Another Phone Card Theory               By Aztech

Many theories have been put forward about the elusive tolling method used
by BTs PhoneCard Payphones,and ways to avoid it.Most of these have been
such shit,its unreal.Eg;Put the card in the Microwave for two minutes after
use,paint it in Nail Varnish,cover it in Tin Foil,etc,etc...
        Although most of these are totally useless,they are heading in the
right direction,ie:To stop any contact between the tolling mechanism and the
card.BUT!If contact between the card and tolling mechanism is not established
at some point,the card will be rejected.To enlighten you as to the method 
used to knock credits off the card,read on:

If you look closely at a used card,you will see two things:

1.A Black or Brown mark on the metallic white strip running across the card
  (ie:across the part that seperates the head and base of the card,the base
  being the space for graphic designs,and the head the issuing company and
  credit identifier.)
2.A very,very vague square around the brown or black mark.

The more electronically minded of you should by now have figured out the 
line of argument I am presenting.If you use a scalpel to remove the white
band covering the milled metallic strip on the card,and then use a multimeter
set to measure diode resistance on the strip,you will notice that the total
resistance stays constant.from left to right.The total resistance on any card 
is directly proportional to the credits left.Therefore,to knock credits off,
the resistance must be gradually increased.This brings us back to the 
brown mark,and the mysterious square.This is caused by a two pin discharge
electrode,whose job it is to individually blow each little mill on the strip
unitl there is a high resistance across the card,making it impossible to
let the card complete the speech circuit by carrying the voltage required
to keep the line 'open'.
        Now,before you start to mutilate the card with scalpels and 
probes,stop.The cards are designed so that any interference will render them
useless,much like warranty seals or safety stickers.So,the only avenue open
to you now is to buy a set of plastic cards and build your own.Use an old
BT card to get the line up of the resistance strip just right,and use strips
of foil or any other germainic conductor,with a resistance of approx 0.5 ohms
to build the strip,making sure each one has approx 0.1mm between it and the
next.Now you must also paint the card with electrically conductive paint,to
establish a ground or earthing voltage.The value of the card can also be 
determined by placing more strips on the tolling strip,but remember,you
must cover the strip with a very faint insulator,able to withstand 0.2 volts
before conduction,to ensure a smooth movement of the electrode head between
strips.Good Luck,But remember,this is only a theory,but one which has brought
expressions of "Who told YOU that?" to BT engineers faces.

(ADDITIONAL:It was later found out after some leafing through BT paraphenalia
 that the card actually uses Optical encoding to hold charge data.
 Unfortunately,we could find no technical reference to this method anywhere
 else,so we are still looking for a way past the tolling mechanism.)

                                        ---=[AZTECH]=---