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its been a few years since i've done any electronics but have been asked to research the viability of an idea and have gotten stuck...

We want to transmit a timing signal from RFID readers which will be picked up by tags and used to control a simple switching of an led. The led will be on a powered circuit, and will fade over a few seconds, but essentially will be turned on when the reader emits the pulse.

I suppose this could be done with a standard RF circuit, but on some of the readers the communication will have to be 2-way (recognising a tag is within range and acting accordingly)

The reading/writing of data to the tags is not necessary at all.

Is it possible to use an RFID tag which can be used as a switch for the LED control circuit?

Would there be a better tech to use for this?

Thanks Sean

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It's possible, but most (if not all) commercially available tags just transmit their code upon reception of the carrier. The antenna is their only I/O.
What you need is a low-power microcontroller which is fed from the energy it receives from the reader's carrier. As a microcontroller it can do anything you like, including sending a code to the reader, and switching any external devices like LEDs. Of course these devices will need their own power source, since the power extracted from the carrier will be hardly enough to power the microcontroller. A good low-power microcontroller is TI's MSP430, "ultra low-power" in TI's marketing speak.
You'll need to have good antenna coupling to get a high enough voltage not only to operate the microcontroller, but especially to control the external logic. You may have to use level shifters which accept very low input voltages. For instance the Fairchild FXLP34 can work with input voltages down to 1V.

This gal made an RDIF tag with an AVR microcontroller and some clever tricks.

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