Thank you for the links!
I've looked in Lynx, and they have small TX/RX pcb mounted that look
really promising (and quite cheap, too. Enough to consider buying
instead of building).

Now I need to find the best way to work with them. They connect straight
to the CPU, no problem with that, I think. But I need these to be
really small. I hoped to be able to use vitually no antena, since I
only need about 10-30 meters. But I never worked with RF. Don't know
if this is possible.

I intend to have a small transmiter just sending a code. And then, I
want a receiver, getting this code. Since I'll be using a PIC in this
receiver, I can do more than just checking the code. But that's
basically what I need.
In the transmitter it's probably easier to use also a PIC, instead of
playing around with other logic...

Any hints you could give me, I appreciate.

  Thank you,
  Nuno.
--
                                             _/^\_            __
_____/\_________________________________oOOo__O-O__oOOo______/oo\____
  /\/ /  Nuno Filipe Freitas Pedrosa                   __ /..\__/
  \ \/\  mailto:Nuno.Pedrosa@oen.siemens.de           /`'\\__/-\ __
   \/\ \ Tel. : 0049-89-72222342                      \__/  \__//@@\
___/ /\/_______________________________________________/OO\_..__\__/_
   \/                                                  \__/