Magnetic Readers - an addendum - 1s and 0s

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From: SFPhysics@aol.com
Date: Sat Feb 23 2002 - 11:11:26 PST 

From: SFPhysics@aol.com
Message-ID: <f4.173ad9fc.29a9435e@aol.com>
Date: Sat, 23 Feb 2002 14:11:26 EST
Subject: Magnetic Readers - an addendum - 1s and 0s

<< Many thanks for the detailed reply Al. Could I trouble you to explain why
 the plastic bag makes the 0s go down and 1s become more distinct?
 i.e. how does the plastic decouple the magnetic circuit?
>Thanks again,
>Tom
>>

Hello again Tom:

The thickness of the plastic bag introduces a gap between the sensor and the
card's magnetic stripe. Where the magnetic domains of the 1s were smeared
into the 0s so the 0s registered before as 1s or indeterminates, the gap
*decouples* the magnetic field so the reader does not detect the lower level
magnetic smear and the 0s are again read as 0s. All it takes to decouple the
magnetic circuit is that few thousands of a millimeter of plastic between the
card and the reader head which will lower the sensitivity of the circuit and
drop the output levels. Thinking in terms of voltage output, digital
circuits normally use 3.5 - 5 Volts to mean a 1, 1.5 Volts or less is
considered a 0. Decoupling a smear signal of 2 Volts output to give 1 Volt
from the circuit lets the AD (analog to digital) converter again "see" a zero.

Best to you and the list,

Al Sefl
Itinerant Physics Philosopher...

PS: To answer an off-list question on-list; yes, cards can be swiped by a
criminal who duplicates the magnetic pattern on another blank card and steals
your card identity. That is why I try to use only cards with PINs (Personal
Identification Numbers) that are not on the magnetic stripe but stored in my
large empty head. ;-)

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