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I realized that a Lego rotation sensor is not so easy to find,

and not at all cheap.

In many cases it's not the best way to control the absolute position of a part of a robot, such as legs, arms or other moving parts...

I came to this idea after visiting many web pages, as Michael Gasperi's page, describing sensors, RCX raw readings and mathematical sofware corrections to those readings.

Since I'm a fool from infance, I had in my electronic workbench some tiny trimmers (variable resistors) taken off an old VCR, that were'nt doing nothing.

"Here, let me show what I'll made of you!" It was a hard work to fit things in there, mostly because I had to arrange a mechanical coupling between that sweet Technic grey pin (a real pleasure to see only it, from outside) and the small screw-head of the trimmmer.

I resolved to use a square part of a PCB (bachelite board for electronics) to do that.

The pin allows perfect compatibility with other Lego, and I assure that it never come out when you pull it (for example, to remove a gear), because the PCB square don't let it slip.

Unluckily, beauty came after functionality, and the 2x4 brick had to be cut on the top.

Patience! I closed and reinforced all up with a 2x2 plate.

The pin, coupled with trimmer, can swing about 270 degrees.

The schematic (nursery school stuff) is shown below.

How to build:

Sorry. Is quite useless that I describe the building process, because everyone could find a trimmer that is slightly different,

and so all the montage would be different.

I'm sure that with more suitable components, the result would be much better. Try by yourself, I hope you'll be more lucky than me.