Stop thief
Question: I have noticed that electrical goods stores use a form of adhesive
metal coil as a security tag. How do these tags set off the door alarms and how
are they deactivated before a legitimate customer leaves the shop?
Answer: The coils are the antennae of a very simple, disposable radio circuit
that is based on a tiny diode or tuned resistor-capacitor circuit. The gates at
the shop door emit a radio signal that the circuit is tuned to, so the circuit
resonates and re-radiates a modified signal. When the gates pick this up they
trigger the…
