ویدیوی بعدی
ویدیوهای پیشنهادی
What Are Those Stickers on Items in Shops
What are all those square (or rectangle) stickers on items in supermarkets? Those are part of Electronic Article Surveillance (EAS). The labels are Electro-magnetic (EM), Acousto-magnetic (AM) or Radio frequency (RF) tags used to prevent shoplifting. They are detected by antennas at the exit of the store and trigger alarm. If the item is paid, the tag is deactivated and doesn't trigger alarm. The RF ones are a simple resonant LC circuit which is being deactivated by a strong signal that shorts out the capacitor. I managed to find one RF tag (this is the square one) that wasn't deactivated, so I tried to build my own simple detection circuit and measure its operating frequency. The nominal frequency of the RF labels in retail stores is 8.2 MHz. I have measured 8.65 MHz. This is pretty close given how inaccurate those low cost disposable resonant circuits must be.
The oscillator in my schematic is built with 74HC14 (6x Schmitt inverter). One inverter as RC oscillator, one inverter as a buffer and 4 inverters in parallel as a power output stage.
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