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wireless rfid reader — Parallax Forums

wireless rfid reader

rogermrogerm Posts: 7
edited 2009-01-11 23:56 in General Discussion
I am developing a wireless RFID reader using the serial card reader (#28140) and an arduino with Xbee shield attached. The reader is hooked up to the arduino's 5V pin.

The project runs fine off USB power and I now want to make it truly wireless. I am struggling to find a safe and sustainable alternative power source. I measure the current for the reader as 100mA (when in active mode) and the arduino as 84mA - total = 184mA.

I have looked at polymer lithium ion batteries, but I can find no off-the-shelf solutions for the required 7.4V. (I am a little hesitant to go down that route on my own given that I am not properly qualified.) Does anyone have any suggestions?

Also, any thoughts on how I might cut down the current consumption? Would it be possible to hook up the reader directly to the xbee?

many thanks

Comments

  • kwinnkwinn Posts: 8,697
    edited 2009-01-07 13:35
    USB usually provides 5V so you should be able to run it off 5V as well. As for reducing power there are several options.

    It may be possible to hook the reader up to the xbee, but I have no expertise on those modules.
    Connect the unit to an AC power supply.
    Use a low power micro that lets you put everything to sleep until a tag is detected (now you need to detect the tag somehow).
    Use a solar cell to keep the battery charged.
    Have a cover/generator over the unit that charges the battery when lifted for access.

    How you power it depends on where it is located and how it is to be used. For more specific advice more detailed info is needed.
  • rogermrogerm Posts: 7
    edited 2009-01-07 14:18
    The wireless reader will be a standalone object, as light and compact as possible. It will be embedded in a soft physical icon as part of a larger interactive exhibit.

    When in use, there can be no cables attached to the reader (physical icon). It will however be possible to charge / swop out batteries when required. It will be used in doors, so that makes solar cells a non option.
  • FranklinFranklin Posts: 4,747
    edited 2009-01-08 02:14
    4 X AA batteries?

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    - Stephen
  • WBA ConsultingWBA Consulting Posts: 2,935
    edited 2009-01-08 09:51
    I had a similar project requirement a while back. Here's my solution:

    I had a 3 handset cordless phone setup that had 6 volt battery packs. I used the battery packs one at a time and plugged them back into the handset in a charging base to recharge the batteries. Each pack lasted about 1.5 hours in the project and would charge mostly full in three hours that the 3 pack cycle created. It worked out great even with one day that required a 17 hour run. The packs were very small, looked like 4 coin cell batteries in a 2x2 configuration. When all was said and done, the phones were able to go back into normal use.

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    Andrew Williams
    WBA Consulting
    IT / Web / PCB / Audio
  • rogermrogerm Posts: 7
    edited 2009-01-08 11:19
    Nice idea! I would prefer not to be swopping out batteries at such short intervals, but it is definitely another avenue to explore. Thanks very much for your thoughts.
  • rogermrogerm Posts: 7
    edited 2009-01-08 11:34
    quote from kwinn's post:
    "USB usually provides 5V so you should be able to run it off 5V as well."


    This is not necessarily true. I have conducted two tests with batteries - a 9V standard alkaline battery and a 3.7V lithium-ion battery regulated to 5V. In both cases there was not sufficient current to power the RFID reader module into active mode. (There was enough current to switch on all the components but not enough to read tags.)

    I think this is a result of the maximum discharging rate of the batteries. Does anyone know what the maximum current through USB is? (assuming there is one)


    Thanks
  • uxoriousuxorious Posts: 126
    edited 2009-01-08 19:02
    Section 7.2 of the 650 page USB 2.0 Specification defines current in the terms of "units" with a unit being equal to 100mA. Low power calls out 1 unit and High power calls out "up to 5 units". So,

    Low power ports: 4.40-5.25V @ 100mA
    High power ports: 4.75-5.25V @ 100-500mA

    The interesting thing is that the spec also defines the over-current protection value as 5.0A. Insane, but it is possible that a PC manufacturer will allow a USB port to supply up to 5 amps before it has to alert the OS that the port is drawing too much current.

    Personally I have seen a USB port push 2.4 amps without causing an OS alert. (We do some intersting thing swith USB at work) Also, one of my portable HDD enclosures has a USB power thief Y style cable. One USB end has all four connections and one only has the 2 power connections. That way, the HDD can actually draw up to 1 amp and still be within standard USB specs.

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  • rogermrogerm Posts: 7
    edited 2009-01-08 19:18
    Thanks very much for the detailed response. VERY HELPFUL!! : )
  • Invent-O-DocInvent-O-Doc Posts: 768
    edited 2009-01-11 23:56
    @ Roger,

    I did not have the same experience as you with the 9V battery. I've powered an SX with an LCD display, an XBEE and the RFID reader module simultaneously and it worked fine. The battery does not last very long if I leave the backlight and reader in active mode continuously, though. I'm looking at 4xAA or a lithium ^6V eventually for more mAH. Time does extend considerable if I run the RFID reader on active with a non-continuous duty cycle. Good luck!

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    Thomas Talbot, MD
    Gunpowder, MD, USA
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