Disclaimer
This page is not connected with or endorsed by The LEGO Company. LEGO, LEGOLAND, LEGO SYSTEM, LEGO TECHNIC, DUPLO, LEGO PRIMO and LEGO MINDSTORMS are trademarks of The LEGO Company.

Before planning about sensors structure is a key passage to make a Lego compatible electric plate:

I had not much will to destroy my few and precious original electric cables, so I studied this solution.

Lego electric terminals have 4 studs, joined together in pairs: so we need only two metal contacts to go!

This version doesn't need original electric plates to cut: it is just a Lego 2x2 Brick, with a hole to make the cable enter.

As you can see, there's no need to hollow the block.

I used a thin copper plate (very easy to find) to make contacts

and a double wire similar in dimensions with Lego one (mine is transparent).

How to build:

Be ready to sacrifice a brick;

Use a drill or a hot soldering iron to make the side hole;

Cut the wire at the lenght desired;

Cut copper to fit inside the brick;

Hot-melt a little of solder on the cable ends and on the contacts;

Solder wire ends with contacts;

Put contacts in the brick and glue them.

If brick mounting on RCX is hard or there's no electric contact, gently bend copper and brick walls out, to enlarge the brick a bit.

If your contact seems like mine (even if you can sure work better than me!), you've finished. I assure it works.

This is my new male electric attachment:

It fits perfectly into female electric connections, both Lego original ones and mine.

How to build:

Use a drill to make an hole in a hard plastic panel, with the diameter of a Lego stud.

Prepare a thin copper foil, cut it at the height of a stud and make a circle of it, inside the hole you made before.

With the soldering gun, put solder in this hole, to fill it

Before it harden, put a terminal in the center of it;

When stain become hard, it is metal, indistructible and perfectly conductive.

It works perfectly and looks not bad.