134khz rfid reader.

Discussion in 'Wireless & RF Design' started by geoffers, Jun 12, 2011.

  1. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Hi, wonder if anyone can help/point me in the right direction.
    I've built a rfid reader using the atmel u2270b front end chip, after a lot of poking about I've got it to reliably read fdx-b protocal animal tags. (I can now identify my sheep!) I bought the iso 11784/5 standards and unfortunatly the rfid tags I have for my cattle are of the hdx type. In the standards it states that the activation field should be turned off to enable the hdx tags to respond, they charge a capacitor then transmit. I've programmed the pic to turn the field on and off via the u2270b and using my cheap scope can see the response of the hdx tag. The signal is taken via a diode to a capacitor divider then to the input of the atmel. I tried putting a op amp in to amplify the hdx signal and was hoping to use a pic with on board comparators to read the signal (which uses fsk). Unfortunatly for me this is where my knowledge lets me down, I've no formal electronics education and have taught myself as I work on stuff like this, so there are quite big gaps in what I know. When the op amp is conected the reader will no longer read fdx-b tags, I guess the modulation of the field is a small signal and gets drained off? I was hoping I could use a transistor to switch the signal to the op amp after the field is deactivated? The pic could switch the transistor at the same time as the field, my question realy is what type of transistor do I need? The field is turned on by a high output from the pic. Sorry for the rambling post.
    Thanks Geoff
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,638
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    Hello,

    Can you post a schematic of what you have upto now?

    Bertus
     
  3. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Hopefully there should be a very basic pdf attached, if I connect the scope across the coil I can see the field start and stop, if I hold a hdx tag next to the coil I can see its response, unfortunatly my scope won't let me zoom in on the tags signal, using the trigger function it starts when the field does. I'm not sure I have the op-amp correctly connected, I found a few pages on the net about rfid and the op-amp is set as a band pass filter? I dont know what that is but was hoping I could isolate the tags signal. However when I connect its input to the u2270b's input the reader will no longer read the fdx-b tag. On a side note, what software do people recomend for drawing schematics?
    Thanks Geoff
     
  4. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Been pluging away at this, I've now put a 2907 transistor in my circuit, its collector is connected to the input of u2270b, and base via a resitor to the pin of the pic thats turning the field on and off, I've read up a bit on op amps and set it as a negative feedback amp, however the problem I have now is the output from my pic isnt a square wave as I would expect. The pic is a 16f818 and i'm using portb,3 as my output. This affects the signal the op amp is reciving. On the plus side the fdx-b part is still working. Any suggestions would be welcome.
    Thanks Geoff
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,638
    2,344
    Hello,

    Do you have a datasheet or a link to the datasheet of this u2270b ?
    I want to have a look at the specs of this.

    Bertus
     
  6. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Hopefully the data sheet should be attached.
    Thanks for looking, Geoff
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,638
    2,344
    Hello,

    Is the system not sensitive enough?
    The standard fequency is 125 kHz.
    This could be the case if the frequency of the 2270 is not correctly adjusted.

    [​IMG]

    With the resistor on the RF pin the frequency can be fine-tuned.

    Bertus
     
  8. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Have just been looking at my circuit, wonder if I'm going about it the wrong way? The signal from the HDX transponder is very small, I can see it across the coil but by the time its been trough the capacitor divider its all but gone? Someone suggested using a PIN diode to connect a op amp to the coil after the field has been turned off, knowing very little about PIN diodes I've looked up a bit of info, unless I've missunderstood it I would have to put 80-90v on the diode to turn it off? The voltage on the coil is around 80v when its active, the signal about 5mv! Alternativly is there a op amp that wont be cooked by 80v, the pic knows when to try and read the signal.
    Thanks Geoff
     
  9. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Hi, the chip works fine to read the fdx transponder which as I understand modulates the activation field. The hdx transponder transmits after the field has decayed, the 2270 wont output this signal. However I can see the signal across the coil, its this signal I want to amplify. (I think!)
    Thanks
     
  10. Jgb5067

    New Member

    Aug 1, 2011
    16
    2
    in response to your question about schematic editors, I like using Eagle. it's free and has a fairly large library available out on the interwebs.
     
  11. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Thanks, had a quick look but didn't look very free! As regards reading the HDX transponder, I think I was making my life to difficult?! I now have a working circiut to read both types. I used the tms3705 from texas to read the hdx and the u2270b to read the fdx.
     
  12. Jgb5067

    New Member

    Aug 1, 2011
    16
    2
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