Subject: Re: fonecards

Date: Thu, 14 Sep 95 18:04:28 GMT

References: <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

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

<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 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.