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.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]