ccplumb@watnot.UUCP (04/07/87)
In article <2634@phri.UUCP> roy@phri.UUCP (Roy Smith) writes: > This is also, BTW, one of the ways those stolen-item detectors in >stores work. The big plastic clips contain some sort of passive tuned >circuit, with a diode detector. The gizmos on either side of the door emit >RF at some frequency (presumably up in the several hundred Mhz range) and >listen for harmonics comming back. The important features of this system >are that the replicated parts (the big plastic clips) 1) are cheap to >produce, 2) require no internal power, 3) are not likely to break, and 4) >can be made physically small. The latter three are properties which also >makes this a nice spy technology. I always wanted to know that. I've seen thing with a similar purpose at bookstores. They looked like strips of aluminized mylar/tinsel, a few inches ling and about 7mm wide. They can be `turned off' (so they don't register to the thing at the exit) by a demagnetizer-like gizmo. Anybody know how these work? (Funny thing: one store I know has one of these, but they always leave it turned off! Kinda ruins the effect if the power lights and switch are visible, don't it?) -- -Colin Plumb (watmath!watnot!ccplumb) Silly quote: That's a horse of a different feather.
mb@camcon.co.uk (Mike Bell) (04/24/87)
A local company, Cotag, produces coded tags which are sometimes used for this purpose - mainly on pricier items. I seem to remember the method of operation was to receive rf on one frequency (somewhere in the 10-100 KHz region) and retransmit a coded echo on twice the input frequency. Not quite as neat (or cheap) as the gdo method, but less susceptible to false alarms and useful for other purposes (security doors/FMS systems etc.) [No I'm not connected with Cotag - hence the vagueness]