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From: John Fields
Organization: Austin Instruments,Inc.
X-Mailer: Mozilla 4.01 [en] (Win95; U)
Subject: Re: Reed Switch "dead spot".
NNTP-Posting-Date: Mon, 02 Sep 2002 11:34:47 CDT
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Date: Mon, 02 Sep 2002 16:34:47 GMT
Travers Waker wrote:
> A while ago I posted my initail ideas for my first electronics project -
> building a sensory chess board that sends the moves made by the pieces to
> the PC via RS232. I planned on using reed switches placed under each square
> and a magnet in the base of each piece to detect the pieces position.
> Over the weekend, I went to a local electronics components store and bought
> 2 reed switches (both the thin glass tube type, one about 12mm long and one
> about 20mm long) to experiment with. I connected up an LED to switches so
> that I could see when the reed switches are turned on. The magnet I used
> was a disc-shaped (coin-shaped) refridgerator magnet that I was using to
> attach paper notes to my refridgerator door. The diameter of the magnet was
> about 12mm, and the thickness about 3mm.
> OK, here's the PROBLEM:
> As I move the magnet along the length of the reed switch's glass tube, The
> LED turns on as expected, but as I continue to move along the tube, it turns
> off as the magnet is directly above the "joint" in the reed switch (the spot
> inside the glass tube where the two halves of the reed overlap and
> presumably make contact). This was surprising to me, because I thought this
> would be the spot where the switch would be most likely to be closed! As I
> continue down the tube, away from the joint in the reed switch, the switch
> turns back on again, until the magnet is out of range.
> I experimented a bit more, and it seems that the "dead spot" above the joint
> in the reed switch extends in a line perpendicular (at 90 degrees) to the
> length of the glass tube. So I guess a "dead line" is a better description.
> Obviously, this is a problem for my chess board design, because a piece
> placed in the middle of a square won't be detected!
> I've thought about some solutions, like using 2 reed switches per square,
> set far enough apart so that the one will "cover for" the other one's dead
> spot, but I'm interested to hear other people's ideas.
> Also, I'd be very grateful if someone could explain to me why this is
> happening. Is there anything else I should know about reed switches? (e.g.
> can they be used with either side "facing up", towards the magnet, or are
> some of them "directional"?)
> Thanks very much for any helful advice/ideas/explanations.
While Chip's explanation of the cause of the problem is correct, using a
bar magnet won't solve the problem because you'll still have a dead line
(actually a dead plane)along the magnetic axis of the magnet when its
magnetix axis is perpendicular to the magnetic axis of the switch.
What works very well (I just tried it) is to take a couple of switches
and mount them at right angles to each other in X and Y with the switch
gaps axial in Z. A bar magnet with its magnetix axis perpendicular to Z
and centered on Z can then be rotated 360° about Z and one switch will
be made while the other one is open. The only caveat seems to be that
either end of the magnet must not be allowed to be skewed past the line
describing the axis of the crossed switches (+, below).
S W I + T C H
Wire the switches in parallel and there ya go!
Professional circuit designer
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