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?
Best Answer
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.