A laser would burn part of a strip for each credit used.
http://technologyandarchives.blogspot.co.uk/2011/05/how-bt-p...
(There must be better descriptions of them out there).
A laser would burn part of a strip for each credit used.
http://technologyandarchives.blogspot.co.uk/2011/05/how-bt-p...
(There must be better descriptions of them out there).