Step 3: How do we emulate a card?

Picture of How do we emulate a card?
So the next step was to identify how to pretend to be a card - I wanted a card that I could type a card number into, so it had to have a microprocessor on it, was well as a keypad to allow the data to be keyed in.

The ATMega  manipulates the 125kHz RF field by using a bridge rectifier.  When the output of the micro is low, the diodes in the bridge are allowed to be turned on by the current induced in the coil,  this effectively short it out.  The reader detects the additional load, and a bit transition is detected.

The job of the micro is simply to turn the output on and off in a way that makes sense to our reader.  So I created a board that had the micro, a power supply, keypad, and some status LEDs on it.

The attached PDF is the full schematic of the project.

You may notice that c6 is 0pF -  That is intentional  c6 is a placeholder component allowing me to either use a 1000pF surface mount cap, or a 1000pF through hole cap.

The coil is 100 turns of fine wire would on an open former that is just smaller than the card border.

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Laserman5953 years ago
drj113 (author)  Laserman5952 years ago
Ahhhhh - it took a while for me to see this..... C4 was not chosen intentionally :-)
wow, just wow. : )
drj113 (author)  techxpert2 years ago
Why Thank you :-) This was a fun project.
diwib3 years ago
Do you know how would I turn this into a 13.56 Mhz card emulator?
kwadwo00 diwib2 years ago
Have you heard anything on how to turn this into a 13.56 Mhz card emulator? i.e. Could you swap out the coil for a 13.56 Mhz antenna?
drj113 (author)  kwadwo002 years ago
I have never experimented with 13Mhz cards - that would be very interesting.
drj113 (author)  diwib3 years ago
That would be a very interesting task - Firstly, the tank coil would have to be resonant at 13.56 Mhz - less turns. Then the delays in the code would have to be reduced to allow the card to run faster.... I will have a look at this when I can get hold of a 13.56 Mhz card and reader to play with. It may not be a task for an Atmega CPU - they may be too slow
flashmandv3 years ago
Hi All,

what I need to change in the schematics and softwarte to make it work with PSK modulation?
Please help, since many rfid readers are using only PSK :(
kokko4 years ago
sorry .. but the capacitor C6 .. mistake or to see and 0pf??
I'm sorry for my ignorance ...

Kokko greetings :-)
drj113 (author)  kokko4 years ago
c6 is 0pF just as a place holder in the schematic editor- I wanted to have EITHER a through hole or a surface mount cap - Just use either c4 or c6
dbell4 years ago
What part number diodes did you use for the switch?

drj113 (author)  dbell4 years ago
Any switching diode is fine - it is not speed critical - I used 1n914