I did such a PIC project once. It was a "key" for opening my garage door
;)

The key, as a tiny aluminium pipe with about 12mm
diameter and it was about 6cm long. Inside this pipe was a PIC 12C508 and
a little electronic stuff.
When I inserted the key into a slot next to the garage door a
communication started and the key sent out a 10byte code.
Before I explain how it worked I will draw the circuit of the Key :

         ---------------------------------
         |                               |
         |     D1                        |
+5V DC   |    |\ |                       |       -------------------------
---------O----| \|------O------------------------| Vdd                   |
         |    |/ |      |                |       |                       |
         |              |                --------| Inpin                 |
         |              |                        |                       |
          \  |          |     |-------|          |                       |
     T1     \|----------|-----|       |----------| Outpin                |
            /|          |     |-------|          |                       |
         |/_ |          |                        |                       |
         |           ------- +  C1               |                       |
         |           ======= -                   |                       |
         |              |                        |                       |
---------O--------------O------------------------| Vss                   |
GND                                              |                       |
                                                 -------------------------

The capacitor C1 has to be big(a few yF) - I took a Tantal capacitor.
The power supply has to be an high Impedance supply (a few hundred Ohms) -
my supply had an impedance of 300 Ohm.
If the PIC wants to send data he just gives a short "HIGH" on the output
pin which switches the transistor (I took a off-the-shelf NPN transitor -
BC548) and shortens the power supply. The PIC will remain working during
the SHORT time where the power supply is shortened because he will be
supplied by the capacitor. So the longer the time when the PIC shortens
its "own" supply should be, so bigger must the capacitance of the
capacitor be.
The other side - which communicates with the PIC - is also able to pull
down the Power line for a short time. So the PIC could also receive data.

The only problem of this way of communication is you will have to play
around how long and how often you could pull down the power line (how fast
the communication could be) - My application transfered the 10 byte in a
few milliseconds ... ouhh .. I think it was 0,012 seconds ...
Its not fast but it's easy and you wont need a lot of electronic stuff for
such a way of communication.

Sorry for my bad english,
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Kraft Bernhard aka. Krufti              /"\
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