On 9/6/18 8:12 PM, croy wrote:
I have a simple little project to do at home, and the security type of magnetic reed switches
would be perfect in two locations, if I could fully understand the terminology as it relates to
normally open and normally closed.

I purchased some 3-terminal switches online, but when I received them, they didn't work for me.
I need the normally-closed function, and when I put my ohm meter across the normally closed
terminals... nothing. When I put the magnet close... nothing. The normally open circuit seems
to work fine--N/O until the magnet comes close, then closed. I sent the switches back as
defective.

It seems that the security world these days doesn't want much to do with N/C switches, and I
understand why. And if I peruse switches on line, they are mostly N/O.

Unless I look at overhead door switches, and then I see that most of them are advertised as
N/C. Are they N/C when the magnet is near? Or when the magnet is absent?

Example:

https://www.amazon.com/Overhead-Magn...tomerRe views

I'm wondering if different folks are describing it differently. I thought that "normally"
meant with no magnet near. Is that correct?

I even saw an overhead door switch advertised as NC/NO, but it only had two leads--how the hell
could that be?

I don't want to buy more stuff and find out it won't work.


A while back, I had to replace the magnetic switch in a low voltage
circuit I installed to turn an indicator light inside the house on when
the garage door was open.

I found conflicting definitions of normally open vs. normally closed.
The issue was did "normal" mean when the switches were in proximity- or
when they were apart.

After receiving the wrong one from an Ebay vendor, I just ordered one of
each from a different vendor- and installed the one that closed the
circuit turning the light on when the door was raised and moved the
switches apart.

To me, that arrangement is "normally open."

--
The fastest way to stop a bad guy with a gun is a good guy with a gun.