Re: Wheelchair proximity detection

[nobody@xxxxxxxxxxxx (Dave Houston)]

Powering any device from the chair battery _may_ void the chair warranty.
People I've worked with in the past (ALS victims and quads) have indicated
this is probable. The manufacturers have to be concerned about product
liability claims.

Most power chairs have two high torgue DC motors that will be
starting/stopping while you are positioning the chair for ingress/egress.
This will create a noisy electrical bus which any attached electronic
devices will need to deal with.

You will probably not find an active RF device that can transmit
continuously on small batteries for more than a matter of hours. At ISM
frequencies, the FCC prohibits continuous transmissions. Look at the mA draw
of the transmitter (4-5mA typical) vs. the mAh battery capacity.

I think this leaves only a switch operated transmitter (where small
batteries can give long life) or a passive device on the chair that
interfaces with some type of reader that is powered from the household AC
circuits. Unless the switch operated device also can warn of weak batteries,
you have a potential failure point.

I still like the concept of a barcode reader (or readers, if you need it
both inside and outside) with the barcode on the chair. Barcode readers
don't require precise alignment. A pressure mat or IR beam can activate the
reader(s). It also is relatively secure if only the barcode on the chair
will automatically open the door.

More esoteric and creepy, there are implantable, passive devices (Digital
Angel) that some have used for automatically opening doors. I haven't looked
at this lately but, as I recall, the range was a foot or two - it depends on
the reader.

"David Norwood" <judapeno@xxxxxxx> wrote:

>I need to trigger an automatic door opener when my wheelchair is within
>about 3 feet.  I would like some sort of radio transmitter on the wheelchair
>and a receiver near the door.  Here are my requirements:
>
>- it must work at about 3 feet.  I can't find an affordable RFID reader that
>works at this range.
>
>- 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?
>
>- it must provide a simple contact closure, or have a serial or usb
>interface and Linux drivers.  Zigbee?
>
>- I'm not too concerned about security.  There are easier ways to get into
>my house than spoofing a radio signal.
>
>- it must not require too much soldering.  I have friends that can help with
>this, but within reason.
>
>I've seen various radio transmitter modules on the web, but don't know how
>much power they require, and how to interface a receiver to my computer.
>I've figured out how to do this with Bluetooth, but I have the power issue.
>
>David
>