Define "Normally Open" vs. "Normally Closed"

[Bitzer]

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?

[salty]

"normally" is whatever state it is in when no outside force is acting upon it.

[Stormin Mormon]

Wiki didn't have the answer to this, so I provided it.

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.

[Stormin Mormon]

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.

[MLD]

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

[MLD]

Your description is about as clear as mud. Try again!

[JohnnyD]

Actually, he is 100% correct and your previous explanation and advice was wrong.

[Existential Angst]

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.

[Bitzer]

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.

[Bob Villa]

You need a NC switch as you describe it.

[Metspitzer]

Normally = Energized = most of the time. Except fire alarm systems

Upload the wiring diagram to be sure.

[Metspitzer]

Or you can look at the switch you pulled out and wire the new one the same way.

[Existential Angst]

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.

[Existential Angst]

Not in relays.

[Existential Angst]

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.

[Metspitzer]

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

[DerbyDad03]

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?

[RBM]

As an electrician, I've been through this numerous times over the years. It clearly depend on who you are talking to, and what trade they're in. IMO, the "normal" position is the state the switch is in when nothing affects it. Alarm people have the opposite take on it. My solution is to always buy devices that come in the "form C" which is both open and closed circuits, this way you can't go wrong

[hrhofmann]

You use a reed switch, with TWO magnets. When the door is down, place the two magnets so they oppose each other, the reed switch will stay open if you position the magnets correctly. one magnet on the door, one on the frame next to the reed relay/switch. When the door goes up, the one magnet will move away from the reed relay/switch and the remaining magnet will close the switch and turn on your alarm/light bulb/whatever. This is high-school 9th grade stuff, not rocket science!!!!

[Metspitzer]

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.