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I am a very novice chiphacker and am asking for ideas on how I might implement something like this.

I am about to start an RFID project, which is the easy part. RFID readers are cheap, arduinos are cheap, and RFID tags are cheap.

But I want the ability to turn the RFID tag "on" or "off". I want to be able to say "when this happens, then allow yourself to be 'excited'".

The project I'm working on is testing for the presence of a liquid (which has ions in it.) So one high-level solution would be "separate the RFID chip from its antenna. When the liquid is spilled, it 'shorts' that connection and now the tag may be excited."

So my question is this: first, is there any mechanism for turning a tag "on"? There are plenty of ways to turn them "off" (basically destroy the tag) but nothing that works in the opposite direction. And how farfetched is the "liquid" idea?

Might there be some resource that could help me understand what possible workarounds would be, or different attributes of different kinds of RFID tags? Or even something that tells me why this won't work?

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    \$\begingroup\$ So basically you want to detect a liquid, and know exactly which object is wet with an Arduino placed in a dry location 4-8 inches away? \$\endgroup\$ – joeforker Jul 30 '10 at 13:54
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    \$\begingroup\$ I've always liked the idea of passive, unpowered sensors. Tangents: micah.navi.cx/2008/09/using-an-avr-as-an-rfid-tag (could be used for more complex responses than just on and off) spybusters.com/Great_Seal_Bug.html (unpowered passive wireless microphone) \$\endgroup\$ – endolith Jul 30 '10 at 21:22
  • \$\begingroup\$ @joeforker basically yes. I'm not worried about "which object" is wet as compared to "oh, this thing is wet now. time to clean up the spill." I want to do it in a cheap/disposable way which isn't bulky (eg, detecting moisture or sweat in clothing), thus my interest in RFID tags as opposed to a perfboard-type thing. \$\endgroup\$ – rascher Jul 31 '10 at 3:11
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    \$\begingroup\$ @Rascher - Do you need to identify the object when it's wet or when it's dry? Your question asks about shorting the chip to its antenna so you only get a signal when it's wet. The converse would be to short the antenna (or other parts of the circuit) so that it won't resonate when wet, which would likely be preferable from a usability standpoint (unless it's supposed to be wet). My guess is that what you'd really like to do is short some "I'm wet!" bit in the output so that you can scan for wet/dry status. Is this correct? \$\endgroup\$ – Kevin Vermeer Aug 4 '10 at 20:32
  • \$\begingroup\$ @reemrevnivek Oh, that's a good idea! I just want to detect when it becomes wet. So "I have just stopped receiving a signal from the tag, it must have gotten wet!" is a completely fine way to solve the problem. So the next question: how? Are existing RFID tags sensitive ("fragile") enough that they'd stop resonating when doused? \$\endgroup\$ – rascher Aug 5 '10 at 23:41
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It might actually be just a little easier to make your own RFID in this case. You will need a very low power microcontroller (an attiny or small PIC is typical) with an antenna and a small number of discrete components. Here's a link to something similar: http://micah.navi.cx/2008/09/using-an-avr-as-an-rfid-tag/

With that, you can arrange for the RFID micro to power on when it gets power from the antenna, perform its sensory operation, and then respond based on the result of the computation.

Obviously, the available current from such a setup is pretty small, so unless the sensor works well on very small voltages and currents, you may have a hard time without an external power source.

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I've heard of RFID tags that have small amounts of read/write memory in them so it might be possible for you to get a tag containing a microcontroller. You could then 'read' the tag and when the tag is being powered by the reader you could have the microcontroller perform a check of your liquid sensor and return an appropriate value to the tag reader for wet or dry.

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There are certainaly RFID tags with sensors coming onto the market soon. I don't know if they exist yet. But batteryless wireless sensor networks are going to be a big thing soon.

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  • \$\begingroup\$ huh - that is interesting. I suppose what I'm trying to do, in a sense, is homebrew my own. \$\endgroup\$ – rascher Aug 5 '10 at 23:43
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One researcher from Canada is cheaply/easily creating his own RFID tags so he can do something similar; to add other components to them to augment their functionality. Really cool stuff!

Check this out.

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