This thing is a simple RFID reader and emulator working with 125 kHz cards format H10301- FSK modulation 26 bits only. These cards (made by HID and other companies) are widely used in US.
The device can read and show facility and card numbers on 16 x 2 LCD. The reading can be done in two modes: active and passive. In passive mode the device do not generate EMF so it mast be close to another reader. The idea is to add ferrite rod antenna and RF amplifier and see how fare from another reader can get card number (not done yet). The read card numbers can be saved – up to 32 card numbers. Every read card is send to computer (if connected) -the connection is RS232 at 57600 baud rate with TTL levels. It looks like:
32 378E : 000000000010000000000110011001000110111100011100
Also you can enter numbers manually.
The device can also act as card when it closer to another reader (emulate it) with numbers entered manually or from memory.
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Step 1: Design
The main part is popular microcontroller PIC16F628A.
Functions of the buttons 1,2,3 (in the middle) is shown on second line on LCD.
The antenna coil has about 60 turns with magnet wire diameter 0.2 mm, and size 80 X 65 mm. number of turns, wire diameter and size of antenna are not critical. If you use small diameter of wire and resistance of antenna coil become high (10 ohms and more) R14 should be 0 ohms. More important is L1, C7 and C8 to be in resonance at 125 kHz. This may be achieved by selection of C7 so that on C9 you have 20V or more (active reading mode).