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Alarm System Contacts
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 |
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