For this project, I used a simple electromagnetic lock and a small wood door to simulate a house entrance.

Some information first:

• RFID is an acronym for radio-frequency identification.
• RFID is the contactless exchange of information. And a very big advantage is that the transmitter (tag) needs no own power. It can be passive.
• These cheap handy transmitters get the needed power from the magnetic field, created by the receiver. Unfortunately the field strength is deceasing inversely proportional to the square. (inverse square law)
• Only short distances possible.
• The signals can get through non metallic materials, such as wood or plastic.
• The well-known RFID chip RC522 works with 13.56 MHz and communicates without encryption. A mikrocontroller talks with the chip over the SPI. (Serial Peripheral interface)

Two RFID tags:

You can also use your credit card if the symbol is on it, or an implant.

This is a screenshot of the pins, when nothing is in range:

• NSS = Slave Select
• MOSI = Master Out / Slave In
• SCK = Clock sent by the board (125 kHz)
• MISO = Master In / Slave Out (Slave sends data to the Master)

If the door is open and you put a tag for 10 sec. to the receiver board, it gets stored. Not very safe.

Such a door mechanism should have an UPS in real life. In case the power grid is down.