How to detect when a card has been removed from the reader

[richardvolpe]

Hi

If a card is put against the reader, the Read.py script will continue to read the same card over and over, and process the output (in this case printing the UID) again and again.

I am looking for a way of only printing the UID out once, and then upon subsequent iterations, if the code detects it is the same card sitting on the reader, then it should not print out the UID.

Any assistance would be greatly appreciated.

[Jbithell]

So here's Read.py from this repo:

#!/usr/bin/env python
# -*- coding: utf8 -*-
#
#    Copyright 2014,2018 Mario Gomez <mario.gomez@teubi.co>
#
#    This file is part of MFRC522-Python
#    MFRC522-Python is a simple Python implementation for
#    the MFRC522 NFC Card Reader for the Raspberry Pi.
#
#    MFRC522-Python is free software: you can redistribute it and/or modify
#    it under the terms of the GNU Lesser General Public License as published by
#    the Free Software Foundation, either version 3 of the License, or
#    (at your option) any later version.
#
#    MFRC522-Python is distributed in the hope that it will be useful,
#    but WITHOUT ANY WARRANTY; without even the implied warranty of
#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#    GNU Lesser General Public License for more details.
#
#    You should have received a copy of the GNU Lesser General Public License
#    along with MFRC522-Python.  If not, see <http://www.gnu.org/licenses/>.
#

import RPi.GPIO as GPIO
import MFRC522
import signal

continue_reading = True

# Capture SIGINT for cleanup when the script is aborted
def end_read(signal,frame):
    global continue_reading
    print "Ctrl+C captured, ending read."
    continue_reading = False
    GPIO.cleanup()

# Hook the SIGINT
signal.signal(signal.SIGINT, end_read)

# Create an object of the class MFRC522
MIFAREReader = MFRC522.MFRC522()

# Welcome message
print "Welcome to the MFRC522 data read example"
print "Press Ctrl-C to stop."

# This loop keeps checking for chips. If one is near it will get the UID and authenticate
while continue_reading:

    # Scan for cards    
    (status,TagType) = MIFAREReader.MFRC522_Request(MIFAREReader.PICC_REQIDL)

    # If a card is found
    if status == MIFAREReader.MI_OK:
        print "Card detected"

    # Get the UID of the card
    (status,uid) = MIFAREReader.MFRC522_Anticoll()

    # If we have the UID, continue
    if status == MIFAREReader.MI_OK:

        # Print UID
        print "Card read UID: %s,%s,%s,%s" % (uid[0], uid[1], uid[2], uid[3])

        # This is the default key for authentication
        key = [0xFF,0xFF,0xFF,0xFF,0xFF,0xFF]

        # Select the scanned tag
        MIFAREReader.MFRC522_SelectTag(uid)

        # Authenticate
        status = MIFAREReader.MFRC522_Auth(MIFAREReader.PICC_AUTHENT1A, 8, key, uid)

        # Check if authenticated
        if status == MIFAREReader.MI_OK:
            MIFAREReader.MFRC522_Read(8)
            MIFAREReader.MFRC522_StopCrypto1()
        else:
            print "Authentication error"

If we add an if statement at the right point we can store the last card that was read and then check if its the same one:

import RPi.GPIO as GPIO
import MFRC522
import signal

continue_reading = True

# Capture SIGINT for cleanup when the script is aborted
def end_read(signal,frame):
    global continue_reading
    print "Ctrl+C captured, ending read."
    continue_reading = False
    GPIO.cleanup()

# Hook the SIGINT
signal.signal(signal.SIGINT, end_read)

# Create an object of the class MFRC522
MIFAREReader = MFRC522.MFRC522()

# Welcome message
print "Welcome to the MFRC522 data read example"
print "Press Ctrl-C to stop."

#Setup a variable to store the last read chip
lastDetected=""

# This loop keeps checking for chips. If one is near it will get the UID and authenticate
while continue_reading:

    # Scan for cards    
    (status,TagType) = MIFAREReader.MFRC522_Request(MIFAREReader.PICC_REQIDL)

    # If a card is found
    if status == MIFAREReader.MI_OK:
        print "Card detected"

    # Get the UID of the card
    (status,uid) = MIFAREReader.MFRC522_Anticoll()

    # If we have the UID, continue
    if status == MIFAREReader.MI_OK and lastDetected != str(uid[0], uid[1], uid[2], uid[3]):

        # Print UID
        print "Card read UID: %s,%s,%s,%s" % (uid[0], uid[1], uid[2], uid[3])
        # Update the variable 
        lastDetected = (uid[0], uid[1], uid[2], uid[3])
        # This is the default key for authentication
        key = [0xFF,0xFF,0xFF,0xFF,0xFF,0xFF]

        # Select the scanned tag
        MIFAREReader.MFRC522_SelectTag(uid)

        # Authenticate
        status = MIFAREReader.MFRC522_Auth(MIFAREReader.PICC_AUTHENT1A, 8, key, uid)

---

Just as a quick heads up - github issues are designed for highlighting bugs or problems on a device with a project in order to alert the contributors so they can investigate and improve the codebase - it's not really designed for open questions or advice on coding - stackoverflow.com is the popular choice for q/a style stuff. I'm not criticising - and hopefully the code above works for you and I'm happy to help if it doesn't, it's just that a few project owners can be a bit funny about people using github issues for this kind of thing from time to time.

[richardvolpe]

Noted re not asking advice in this issues section. Will be aware for next time! Thank you anyway for your help - makes perfect sense. Cheers.