How BT phone cards works

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
-----------------------   ----------------------   --------

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
        ---------------------------------------------------------
                           <0.6mm>

                    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!).

Archaos.

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