Detect the absence of an RFID card

Hi @blackghost,

  Sorry I don't understand your question.

You say...

My target is if the tag removed power is off ... how to check if tag is still present or removed

So far as I am aware, RC522 tags are 'unpowered', in the sense they do not have a battery or other internal power source, nor do they have an on/off switch ... they rely on being close enough to the 'tag reader' antenna to pick up the inductive power from the 'tag reader' unit.

And as a consequence of needing to receive power by induction, they must be physically close to the antenna. The actual range depends on the setup, but typical max is 5cm.

e.g. https://www.nxp.com/docs/en/data-sheet/MFRC522.pdf

Such systems are often deliberately 'range limited' to a few cm, so they cannot be used to intrude on privacy, e.g. recording everyone walking into a shop with a card in their wallet.

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Some tags clearly have a longer range than a few cm, but I don't know if that includes a variant of RFC522. Perhaps you could provide a reference to a specific tag that specifies the range that your application needs?

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and is there any way to add antenna to rc522

Both the 'tag reader unit' and the tag (e.g. card) e.g have an antenna ... that is how they transfer both energising power and data.

Clearly you could consider replacing either of these antennas with an alternate one, though you might have to be careful about legal regulatory limitations.

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I have a suspicision you are trying to do something the RFC522 system is not intended to do.

Perhaps you would like to clarify your question?

Best wishes, Dave

So are you trying to do something like an emergency stop (e.g. if you pull the wrist-strapped plug by falling off, then the jet-ski turns off)?

If so, then it seems you are just polling the RFID reader repeatedly as you pass through the main Loop().   If you "see" the tags as expected, then do nothing until the next loop.  If you cannot "see" an expected tag, then turn off whatever it is you need to.

My rocket launch controller has to do the same type of polling for safety switches....poll, poll, poll, oh there was a change, take action...

thank you all for try to help

Posted by: @lydara

trying to do something like an emergency stop

that was my target