> hmm.. what about something like a ski lift pass?
>> (I know you probably don't want such a huge yoke in the hallway, but
> still.. we tried to figure out what it does, and ended up thinking
> the magetic field the machine generated caused enough power in the
> card to send a message to the detector (they have ID's, because
> you can time them out etc))
>>They're true RF ID readers and tokens. They actually work like little
The reader is the primary and is powered by a 125kHz signal (this frequency
has been used for this purpose for years but there are some new systems
coming out using higher frequencies).
The token (the thing in your hand) is the secondary. When you place it close
to the reader the 125kHz signal is coupled into the electronics in the
The secondary is actually split into several tapped coils. One is used to
pick up the 125kHz, which is then rectified and used to power the
electronics, another tap is squared up and the resultant 125kHz square wave
is used to clock a little ASIC in the token that is programmed with a unique
number at time of manufacture (or can be programmed in the field on some
systems). The clock just causes the unique number to be fed out serially
over and over whilst the token is in range of the reader.
This code then feeds onto the gate of a FET which connects across yet
another tap on the coil, so that a '1' shorts the coil out and a '0' leaves
it open circuit.
As everyone knows (hee hee) if you change the impedance of the secondary of
a transformer, the impedance of the primary also changes by an amount
defined by the transformer's turns ratio.
The reader detects this change in impedance at the primary and is hence able
to read the number programmed into the token.
Yes, there are plenty of access control and ID systems around that use this
principle but it's still not at all cheap and to get any decent read range
(like a complete doorway) you need a really big aerial.
If you use a small reader with a short reader range then you're going to
have to wave the token in the direction of the reader each time you go
through the door. If you have to do that then you might as well use simpler
and cheaper technology.
Then of course there's the problem of how the system works out if you're
coming in or going out....
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