🚀 go-pugleaf

There are several systems in use.  Some systems use a small
resonant foil print on the back of the price tags.  When you check
out, the clerk uses a probe passed trhough holes in the tag to
touch the foil and burn out the resonant circuit.  The sensors by
the dorr operate as a grid dip meter to detect the non burned out
tags.

Our local library uses magnetic strips slipped into the spine of
the book.  The strip can be magetized and degaussed to control its
resonant freqnecy.

One store I visted used tags that actually had diode chips attached
to the back of the tages.  My presumption is the diode operates as
a mixer when the tag is pulsed generating an image frequency that
is easy to detect.  A postmortem on one of the price tags showed
that this system is similar to the first, but it is the diode chip
that is destroyed rather than the antenna foil pattern.

-- 
Bill Mayhew      NEOUCOM Computer Services Department
Rootstown, OH  44272-9995  USA    phone: 216-325-2511
wtm@uhura.neoucom.edu   (140.220.1.1)