mount the coupler to the train. You can use what ever you want to use to mount the coupler. Your brick collection. In case you don't have one, it's sold online on various sites

Contact and Copyright:
webmaster@flemmbrav.de

latest change: happened