Security Alarm Reed Switches

[croy]

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:

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.

[Dean Hoffman]

Normal, at least in my world, is when there is no outside force acting on the device.

[gfretwell]

When I was in the biz a N/C had a little magnet on it and a bigger magnet polarized the other way bucked it and opened the switch Adjustment was critical tho. I am guessing they may have better technology now.

[trader_4]

gnetic reed switches

ology as it relates to

ey didn't work for me.

he normally closed

lly open circuit seems

switches back as

N/C switches, and I

are advertised as

t that "normally"

wo leads--how the hell

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. B oth 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 pro bably find them for a couple bucks on Ebay from China, but I'd only buy one s there that say they have both contacts. If they don't, typically they wil l give you your money back and you don't need to send them back.

[Wade Garrett]

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."