Wheelchair proximity detection

["David Norwood" <judapeno@xxxxxxx>]

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