Well, you could likely kill two birds with one stone.  If I understand you
correctly, you need the device in the shoe to know when it is within reange of a
receiver, and also to be charged through some sort of non-contact (possibly
inductive) setup.

All you need to do is wind a coil around your device, tuned to a given
frequency, rectify it and charge your batteries any time you are within the
chargin coil.  You can use a few discrete components to tell you when you are
being charged, and therefore are within range of the receiver.  No need to go to
RFID or other systems.

If you can get your subjects to stand on foot-print marks on the floor, it
becomes even easier (or tell them to put their shoes there for the evening).

Not only that, but you could also use the same coil for bi-directional
communication.

The shoe stand is always sending out a weak magnetic field at a given
frequency.  When the shoe is placed on it, the shoe can detect the field.  (the
stand could also detect the shoe, but that won't be necessary here)

When the shoe detects the field, it sends a commend to the stand to enter
communication mode.  Since they are in very close proximity, very low power may
be used to send information back and forth on a master-slave protocol of some
sort.

When done, it tells the stand to charge it (probably for a pre-determined amount
of time), and goes to sleep until the charging is done (either because the shoes
were removed, or the charging cycle has ended.)  If it isn't charged enough, and
it's still on the stand, it can request more energy.

Chances are good that your subjects remove their shoes for several hours a day,
and can remember to put them on a stand overnight.  If not, the entire cycle
except charging could be carried out in a hallway witha larger coil as the walk
past it, etc.

Benefits of this would include low radiation magnetic fields, no wasted power on
the portable device reading for RFID or another tag, very low poer for
transmission, etc.

-Adam

"Dr. Chris Kirtley" wrote:
>
> Dear all,
>
> Sorry to keep asking questions, but I'm trying to kill several birds
> with one stone. I want to sense proximity to a receiver so that I can
> transmit telemetry data (hence the reason I asked about RFID tags), and
> also to charge the battery inductively. I wonder if anyone has any tips
> on the latter? The battery is very low capacity (20mA 3V) Vanadium
> Pentoxide Lithium.
>
> Thanks in anticipation,
>
> Chris
> --
> Dr. Chris Kirtley MD PhD
> Associate Professor
> HomeCare Technologies for the 21st Century (Whitaker Foundation)
> NIDRR Rehabilitation Engineering Research Center on TeleRehabilitation
> Dept. of Biomedical Engineering, Pangborn 105B
> Catholic University of America
> 620 Michigan Ave NE
> Washington, DC 20064
> Tel. 202-319-6247,  fax 202-319-4287
> Email: kirtley@cua.edu
> http://engineering.cua.edu/biomedical
>
> Clinical Gait Analysis: http://guardian.curtin.edu.au/cga
> Send subscribe/unsubscribe to listproc@info.curtin.edu.au
>
> --
> http://www.piclist.com hint: The list server can filter out subtopics
> (like ads or off topics) for you. See http://www.piclist.com/#topics

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu