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RFID/NFC Tag Front Door Lock

RFID reader

This is a classy RFID reader to unlock our home's front door. It supports groups and has an optional Android app.

I live near Boston and our public transit system uses RFID cards for regular users of the subway. This means that everyone I know has one already in their pocket, so it makes it very easy to give any of my friends access. Also, the city gives out the cards for free, so it's easy to get your hands on unused ones just by looking for them in the subway stations.

Using their existing RFID tag, a person can unlock a door if they're in the right group. It's easy to switch which current group is allowed access, so groups can be set based on who's allowed in at a given point in time.

For example, a close group of friends who have regular parties could all be added to a group, which could be switched to on when that party is happening. Or one could use it to only allow a limited number people in during the evening, but allow more in during the day.

Below are some details if you'd like to build one yourself. I was going for style, compactness, and better control over a quick hack, so there are many details here that can be omitted if desired.

RFIDs / NFC Tags

The system was designed to work with 13.56MHz IEC 14443 Mifare Classic and other similar NFC tags, but one could use another reader or tag type if desired.

Up to 50 tags can be stored on an atmega168 chip or 100 for a mega328 (check the printing on the main chip on your Arduino board).

Just a word on terminology: "RFID" and "NFC tag" are interchangeable for this use case, much in the same way that "smartphone" and "Android device" are often interchangeable. However one can have smartphones that aren't Android devices and also Android devices (such as a Google TV or smart watch) which aren't smartphones. I use "RFID" here to refer to the use of an NFC tag for its public ID number.


There are seven groups which function as an access control list. Only one group can be active at a time and the currently active group's number is displayed on the LED display.

Each RFID can be added to any of those groups.

All the RFIDs and groups are remembered in EEPROM, so they'll survive power cycling.

The currently active group, however, is not stored in EEPROM, so it will revert to group 1 if the power is cycled. You should ensure that group 1 is a safe group to be active in this case.

Physical Interface

The setup has a 7-segment display and a single button. The 7-segment display shows the active group number and the button advances through the groups.

To add a card to a group, long-press the button until it says, "scan card". Once you tap the card on the reader, it'll be either added or removed (if it's already in the group).

Serial Control Interface

Besides the physical interface, there is a textual control interface that can be used over USB or Bluetooth. The Android app speaks this interface over Bluetooth to manipulate the groups and activate the relay.


The command language is simple: just one character followed by some arguments and terminated with a newline. There is no whitespace between the command and its arguments, although it's possible some commands will ignore it. It supports CR, LF, or CRLF, so it should be usable from a simple terminal app or the Arduino serial console.

'l' command

List all the records.


l <newline>

Output is in the following format:

group <tab> ID

group is a bitfield of all the groups the ID is in. Group 1 is the least significant bit, group 7 is the second-to-most significant bit.

ID is the ID, chunked into bytes, printed in hex and separated with ":"s. Eg.:


'c' command

Erases all records. Doesn't prompt for confirmation, so be careful with it.


c <newline>

'a' command

Add the given ID to the currently active group.


a <id> <newline>

Adds an ID. <id> should be conveyed in hex, much like the output of l (although the ":" are optional). This adds the ID to the currently active group.

'A' command

Adds the next scanned card to the current group.


A <newline>

'd' command

Deletes the ID from all groups.


d <id> <newline>

<id> is in the same format as the "a" command.

'r' command

Remove the ID from the current group.


r <id> <newline>

'g' command

Sets the currently active group.


g <group> <newline>

<group> is "1"-"7" and can be omitted to just retrieve the current group.

Prints the current group.

'o' command

Opens the door.


o <id> <newline>

'v' command

Prints version info.


v <newline>

Android App

There's an Android app included in this project to allow one to add/remove cards from the groups, switch the currently active group, and trigger the relay. If you have NFC on your phone, it lets you scan cards from the phone to add them directly to the list.


I built this with some parts I had lying around, but I think they're actually fairly well suited for building similar setups. The shift register makes driving the 7-segment display easier, but isn't necessary - one could probably find enough spare pins or use an LED driver or some such.

I decided to use the Grove relay board because I wanted to have the relay in a different location than the main interface board. Obviously this is overkill for many setups and a simple relay (+ transistor) would suffice.


RFID reader opened

More photos in the RFID front door photoset.

One of the things that I love is hardware with beautiful housing. So I attempted to create a wooden housing based on examples of ornately decorated objects that I found. I'm going to be looking at this every day, so I wanted it to be pleasant and not as impersonal as so much technology feels.

housing SVG reader housing SVG

There are two housings: one to hold the Arduino + ProtoShield, RFID reader board and Bluetooth transceiver; and another to decorate the RFID reader itself. Both are intended to be lasercut.

The colors in the housing SVG files represent the following:

  • Red: cut
  • Green: medium raster etch
  • Blue: deep raster etch (to fit the antenna)
  • other colors: informational; don't cut

The main box was made with BoxMaker, which is an excellent tool for making laser cut boxes that fit together nicely.

This entire project is stored in git and can be easily cloned:

git clone git://

Or you can browse the RFID Front Door repository online.

If you'd like to contribute changes, just send me an email with your changes committed to a public repository (you can always use Gitorious or Github).


All source code is licensed under the GPLv3 unless otherwise noted; all photos, SVG documents, documentation, and art are CC-by-sa.


RFID/NFC tag front door scanner







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