>Hi,
>
>Does anyone have any informations about transponder, like the one used
>in cars to pay toll? Is it possible to do one with PIC?
>
>Cheers,
>
>Octavio

In Norway we have a system built on the SAW (Surface Acoustic Waves) principle.

The transponders contain a PCB and a single SAW chip.
The chip contains a pattern that corresponds to a unique serial number.

When a car passes, the system sends out a powerful RF signal
>from an antenna pointed at the car, and the transponder reflects a slightly
modified signal (modified according to the serial number) that is
received by the system.

The transponder contains no batteries or other power source.

A database holds information about payments/number of passings etc for each 
serial number.

I don't know how this is done in other countries.

I know of people that pull down the sunscreen, turn on full lights, and steps
on the loud pedal before passing....

Anyway, I don't think it is possible to use a PIC to a thing like this.

If it was possible, you would have to get hold of someone elses serial number
and then they would hav to pay for your passings.... 

Remember, the above applies only to the system commonly used in Norway.

-Oyvind