UPDATE: Here is a link to an Arduino Mini shield based on these instructions http://wiki.smallroom.net/doku.php?id=terd:projects:rfidspoofer .
Step 1: Parts
*Some enamel coated solid core copper wire (I used the green spool from the 3 spool set Radio Shack carries).
*A NPN transistor, I used a 2N3904
*A 10 K Ohm Resistor
*A 10 nF capacitor (0.01 uF). I'm using a Metalized polyester film cap I got from Radio Shack, others should work though
*A toilet paper roll to wind the wire on
I tested my circuit using a Parallax RFID serial reader connected to a second Arduino
Step 2: RFID background
125 KHz cards use manchester encoding to encode the data to send it to the reader. Manchester encoding basically takes the XOR of the bit that needs to be transmitted and the clock value. So if the clock value is low (0) and the value to transmit is 1 then it would be 0 XOR 1 which is 1. This has to be done on every clock cycle. For more information on manchester encoding see en.wikipedia.org/wiki/Manchester_code.
Step 3: The Data
The serial number of a tag is sent over using a fairly simple protocol.
It starts by sending 9 one's
Then it sends 10 sets of 4 bits, then one parity bit (it's using even parity)
Then it sends "column" parity bits (even parity of the rows in the previous step)
Last it sends a 0 stop bit
So an example looks like this:
(10 rows of data - the card serial number)
(the first 4 bits are the data, the last is the even parity bit)
(then it sends the column parity bits, even parity of the rows above)
(last a 0 stop bit)
See the pdf in the first link in the references section for more details on this
Image provided by Flickr user at www.flickr.com/photos/kurtisscaletta/2473469841/ and used under the creative commons license.
Step 4: Building the circuit
After you have your coil you can connect it to your circuit. The schematic is pretty easy. Just connect pin 9 on the Arudino to a 10 K Ohm resistor, then to the base of the transistor. Next you can put your capacitor between the collector and emitter of the transistor. The emitter also needs to be connected to ground. Next connect the coil the the emitter and collector of the transistor.
Step 5: The code
Step 6: Testing
Once it's working at really short ranges (touching the reader) you can mess with the coil some more to tune the antenna better and you should be able to get a range of a few inches.
Step 7: The Video
Step 8: Elephants in the Room
The second issue is the form factor of the antenna. It should be easy to modify this though by simply collapsing the coil. At that point though you'll need to use a different calculator that does multi-level coils to figure out how to wind it.
Last, there's the range. By experimenting with the coil winds and the capacitor you should be able to get a few inches of range. More range would probably need some type of an amplified coil.
The image for this step is from www.flickr.com/photos/exfordy/123900378/ used under the creative commons license.
PDF on a similar project, good discussion of how it all works and schematic
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