Re: Wheelchair proximity detection

["Robert Green" <ROBERT_GREEN1963@xxxxxxxxx>]

"David Norwood" <judapeno@xxxxxxx> wrote in message

<stuff snipped>

> As long as I can calibrate the sensitivity relatively accurately, I don't
> think accidental door openings will be a problem.  I can stay away from
the
> door.

I'm not sure how well these things can be calibrated, give the amount of
trouble my local Target has with supposedly deactived tags going off at the
exit sensor.

I've been looking at some unit at Ebay, particularly this kit:

* 13.56MHz
* Dimensions: (58mm X 34.5mm or 2.3?x 1.4?)
* Embedded antenna
* Read Range up to 75mm (embedded antenna), or 250mm (external antenna)
* Build in Multi Interface with Processors
   SPI (up to 1Mbps) for high-speed application
   Or RS232 with 9600-57600bps  for long distance application
* 5V DC or Battery power for mobile operations. Power consumption as low as
7mA.
* Multi-protocol firmware enables read and write of ISO standard and Philips
Mifare Tag/Card.
* LED and BUZZER output for industry application.

The kit is listed at a starting bid of $50.  You'd have to add an external
antenna and even with it, the range is only a foot.  However,  it may be
possible to mount the antenna near the door and low to the ground on one
side.  Two different tags, each mounted on opposite sides of the chair,
could be used to determine if you are leaving (right tag detected - close
the door behind you, left tag detected,
your are outside approaching the door).

I'd look at my entryway to see if I could figure out whether I could conceal
the antenna close enough to the passing wheelchair that it came within a
foot.  At my house, there's no way to approach the door without going
through a narrow porch opening made of brick.  If I mounted an RFID antenna
there, it would almost impossible for chair-mounted tag to NOT get within
range.

> >> - it must work at about 3 feet.  I can't find an affordable RFID reader
> > that
> >> works at this range.

Where do you want to mount the reader or its antenna?  I am assuming inside
the house, on the other side of the door and  that's why you need a
three-foot range.  If you can rethink that requirement and find some way to
mount the unit to be close to the tag, you could use the much cheaper
passive RFID tags and eliminate the siphoning circuit.

I'd probably design a sensor swing arm that would almost act as a inverse
"curb feeler" and would deflect when the wheelchair edge hit it. The swing
arm would then follow along the edge of the chair and would swing back like
a saloon door to its original position once the chair has passed by.  Mount
the RFID tag at the height of the sensor arm and it should get close enough
to activate it.

> > Have you checked Ebay for used commercial equipment?  Marc H. found a
> > place
> > on the web that was selling video MUX's for about 1/10 the cost of a new
> > one.  (Thanks, Marc!)  Good deals abound if you know where to look and
how
> > to get support from other than the vendor, who's probably just a
> > liquidator.
>
> See my other post about commercial RFID readers.

If used commercial equipment is anything like the incredibly marked down
MUXes Marc found, they only appear every few months or so.  It might take
some serious searching via their email notification service to find the
right device.  Anyone looking for a 100 Wasp employee tags will find them on
Ebay at about 1/10 the cheapest price I have seen anywhere else.  You're
right, though, the descriptions are deliberately brief.  Most likely to
avoid any issues about the item not being as represented.  You have to do
the research, model number by model number.

> >> - the transmitter must operate on a battery for at least two months
> >> continuously.  I like Bluetooth, but can't find a low power
transmitter.
> >> What do they use on those automatic pet doors?
> >
> > Some that I have seen use a magnet inbedded in the pet collar.  A small
> > neodymium magnet can put out a field detectable from several feet away.
> > Hall-effect sensors allows contactless sensing of an external magnet or
a
> > ferrous object.
> >
> > http://www.allegromicro.com/hall/
> >
> > Whether you could or would even want to use that for *your* door is
> > another
> > question.  It is, however, how many pet doors work since they want the
> > "transmitter" part to be as cheap and passive as it can be.
> >
> > Other forms of pet doors use IR in addition to magnets:
> >
> > http://www.petdoors.com/cat_mate_electronics.htm,
>
> Thanks for the information about the pet doors.  I didn't know they were
> passive.  Maybe I can hack one.

That's pretty low security!   Perhaps lower than you want to go.  Not just
any moving thing can get into your house, true.  But anything moving thing
with a magnet could.

<stuff snipped>

> >> I've figured out how to do this with Bluetooth, but I have the power
> > issue.
> >
> > Is this a power chair or a manual wheel chair?
>
> It's a power wheelchair, so I do have access to 24 volt power (two 12 volt
> gel batteries in series).  It sounds like there are no radio transmitters
> that can operate continuously off a small battery, so my Plan B is to
attach
> a Bluetooth device to my chair power.
>
> Is there a < $50 module that converts 24 vdc to 5 vdc?

Yes.  That's the easiest part of the project.  And with the enormous
capacity of wheelchair batteries compared to, say a 9 volt transistor
battery, siphoning off power from the chair batteries would be the way I'd
go.

There's an outfit:

http://www.rfidexchange.com/purchasing.aspx

That has a lot of pricing info.  As you already know, active RFID stuff
clocks in at the "thousands of dollars" price point although they have some
readers listed for under $500 that will do the range you want.  But not much
under that and it's not clear what else you might need to make it work.

It's very possible that if you contacted the PR department of one of the big
active RFID players and explained what you are doing that they would
contribute to the cost of the project.  They might be more than willing to
help if they could get it written up in the trades.  They might also see it
as a potential new market for active RFID.

--
Bobby G.