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