This project emulates an EM4102 RFID tag. Using the three switches allows to select one of the three RFID codes to be sent.
How does it work?
The coil together with the two parallel capacitor form an LC-circuit with a resonance frequency of 125kHz. Once the the RFID reader is within range, there will be a 125kHz voltage sine wave over the LC. The two schottky components will rectify that voltage and charge the power supply capacitor. Once this capacitor has enough charge, the microcontroller starts up. The two schottky diodes are optional. If not placed, the internal ESD-protection-diodes of the microcontroller will rectify the sine wave.
After start up , the microcontroller will poll the switches. If a button is pushed, the corresponding RFID code will be sent. The data transmission is done in the microcontroller by periodically shorting the LC-circuit. The RFID-reader will detect the load switching of the tag and will decode the data.
- In the attachment, you can find the artwork that has been used to create this PCB.
- Via's have been soldered using a Roadrunner Wiring Pencil.
- Most components can easily be soldered. The inductor should be soldered last. Be sure not to forget to cut the trace on the PCB of the inductor. This ensures that the original circuit of the inductor is non-functional.
The microcontroller can be programmed in-circuit using some simple tools:
- Pomona test clip 5250
- AVRISP Mk.II
- 5V Power supply
- 5x Breadboard jumper wires
The programmer doesn't power the device. Connect the 5V power supply to VCC & GND.
Remark that the fuses also will be programmed. Once this is done, an external clock will be needed to re-program the device. An easy solution is to put the device on the RFID-reader during re-programming.
A key fob housing (CamdenBoss 2955-20R-3) is used. Remark that on the inside a mounting stud has to be partially cut away to allow the inductor to be mounted. The antenna should be glued to the PCB. This will ensure that the connecting wires don't crack the PCB tracks. In an improved design, the connecting wires of the antenna should get bigger pads on the PCB: on top side as well as on bottom side.