TUCoPS :: Phreaking Technical System Info :: bt_phcrd.txt

How BT Phone Cards work, by Archaos

Subject: Re: fonecards
Date: Thu, 14 Sep 95 18:04:28 GMT
References: <DEwv47.Cv8@spuddy.mew.co.uk> <43blpt$9ke@columbia.acc.brad.ac.uk>

Here Ya go:

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HOW BT PHONE CARDS WORK - Transcribed by Archaos - June '93
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Contrary to popular belief, BT phonecards do not work using a magnetic 
strip system. The reason for this being that a magnetic strip would be 
read only.

So how do they work then?

Well, examine a phonecard - preferably a used one if you are going to 
scratch it or dissect it. If you look on the printed surface (the green 
side - which is the front) you will find two lines which form a thick band. 
Underneath this area is a "track" which holds the information about the 
number of units used up and how many are left. A used phonecard will have 
some tiny bars marked on the track near one end.

On the reverse side of the phonecard (the black side) you can see a shiny 
black strip in contrast to the matt black which has text on it (on older 
phone cards the whole of this side is shiny black). Anyway, this shiny strip 
is "opposite" the band on the front and acts as a "window" to the information 
on the track - for the simple reason that it is no ordinary shiny black 
plastic. This special black plastic is not like all the others (which do 
not let normal light or infra-red light pass through) but is transparent to 
infra-red light. When a phonecard is in the machine, an infra-red beam is 
shone through the back of the card and the reflected beam is checked to 
detect the time units remaining.

Now to explain the track itself which is protected by a layer of paint that 
also serves as the base for printing text and figures visible to the user. 
On a 20-unit card, the track has 20 tiny rectangular areas (called 
diffraction gratings - you might have come across them if you took physics) 
which affect the light reflected by the cards. As the time units are used up, 
the ares are destroyed by an eraser head. The design of the assembly enables 
the progress of the erasing operation to be checked. in fact, the 20 
rectangular areas touch each other and form a continuous strip on the card.

The area which is read is wider than the track. This makes it possible to 
detect a reduction in track width.

Each unit is separated from its neighbour by a distance of 0.6mm. the erase 
area is greater than the width of the track so that the unit is always 
completely erased. The dimensions of both the card and the time units 
suggest 140 as the theoretical maximum number of units possible.

The read-and-erase mechanism consists of a moving carriage on which are 
fixed the eraser head and the optical components for reading. the carriage 
is driven by a stepping device which moves along the track to determine 
whether each unit is god or erased. when a unit has been consumed by the 
cardphone, the area is erased in its turn and the carriage moves on one step.

OK, for those that weant to know, here is an ascii graphical representation 
of the read and erase geometry : 

                      Time units
         Track |     |     |     |     |     |     |     |     | 1.2mm

                    Area read              Area erased
                       ***                  ********* 
         |     |     | *** |     |     |    *|*****|*    |     |   1.6mm
                       ***                  *********
                      0.4mm                   0.7mm

Well I hope you all understood that! Most of the information in this text 
file was obtained from British Telecom <spit> sources so is quite likely to 
be correct (after all, they should know their own cardphones!).



Hope I didn't offend anyone by upping this. 
                             -- DreamshadoW --
                       Damn me father, for I must sin
                                                       Finger for PGP Key.

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