Andrew Gabriel wrote:
In article ,
PCPaul writes:
On Sat, 19 Jan 2008 23:04:26 +0000, Roger Mills wrote:

Alternatively yet again, doing a quick Google for 'cat flap'
suggests that there are locking cat flaps - which may fit the bill
- and those which only respond to *your* cat by reacting to a
magnetic gizmo on its collar. So, presumably, if you remove the
collar, the flap won't open when the cat approaches.

You have to watch the magnetic ones... I'm reposting this story here
as the content of the site may be a little strong for the
weaker-spirited amongst you dear readers ;-) (cue a rush to read the
rest of the site...)

The original is at http://b3ta.com/questions/animalcruelty/

Long before these existed commercially, I tried to make one with
a magnet and reed switch. Got to the point where it worked quite
well on the bench, so I fitted the magnet to the collar. Next
time the cat came in, there was a trail of rusty old ironmongary
trailing from her collar. I gave up on the idea.

I have a mental picture of a lawnmower, and several plate-sized cogs....
)