Nobody wrote:

On Sun, 08 Feb 2009 20:01:18 -0800, David Nebenzahl wrote:

Just one thing: did someone here say that switches like this were
typically N/O rather than N/C? And that such could be used for an alarm
circuit?

Think about it: if you have a N/O switch as an alarm sensor, all the bad
guy has to do to get in is to cut it out of the circuit. There's a
reason alarm switches are normally closed.

Phil Allison said:

** It's a better than even money bet Terry wants the switch to be closed
with a magnet nearby.

Which IS the NORMAL situation in an alarm installation.

IOW, it's a question of how you define "normally".

Reed switches are open in the absence of a magnetic field, and closed when
one is present.

For a relay, you would call such contacts "normally open".

But for a burglar alarm, the "normal" situation is for the magnetic field
to be present, from a permanent magnet in a door or window which is
"normally" adjacent to the reed switch. In this situation, the contacts
will be held closed.


Thanks for the responses and my apologies for not responding sooner.

Yes, as many have pointed out, I was mistaken about the N/C
requirement. It was indeed the ambiguity over the meaning of 'normally
open' that caused my mistake. I'd forgotten that the accompanying
magnet normally(!) kept it closed. IOW I was regarding 'normality' as
the state when my doors are closed, not when the potential burglar was
gaining access ;-)

I did realise my mistake soon after posting and duly ordered a few
from Rapid Electronics
http://www.rapidonline.com/Electroni...354/kw/78-0797


--
Terry, East Grinstead, UK