Electronic – Continually reading an RFID Tag

rfid

I am attempting to build a system which stays active when an RFID tag is present and then deactivates when it's removed.

RFID readers seem to have an output that fires once when the tag is in range and then doesn't run again until the tag is moved away then then rescanned.

My initial idea was to use a standard reader and then reset it forcing a new tag to come through each time. The ID-20 reader has a reset pin which I believe supports this.

I would prefer to use a cheaper reader for this project as I have quite a few to build so I am currently working with this device.

The cheaper reader doesn't have a reset pin so I was going to toggle the power to the reader when I wanted a new read. Sadly this didn't work as planned, this cheaper reader only seems to read when a tag is moving through the field.

Has anyone done anything like this before and can offer advice or would anyone know why the cheaper reader wouldn't work when the tag is static?

Best Answer

This is a reader firmware problem. Most access control readers default to emulating old-fashioned magstripe readers, so they only report once per "insertion" and mask the rest of their reads, as long as the tag is still visible.

Internally, the tag is constantly repeating its ID -- otherwise, the system would be much less reliable, since there would only be one chance to catch the message.

The "right" solution is to find a reader that's configurable for repeating reports, such as the Olimex MOD-RFID125, but I don't know if any meet your price point.

It's strange that the cheaper reader doesn't report a tag on powerup. It's possible that it has a sloppy RF powerup ramp, so the tag gets into a weird state because it didn't cleanly initialize. If you have an oscilloscope and a loop of wire, you can try to put it near the reader and tag to try observing the signal. You're working with a 125 kHz carrier, which should be slow enough for most scopes to capture.

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