On 5/25/2011 12:45 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


There was a thread on this about a year ago.

RBM's reply was:
As an electrician, I've been through this numerous times over the years.
It clearly depend on who you are talking to, and what trade they're in.
IMO, the "normal" position is the state the switch is in when nothing
affects it. Alarm people have the opposite take on it.


My reply was:
I agree.

In all the following the contacts are closed with the magnet next to the
reed switch.

From an alarm perspective *NC* ("normal" is when the door is closed):
http://en.wikipedia.org/wiki/Burglar..._control_panel
"Most switching devices are N.C. (normally closed) circuits, so when the
device is not in an alarm condition, the circuit is closed."

http://www.alibaba.com/product-gs/21...or_Alarm_Switc...
"Output signal: normally closed (switch contact is closed when the door
closed)"

From a component manufacturer *NO*
C&K: "ELECTRICAL CIRCUIT: SPST NO (Contact Form A). Reed switch
opens when magnet is removed from proximity. Contacts are
held closed when magnet is within actuation range."

To know what the switch does you need to have a description like all of
the above.
end reply.

--
bud--