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Message-ID: <4F75191E.9060408@sbcglobal.net>
Date: Thu, 29 Mar 2012 19:23:26 -0700
From: "Girvin R. Herr" <girvin DOT herr AT sbcglobal DOT net>
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To: geda-user AT delorie DOT com
Subject: Re: [geda-user] Reed Switch vs Hall Sensor
References: <CALSZ9grznvM0i=H_5hL3PXZy-UfCtC_aw_RGT112tpmfu2UgAQ AT mail DOT gmail DOT com>
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Reply-To: geda-user AT delorie DOT com

Rob,
Just a few considerations.  I don't know what reed switch you are using, 
but it has been my experience that reed switches are normally open types 
by nature, requiring a magnetic field to close.  Manufacturers add a 
magnet inside the reed switch package to turn it into a normally closed 
switch.  With this in mind, when you add an external magnet, be careful 
that your magnet is correctly polarized to cancel the magnetic force of 
the internal magnet in order to open the reed switch.  Also, make sure 
your external magnet does not overpower the internal magnet, or the reed 
switch may close again as your external magnetic field first cancels, 
then overpowers the internal magnet.  The data from your switch's 
manufacturer should have specs on the magnetic fields required.  The 
switch manufacturer may even have external magnets available or have 
suggested magnet suppliers which would meet these requirements.

As for reed switch reliability, I once used a piece of test equipment 
that ran reed switches at 60 Hertz to switch 75 Ohm RF signals between a 
device under test and a reference signal.  I never had any problem with 
the unit in the several years I worked with it.  Reed switches are 
sealed inside a glass tube, so unless they are run outside their specs 
and the contacts burn, they should last a long time.
Girvin Herr


Rob Butts wrote:
> I'm doing a simple proximity alarm for when a pouch is opened.  In my 
> original design I power a small 8 pin microchip 8 bit microcontroller 
> with a 3 volt coin battery along with a spdt reed switch and a small 
> smt buzzer.  When the pouch is closed a magnet is against the reed 
> switch.  I pull the normally closed side of the switch (open with 
> magnet) high and monitor the switch with the micro.  When the pouch is 
> opened the switch closes and triggers a change of state on the micro 
> input.  If the pouch is opened the micro beeps the buzzer 4 times, if 
> closed - twice.
>  
> I have been told hall switches are more reliable but they are all 
> active low.  Since this is powered by a small 3 volt coin battery I am 
> trying to reduce the power consumption.  My delemma is that hall 
> sensors draw on average 7mA in the active state which it will always 
> be in.  Does anyone know if reed switches are really unreliable?  Any 
> ideas on how to do this in low power?
>  
> Thanks