FULL SUCCESS!

Everything works!
The arduino mini plus 168 3.3v sends through bluetooth the data of the honywell compass to the n900!

(After a mistake with my reset line everything went very well.)

The seller on ebay had a link to the arduino libraries for the compass. There was a useful example and it compiled well for the arduino.

There was a script at
http://www.dasskript.com/wiki/the_arduino_and_the_n900

It worked perfectly with my adapted mac adress on the n900.

Now my n900 shows the directions of my bluetooth compass.



... and what's the best:
I didn't find a suitable little 3.3v battery.
So finally I bought a very cheap china gadget:
A solar key chain with light: 3.7v.
It did open easily (no screws, no glue).
The solar module has the size of the arduino (1/3 of a little match box.)

Now my arduino works with a solar battery.

The whole construction can easily be put into a match box.


What is missing:
- A beautiful compass application
- system integration of the compass


Prospective:
Now I can add every sensor or i2c device I want. (E.g. gas sensor, temperature, balance, adapter to garmin...)