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Topic: anyone know details on 125 kHz RFID reader? (Read 2858 times) previous topic - next topic

gardner

I am hoping to contact someone about 125 kHz RFID readers.  I am presently messing with the SeeedStudio RDM630, which works okay for what it is, but I would like options for a couple of things that this one doesn't do.

I need a larger antenna (170mm square) and I can find no guidelines about designing and optimizing a new antenna for the RDM630.  This reader appears to be a bit-banging type implementation -- I do not see any commercial RFID parts on it, just an AVR, a counter/divider and an amplifier.  Consequently it's not possible to look at the chip manufacturers specs.

Have folks designed new antennas for this or similar RFID readers?  What's one gotta do to ballpark a design and tune it to optimum?  Is there any way to judge if the antenna characteristics are ideal or sub-optimal?

Are there other 125 kHz RFID readers similar to the SeeedStudio one that I should look into?  Maybe ones based on RFID chipsets and/or with documented designs?

johnwasser

Perhaps the open-source SnifferNano 2.0 would be easier to customize:

Firmware:
http://code.google.com/p/rfidsniffernano/

Schematic:
http://iteadstudio.com/store/images/produce/RFID/SnifferNano/SnifferNanov2.0sch.pdf
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gardner

Interesting.  Thanks.  I hadn't seen that one.  The antenna shown looks identical to the one with the RDM630.  I also wound up here -- http://iteadstudio.com/produce/rdm630-new-version-and-sniffer-nano-v2-0/ -- where there is a bit of info about the unit I have that I hadn't found before. Next, on the RDM web pages I found this list of antennas for the RDM series of readers -- http://www.datarfid.com/doce/Product/pic_263.html
Here I see the design spec for the 42mm x 30mm antenna is 475 uH, which at least give me a starting point.

After a bit of research, I think an ID Innovations ID-2 module (http://www.sparkfun.com/datasheets/Sensors/ID-12-Datasheet.pdf) is probably a good bet.  It's still on the edge at the antenna size I want, but would probably be okay.  At $25 it is a reasonable price.  The datasheet has good pointers about designing and optimizing the antenna.

I think what I will do is wind-up an ~475 uH coil and then follow the ID Innovations recommendations for tuning it in.

markB

gardner

Have a look at Roberts site. http://www.priority1design.com.au/shopfront/
I've purchased one (and some of the chips) for an 134.2Khz project, but the chips do the same.
He has larger antenna available, freight/postage is very quick (well across the water for us), and it's based on the chips he sells as well.

He has been involved in a number of projects involving much larger antenna, and phasing them, so drop him a line outlining what you wish to do.

I agree with your summation of the Seeedstudio version...
Interestingly the 134.2 Khz tuned one from priority one was more sensitive to the 125Khz tags, than the 125Khz Seeedstudio version....by nearly three times the distance.

Mark

gardner

Hi Mark.  I'd seen that web site, but there was not too much on design parameters that I could glean.

I went ahead and wound a new 120mm coil for grins.  Using the info from the RDM web-site I started by targeting 475 uH.  A couple of different coil calculators pointed me in the 45-turns range, so I wound up that much.  Then I hooked it to the RDM630 and my scope.

I could see that the voltage was lower than before by a good 10V.  I used the procedure from ID Innovations to estimate whether the inductance was too high using an old aluminium disk drive chassis.  Bringing this in effectively drops the inductance and in my case boosted the voltage, so I backed out a couple of turns and my coil was carrying the same voltage as the old one.

After I soldered on an 80cm twisted pair lead though, I think I have messed up the tuning again as the voltage is back down.  I suspect the capacitance of the twisted pair has affected the resonance.  Tonight's project is to futz with the tuning some more.  Probably shorten the leads and knock another turn off the coil.

If this works out, I will have a try at the 170mm square that I really want.

Meanwhile I have ordered an ID Innovations ID-2 module for good measure.

markB

I stumbled across some additional data, while searching for priority1 ...can't remember where.
It was suggested a larger coil could do what I wanted, and Robert suggested
Quote
In you're case you'll be looking at a 30-35 cm coil. 
I'm thinking about 50 turns, with 0.25mm wire. Just add or remove turns until the coil resonates at 134khz. You'll want the coil voltage to be sitting at about 300v peak to peak.


His other coil details are here http://www.priority1design.com.au/rfid_coil_antenna.html

You seem to be on the right track, and I suspect you will have more luck with the ID-2, as I have read they are very good.
Some other details I found here http://www.wethepeoplewillnotbechipped.com/main/viewpage.php?page_id=79 which use a ferrite rod.

Let me know how you get on m(dot)beckett(at)amuri(dot)net
Thanks
Mark

Grumpy_Mike

Quote
I do not see any commercial RFID parts on it, just an AVR, a counter/divider and an amplifier.  Consequently it's not possible to look at the chip manufacturers specs.

A lot of commercial RFID readers don't either. I used to design them for a living.

It is just a matter of winding a coil with the right resonant frequency. This does not have to be 125KHz but something either side of it. The response is symmetrical so you can go up or down. I would go for something like 130KHz to maximize the range. Tune it by either taking turns off the coil or adding / subtracting capacitance. If you do the latter you have to know if the reader has a parallel LC tuned circuit or a series one, so you know how to add more capacatance.

gardner

#7
Jun 09, 2011, 05:49 pm Last Edit: Jun 09, 2011, 05:55 pm by gardner Reason: 1
Thanks Mike.


If you do the latter you have to know if the reader has a parallel LC tuned circuit or a series one, so you know how to add more capacitance.


This is kinda where things fall down with the RDM reader -- they just don't give you much to go on, to get your coil design right.  I'm looking forward to getting my hands on the ID-2 since they do spell out this aspect of the design.  Unfortunately they appear to be back ordered.

...and thanks Mark.


Quote
In you're case you'll be looking at a 30-35 cm coil.
...
You'll want the coil voltage to be sitting at about 300v peak to peak.



Can you mention which reader circuit Robert is referring to here?  EM4095 or U2270B something else?  I'm not above wiring up my own reader using one of these chips, but I am not keen on ordering them all the way from Oz.

markB

I brought a RFIDRW-E-232 and it uses the EM4095 in more of less the same configuration as the datasheet.

They could be available closer to your home.

Mark

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