Subject: Re: fonecardsDate: Thu, 14 Sep 95 18:04:28 GMTReferences: <DEwv47.Cv8@spuddy.mew.co.uk> <43blpt$9ke@columbia.acc.brad.ac.uk>Here Ya go:---------START OF TEXT -----------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                   0.7mmWell 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.------EOF--------------okay?Hope I didn't offend anyone by upping this.                              -- DreamshadoW --                       Damn me father, for I must sin                                                       Finger for PGP Key.