On May 25, 3:33*pm, jamesgangnc wrote:
On May 25, 3:12*pm, Evan wrote:



On May 25, 1:45*pm, wrote:

Another thread discussing whether a switch in something was normally
open or normally closed reminded me of a similar question I have.

If I want to buy a reed switch/magnet pair to use as a door or window
contact in an alarm system and the system requires that the contact be
closed when the window or door is closed, do I buy a N.O. switch or
N.C. switch? *I assumed N.C. until I started to over-think it. *A
normally open switch is one that is open until it gets activated. Reed
switches are activated by the magnet getting close. *So, a normally
open switch would be closed when the magnet is near and that happens
when the window is closed - which is normal - so normally, a normally
open switch is closed - aaaaaargh! *I need a replacement switch a few
months ago and ended up paying extra for a double throw switch. *It
wasn't labeled so I still don't know if I wanted N.O. or N.C. *I just
used the contact that worked. *

So, what is "normal" for a reed switch? *When the magnet is near or
not?

Pat

@Pat:

N.O., or normally open, means the switch is open until a magnetic
force is applied to close the circuit...

(i.e. closing the window to align the magnet/switch pair)

N.C., or normally closed, means the switch is closed until a magnetic
force is applied to open the circuit...

As others have stated the requirements for one kind of switch or
the other will vary by: the alarm panel being used, the type of
switch being used by other devices wired in a series circuit and
the type of input being used on the given alarm panel...

~~ Evan- Hide quoted text -

- Show quoted text -

Typically you want to use a normally open switch so that when the
magnet is close to it it closes. *That way the circuit is complete
when the windo is closed. *Anyone attempting to defeat the protection
by cutting the circuit anywhere trips it. *Still can be defaeated by
shorting it somewhere else but that takes more than a pair of wire
cutters.

I have seen both types installed with the same magnet to operate
different circuits...

The N.O. switch for the alarm system...

The N.C. switch used on an unrelated and unconnected indicator
panel where when a door or window is opened the indicator light
goes on so some guard/attendant can investigate... All lights
out meant "all openings closed"...

Some of the more elaborate installations of the indicator system
used a graphic annunciator panel with a plan of the building on it...

~~ Evan