>One other idea is to use mag swipe cards everyone has (fly buys, Medicare,
>bank cards etc) you use track 3 which contains the card number and expiry
>date, which is safe and non intrusive. commercial mag stripe readers are
>available form inner range security products for about au$30 and it sends
>the data out in a serial format.

I guess you are looking for a real cheap solution so it does not matter if
the cards do not come back.

One scheme I have seen used was for room registration in a Youth Hostel,
where your "card" was a heavy paper ticket printed with a bar code. There
was a barcode reader associated with a block of about 4 or 5 doors, and
inserting the card released a security door style magnetic lock. The barcode
was associated with an expiry time in the computer booking system, so you
were automatically "booked out" when it expired.

The disadvantage is that all the doors and readers need wiring back to some
central computing system, but I suspect you were looking at that anyway if
looking into smart cards.

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