[Boatanchors] magnetic reed switches

[Robert Nickels]

CLIVE COLLINS wrote:
> Good evening Genntlemen,
>   I am searching for a dozen or so reed switches that are to be included in a project.
>   They have to be as small as possible and normally closed.  
>   
Clive,

The basic reed switch has either Form A (SPST-NO) contacts or Form C 
(SPDT) contacts.   There are two ways to get Normally Closed (Form B) 
contacts:

1.  Use a Form C switch and just don't use the N.O. contact.  Only 
problem is the Form C reed switches are a lot more expensive.

2.  Use a small bias magnet to close a Form A switch, then apply a much 
stronger magnet with opposite N-S orientation to cancel the bias magnet 
field. causing the switch to open.  This is the method used for 
reed-type security contacts that  require Form B operation (most alarm 
loops require Form A, as the magnet attached to the door or window keeps 
the switch closed when in the secured position).

Hope this helps.  A small piece of cut from a fridge-type rubber magnet  
could be  used as a bias magnet, held in proper position  with  shrink 
tubing.

Or you can read more here:  
http://www.reed-sensor.com/Notes/General_Reed_Switch_Theory.htm
 
73, Bob W9RAN