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The magnetic cat door contains a battery, a reed switch and an electro-magnetically operated catch. When the magnet, attached to the cat's collar, gets near to the reed switch, this switch closes allowing the battery to connect to the electromagnet which then unlatches the door for pussy to push open. See 4QD-TEC's article on reed switches for how they operate. There is already a small magnet associated with the reed, to bias it to just below its operating point, so the cat collar magnet has a front and a back (or, more exactly a left and a right - for the magnet is mounted sideways to the cat's collar and the door). If the magnet is fitted the wrong way round, the magnet has to be a closer to the reed to operate it.
If you have a magnetically operated cat flap and have ever tried to get a suitable collar, you will know how difficult it is to find a good quality cat collar which will accept one of the standard magnets you get with the cat door. And how expensive spare magnets are!
So I solved both of these problems by making my own cat collar magnet.
I used a 10mm diam x 5 mm thick super-magnet from Supermagnete The magnet on its own will operate the reed switch in the cat flap from a distance of about 30mm, but is an awkward shape to use (mainly because the strap I was using is significantly wider than the magnet). So I cut a pole-piece of about 12mm length from a mild steel rod, as shown in the diagram, right.
This pole-piece not only makes the magnet easier to mount in the strap but also makes the operating distance greater, by effectively widening the distance between the poles of the resulting magnet. Because these magnets are so powerful, the dimensions don't have to be at all accurate.
I then cut a piece of 12mm wide strap about 90mm long and formed it into a loop: I used a length from a lead which had been supplied with one of the cat harnesses we use when walking our cats. Melting the ends together with a soldering iron made it neater and easier to sit the magnet in before heat-shrinking the cover on, but is otherwise not needed. The leads are commonly made of nylon so are easy to cut and fuse with a soldering iron.
When heatshrinking, take care not to overheat the magnets. They can stand 80°C - the heatshrink needs about 50°C.
For the cover, I used a 20mm length of heat-shrink tubing.
The resulting magnet is being modelled below by our cat Misty. It's significantly better than the 'official' operating magnet and is also smaller and lighter, as well as being significantly cheaper and fitting the collar better. Misty seems well pleased with it! Click on the thumbnail for a full sized picture.
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© 2007-2009 Richard Torrens.
Page first published 27th February 2007.
Last modified: Mon, 05 Feb 2018 08:44:35 GMT
Document URI: www.torrens.org/Cats/magnet.html