Step 3: 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.