3

Build an NFC-powered door lock

TUTORIAL

#!/usr/bin/env python3

from evdev import InputDevice

from select import select

The first line specifies the environment in which

you’ll be working and that you want to use Python 3.

Then, we need to import InputDevice and select,

both of which will allow us to read the data from the

RFID fobs.

Next, add the following three lines of code:

rfid_presented = “”

keys = “X^1234567890XXXXqwertzuiopXXXXasdfghjkl

XXXXXyxcvbnmXXXXXXXXXXXXXXXXXXXXXXX”

dev = InputDevice(‘/dev/input/event0’)

The first line creates and initialises an empty variable

in which we’ll store the characters that make up the

data string from the key fob, one by one. The second

line defines a string of characters which later code

will refer to in order to establish which character has

been typed. The third line defines your input device

so the code knows which to listen to. You’ll need

to check what ID your input device has been given

by the Raspbian operating system and modify your

code accordingly. To do this, type the following into a

Terminal window:

ls -lah /dev/input/by-id

You should see a list of all your Raspberry Pi’s

input devices, with a symlink for each one giving it a

friendlier name. Look for the long name which is clearly

your RFID reader – in this case it’s ‘event0’ (because

I’m shelled into the Pi via SSH and there are no other

USB devices attached, such as a keyboard and mouse).

ALWAYS LISTENING...

The system needs to continuously listen for input from

the reader, so we achieve this by defining an infinite

loop like this:

while True:

Everything else in the program takes place within

that loop, so it’s now constantly listening for an event.

To handle the event, add these lines to the while loop:

while True:

r,w,x = select([dev], [], [])

for event in dev.read():

if event.type==1 and event.value==1:

if event.code==28:

if rfid_presented==”0001155844”:

# Unlock Door

print(“Unlocking Door.”)

GPIO.output(13,GPIO.HIGH)

time.sleep(5)

# Lock Door again

print(“Locking Door Again.”)

GPIO.output(13,GPIO.LOW)

else:

print(“Access Denied.”)

rfid_presented = “”

else:

rfid_presented += keys[ event.code ]

Inside this loop, we’re listening for events of type

1 which means a (virtual) key has been pressed

and a character entered. Each time that happens,

we look at the event code to see if it’s 28, which

corresponds to the newline character, which we

know from before means the end of the data being

read from the fob. If the data that has been entered

isn’t a newline, we take that data and add it to

the rfid_presented variable using the ‘+=’ syntax,

gradually building it up with a character at a time to

create the string of text. Once we get a character

which is 28, i.e. the newline, we stop building the

text string up and see if it’s the data that we’re after.

For the sake of clarity in this example, I’ve hard-

coded the data we’re looking for into the if/else

statements (see the ‘Building in flexibility’ box for a

better real-world alternative). You’ll obviously need

to substitute that with the data from your own key

fob. If the string read from the key fob is the one

we’re looking for, then we jump into action and do

five things:

1. Print a line of text to the Terminal window
   saying what we’re doing (which is very useful
   for debugging)

Instead of hard-coding the data from the RFID fob into your code, consider storing a list

of authorised codes in a MySQL database instead, and modify the Python code with an

SQL query to look up the presented code in the database to check validity. This opens

up a world of possibilities; you could create a web interface to manage access remotely

and assign owners’ names to fobs, or you could add extra columns to your database

table to restrict access to certain days or times... ideal for cleaners or dog walkers, etc.

BUILDING IN FLEXIBILITY

Right

The RFID reader

inputs data it reads

from the NFC tags

(key fobs) into the

operating system

like a keyboard

inputting text