On Thursday, September 6, 2018 at 8:12:44 PM UTC-4, 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.

--
tbl

I'd suggest looking at a real electronics supplier, eg Digikey or an alarm supplier where the description is likely to be more accurate and correct. Both types are available, there are lots of applications other than security.. Depending on what you want and if you can wait a couple weeks you can probably find them for a couple bucks on Ebay from China, but I'd only buy ones there that say they have both contacts. If they don't, typically they will give you your money back and you don't need to send them back.