Define "Normally Open" vs. "Normally Closed"

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I'm replacing a broken surface mount magnetic switch mounted on my garage door that controls a signal light in the house that comes on when the door is open.
Thus, I need a switch that is OPEN (meaning no current flows) when the magnet is near the switch and CLOSED (meaning current flows) when the magnet is removed from the switch.
So do I need a normally open switch or a normally closed one? Different manufacturers/vendors seem to use the terms differently. Is it "normal" when the magnet is near the switch-- or away from it?
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"normally" is whatever state it is in when no outside force is acting upon it.
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Wiki didn't have the answer to this, so I provided it. http://wiki.answers.com/Q/What_is_the_difference_between_a_normally_open_and_a_normally_closed_switch&updated=1&waNoAnsSet=1&status=VGhhbmtzIGZvciBzaGFyaW5nIHlvdXIgYW5zd2VyLiA8YSBjbGFzcz0idW5kZXJsaW5lIiBocmVmPSJqYXZhc2NyaXB0Om5ld19sb2dpbigncmVnaXN0ZXInKTsiIG9uY2xpY2s9Il9oYkxpbmsoJ0Fuc3dlclRoYW5rVScsICdOb3RMZ2RfQW5zUVBnJykiPlJlZ2lzdGVyPC9hPiB0byBlYXJuIHRydXN0IHBvaW50cyBmb3IgeW91ciBhbnN3ZXJzIQ =
In switch language, open means to disconnect, or break the circuit. Some people call this "turned off". Closed, is to connect, or make the circuit. Some people call this turned on.
In the case of a normally open switch, the switch is open (off) unless something takes action on the switch. Pushes a button for example, or holds a magnet near the switch, or other active force. A normally closed switch is closed (on) unless some force causes the switch to be opened.
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Normal is when the magnet is away. Which is the unactivated condition. When the magnet is near the reed switch, that's not considered normal -- it's considered activated.
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Actually, he is 100% correct and your previous explanation and advice was wrong.
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Forget current flows or doesn't flow--that is confusing. Instead, do you have to close the switch in order to turn on the light? If so, then you need a normally open switch. Conversely, if the switch is open when the light is on, then you need a normally closed switch. From what you describe, I would think that you need a normally open switch---light off when door is closed. MLD
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Actually, not correct, either in analogy or in answer.
The answer is normally closed -- as others have said, the state with no outside force acting on it. Same thing with relays -- IF the relay is "single throw". If the relay is "double throw", then again, NO or NC doesn't apply, as both states can be wired in as normal.
A toggle light switch also has two "normal" states, even tho it single throw -- because the "outside force" is, well, you, so there is no natural "return position".. Ditto with any rotary-type multi-position switch, for fan speeds, etc.: every position is normal.
To the OP, curious as to where you would get this kind of switch -- oem, from the garage door mfr?? How much $$?? Wouldn't they be able to just supply the right part, or are you kluging your own solution?
Remarkably, Stormin brought up a good point semantically: "closed" in switching has the opposite meaning to a valve, ito flow. Go figger -- both the semantic snafu, and that Stormin actually grokked it.
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Existential Angst wrote:

I bought the switch for about two bucks 15-20 years ago at Radio Shack-- along with the wire, transformer and LED indicator light i needed for my custome designed system. A contractor bashed the existing switch so I've been to a few Radio Shack stores around town but the clerks didn't know wht I was talking about. I showed the broken one to one clerk who thought it was a doorbell. RS seems to mainly sell cell phones and electronic toys these days-- no more electronic components.
I see lots of them on line and on Ebay for around $10 or so-- but it's really unclear if they will keep the circuit open when the magnet is near or away. I guess I'll just buy one of each kind-- an NO and an NC-- and toss the one I don't need.
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Radio shack ain't what it used to be.
Oh, you know, these magnetic switches were common in the old alarms systems, that you would put on doors, windows, etc.
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If you find one that says SPDT or DPDT, then it can be used as either NC or NO. Often, the screw terminals or lugs themselves are labeled NO and NC, but not the switch as a whole.
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re: "Radio Shack seems to mainly sell cell phones and electronic toys these days-- no more electronic components."
Well, since you brought it up...
I was working at an event this weekend and somebody broke the 1/4 mono jack for one of the loudspeakers.
I went over to the nearest Radio Shack to get a new jack. Since the young lady was nice enough to approach me and ask if she could help, I started to explain to her what I needed - quickly noticing the glaze that was coming over her eyes.
Her response was "You'll have to speak to one of those guys. I know a lot about cell phones, but not much about that kind of stuff."
So what's the deal? Does Radio Shack *expect* that everyone that walks into the store wants a cell phone? If not, why would someone who knows nothing about their other products approach me?
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On Thu, 29 Apr 2010 13:49:12 -0700 (PDT), DerbyDad03

Radio Shack should expect everyone to buy a cellphone.
They left me with a bad taste in my mouth when I bought my first compute from them.
I used to buy everything at Radio Shack. I can't remember the last time I have been in one of their stores. Several years.
I know your situation was that you needed something that day. I let Newegg bring the stuff to my door.
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Metspitzer wrote:

I went to a Shack store in a small town outside of Birmingham the other day to get a toggle switch to repair a neon sign for a store belonging to a customer of mine. The Shack (new name) has nowhere near the number of electronic parts they used to carry but I can usually find what I need because the morons there have no clue what parts they carry. I worked for their repair division more than 30 years ago, I wound up with all kinds of discrete components.
TDD
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re: "I let Newegg bring the stuff to my door."
As you surmised, I needed the jack rather quickly.
"Normally" (thread-based pun intended) I go to the one remaining *real* electronics parts store in my town where if they don't have it, they can get it the next day - without trying to sell me a cell phone.
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normally closed magnetic alarm switch. Install it so the magnet (on the door) is in proximity to the switch with the door closed, and the light will come on when the magnet leaves the switch.
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You need a NC switch as you describe it.
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On Thu, 29 Apr 2010 09:32:45 -0700 (PDT), Bob Villa

Normally = Energized = most of the time. Except fire alarm systems
Upload the wiring diagram to be sure.
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wrote:

Not in relays.
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> Except fire alarm systems
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On Thu, 29 Apr 2010 13:29:25 -0400, "Existential Angst"

I meant De energized in motor relays and Energized in fire alarm systems.
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