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If put a reed switch behind a metal plate in plastic casing and have the bar magnet sweep across the plate in parallel, would the reed switch activate? I would try it myself but the reed switches that I ordered from amazon are not here yet. But conceptually, when I think about it, and this is where I'm not sure about, the metal plate will redirect the magnetic fields away from the reed switch (something like tinfoil-hat-effect that some believe in). Or would the reed switch work regarless?

Zuzlx
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    What is the metal plate made of? How thick is it? – Kevin White Oct 12 '22 at 22:23
  • @KevinWhite 3mm/.12inch. Not sure of its composition. But magnet sticks to it. It's a computer stand. – Zuzlx Oct 12 '22 at 22:26
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    Ferromagnetism is nothing like tinfoil-hat-effect, which is presumably intended to be like a Faraday shield. – JYelton Oct 12 '22 at 22:29
  • put a piece of steel where the reed switch will be mounted ... if the magnet attracts the piece of steel then it will probably activate the reed switch – jsotola Oct 12 '22 at 22:32
  • @JYelton Sorry - wrong term, bad example. At the end, I need to know if placing a reed switch behind plate is sensible. Any thoughts on that? Thank you. – Zuzlx Oct 12 '22 at 22:32
  • @jsotola. thank you. Will try that – Zuzlx Oct 12 '22 at 22:33
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    @Zuzlx - a 3mm piece of steel will probably reduce the effect of the magnet to the point that it will not activate the reed switch. – Kevin White Oct 12 '22 at 23:01
  • @KevinWhite I tend to agree with you. But I'll try that first. Thankfully, 3mm is thin enough to put a drill to it and expose the reed switch from behind the plate. – Zuzlx Oct 12 '22 at 23:09
  • if it doesn't work: use a stronger magnet. – Jasen Слава Україні Oct 12 '22 at 23:27
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    This picture is a pretty good illustration of what your ferromagnetic plate will do to the magnetic field of a permanent magnet: http://www.schermatura.it/wp-content/uploads/2014/12/campo_magnetico2.jpg – Solomon Slow Oct 13 '22 at 01:57
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    Additional query - will the metal plate affect the "sweep" that you mention, or is the magnet going to stick to the plate? – MikeB Oct 13 '22 at 09:40
  • @MikeBrockington I have a rotating body on the other side of the plate with 1/4 gap. – Zuzlx Oct 13 '22 at 14:54

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A steel plate is going to block most of the magnetic field of the magnet.

There is no chance that a reed switch will work with the supplied magnet through 3mm of steel, but heroic efforts may be able to yield success.

I'm thinking neodymium magnets the size of a soda can, or maybe a 'D' cell. It's probably going to be easier to use a different material like aluminium, non-magnetic stainless steel, or titanium. Perhaps cut a hole and weld in a patch of stainless steel.

Jasen Слава Україні
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    I'm not sure "block" is the correct term. It's more diverted. Supeconductors can actually exclude a magnetic field (Meissner effect). – Spehro Pefhany Oct 13 '22 at 07:48
  • You were spot on. Magnetism has failed me. Nothing happens. In fact, I have a pretty strong magnet. When I place it even directly on the plate itself, the other side of the plate is barely magnetize and too weak for it to be useful. Back to mechanical rolling switch solution. – Zuzlx Oct 13 '22 at 15:00
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If you can afford to have a small gap in the steel plate (~3 .. 10 mm, depending on the size of the reed switch), located right in front of the reed switch, it will indeed work. Fitting the switch right inside the gap would work even better.

You'll still have your steel plate (two half-plates actually, separated by the gap) for whatever purpose you wanted to have it. For instance, your device will still stick to magnets just fine, though the force will be a bit weaker than what it would have been with a solid plate.

Magnetic flux is in many ways similar to electric current, to the point people talk about magnetic circuits. The reed switch needs some of the magnetic flux to activate, and the metal plate acts as a parallel shunt which channels most of the flux via a low-reluctance path. An air gap increases that reluctance, which redistributes some of the flux back into the reed switch.

Dmitry Grigoryev
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  • For cosmetic or environmental sealing purposes, it would also be perfectly feasible to fill the gap with rubber or plastic or something (or even a non-ferromagnetic metal, like aluminum or certain stainless steels, though that would be more difficult). The switch would still work through the gap. – Hearth Oct 13 '22 at 14:21
  • @Hearth in many applications just using aluminium for the whole plate would be easiest - without more details of course we can't know – Chris H Oct 13 '22 at 14:42
  • @Hearth As I understand it, the metal plate will be inside the device, which itself will likely be in a plastic case. – Dmitry Grigoryev Oct 14 '22 at 06:38
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Can confirm the hard way that it will not work when 3mm sheet metal separates the reed switch and large magnet. Like a coin operated washing machine

whoops
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