RFID sniffer

RFID sniffer

About this project

RFID is everywhere. Use the easy to build RFID sniffer to find out if objects are tagged.
The RFID sniffer is a simple analog electronic circuit which can detect the presence of 13.56 MHz RFID tags. These tags are commonly used in all kinds of plastic cards like access badges, bank cards, library cards, loyalty cards and so on. Also many other objects may carry RFID tags without you knowing it. Books, toys, and even clothing might be tagged. Carrying tagged objects with you can reveal your identity or whereabouts to anyone equipped with the appropiate tools to read RFID tags.
The RFID sniffer helps you identify which objects are tagged, and which are not.


RFID snifferPosted by marc 2007-11-01 14:09
I started off with a schematic found in an article on the website of German c't magazine.

This RFID tag finder is based on a Clapp oscillator which generates a 13.56 MHz sine wave. The inductor of the oscillator is formed by a loop antenna, which I constructed by winding 5 turns of insulated wire on a spool of fishing line that I had lying around. When the circuit is powered on, the loop antenna radiates a electromagnetic field. When a RFID tag is placed near the loop, a current is induced in the tag's antenna, which will draw power from the oscillator, decreasing the amplitude of the sine wave.
The original schematic detects the presence of a tag by monitoring the supply current of the oscillator, which develops a voltage drop across a current sensing resistor. However, this voltage drop is quite small, and is also dependent on hands or metal objects in the vicinity of the loop antenna.

RFID sniffer breadboard
I modified the circuit to measure the amplitude of the sine wave directly, eliminating the current sensing resistor and adding a envelope detector circuit, consisting of a diode, a capacitor and a resistor. The resulting voltage drops significantly when a tag is present, but doesn't change as much when hands or metal objects are placed on the antenna.
A voltage comparator switches on a LED when the voltage drops under a certain reference level, which is adjustable by means of a potentiometer. The level is adjusted as to light the LED only when a tag is present.

RFID sniffer breadboard (detail)

The result is a detector which is insensitive to hands or metal objects, but can detect RFID tags when placed within 3 or 4 cm distance from the loop antenna.

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