This photograph was shot at Sainsbury’s some time ago when I was trying to buy a hot pizza. Obviously, this is an LC oscillation circuit. It is intended to be concealed underneath this white sticker I assume, but unfortunately, the plastic box which they use to place the bar codes for pizza was transparent, which enabled me to have a look at its original appearance.
My best guess of the purpose of this circuit is Sainsbury’s security device. An LC circuit reaches resonance at a certain frequency which is given by \frac{1}{\sqrt{L \cdot C}} (hope anyone reading this has some knowledge in LaTeX, I still have to work out how to make this expression display correctly in my blog). The scanners at the exits generate EM wave at this frequency, and when this wave gets picked up by the resonance circuit, current within the oscillation circuit peaks and this periodical current flowing inside the circuit generates EM wave. This EM wave can, in turn, be picked up by some detectors which sound the alarm.
However, obviously this sticker with a circuit hidden underneath is not attached to all the products, so does this mean sneaking something without a sticker out of Sainsbury’s will not sound the alarm? I doubt if this is the case, and more thoughts are on the way.