Define "Normally Open" vs. "Normally Closed"

[Bitzer]

If I recall, the first thing they taught back in 9th grade was to answer the question that was asked, not tell the inquirer how to build a convoluted Rube Goldberg device ;-)

[Metspitzer]

Yeah, then you can put one wire on the "common" and flip a coin. :)

Wire it NO and if the light goes off when it should come on then wire it NC.

My best guess is that you should wire it NO and the magnet will close the switch.

[Nonny]

NC

[clare]

Go to the Borg or any other reasonably well stocked shop and get a 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.

[clare]

Actually, even with fire alarms you can have series string (normally closed) or parallel string(normally open) circuits.

[clare]

That's fine if he wants to know when the door is CLOSED - Generally people set it up that they know when it is OPEN. (or NOT closed)

[Metspitzer]

Fire alarm relays are "normal" in the energized state so they fail safe.

[Metspitzer]

No. I think you and I disagree whether the switch is "normal" with the magnet near or far.

I don't know. I am guessing.

[Metspitzer]

Oh yeah...........never mind :)

Reminds me of when we installed some lights in an underpass. The emergency lights burned all the time and the main lights came on in the daytime. :)

[The Daring Dufas]

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

[bud--]

I agree.

In all the following the contacts are closed with the magnet next to the reed switch.

From an alarm perspective *NC* ("normal" is when the door is closed):

"Most switching devices are N.C. (normally closed) circuits, so when the device is not in an alarm condition, the circuit is closed."

"Output signal: normally closed (switch contact is closed when the door closed)"

From a component manufacturer *NO*

C&K: "ELECTRICAL CIRCUIT: SPST NO (Contact Form A). Reed switch opens when magnet is removed from proximity. Contacts are held closed when magnet is within actuation range."

To know what the switch does you need to have a description like all of the above.

For the magnetic switches I looked at from alarm companies, none said NO or NC. A few said NC loop. They were probably all "NC" from an alarm perspective. Except a few were Form C.

[DerbyDad03]

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.

[clare]

Well, I'm not guessing at what "normally open" or "Normally closed" mean on rees switches or momentary contact switches of any type., so it's possible we do dissagree. A "normally closed" switch conducts electricity in it'e un-activated state. A reed switch proximity switch is unactivated when there is no magnet close.

[clare]

Not quite right. An alarm SYSTEM being normally closed requires normally open SWITCHES. This is a series string system, where when one

- any one - switch opens, the "normally closed" loop is open and the alarm sounds. The SWITCHES are normally open, and closed when the magnet activates the switch.

A Normally OPEN system is a parallel string circuit - where the normally CLOSED switches are activated when the doores are closed and the magnet is close - and ANY ONE switch being CLOSED activates the system.

The techs (if they know what they are doing and talking about) KNOW this.

[trader4]

I think Bud and RBM who are both electricians have this one right. There is no consistency on this. At the very least, I can show you an alarm switch I quickly found that clearly calls switches that are CLOSED when the magnet is next to them NORMALLY CLOSED SWITCHES. You can pull up the PDF "manual" as well and it is consistent with that terminology. It didn't take long to find this, it was the first place I looked after googling alarm switch.

I think Buds statement that you have to carefully read the full description to figure it out is spot on. And even then, in some descriptions you may not be able to tell because they really don't say. In this one, it is very clear:

"Surface Mount Magnetic Contact Switch, Normally Closed (10-Pack

Essential Info The most basic sensors of any alarm system are the door and window Surface Mount Magnetic Contact Switches. There are 10 pairs of switches included in this kit allowing you to monitor multiple areas around your home. This type of contact switch is for normally closed circuits; this means that the when the magnets are lined up, the switch is closed."

Note that they call the circuit normally closed and also call the switch for that circuit normally closed.

It all gets back to what your definition of "normal" is. If it means the condition with no outside signal or magnet on it, which is reasonable and how we approach relays then I agree with your interpretation. But I also see how what RBM said is true. That in the alarm industry, which BTW is an obvious big user of these devices, they call a switch "normal" if the window is shut and the magnet is next to it. It becomes not-normal when the window is moved away and it trips the alarm.

[clare]

No, they do not call the switch a "normally closed" switch. They say when the magnets are lined up they are normally closed. The definition of a "normally open" switch meets that spec.

You DO have to read carefully what they are saying - and you have to understand what is really going on.

[trader4]

Yeah YOU sure do need to read carefully what they are saying. And you obviously didn't. Because if your read the description in the link I provided, they CLEARLY call that switch normally closed. It's right in the freaking one line TITLE:

"Surface Mount Magnetic Contact Switch, Normally Closed (10-Pack)"

Here's another link to a switch supplier that discusses the issue and says this:

"For unknown reasons, there are some companies classifying their magnetic switches against the grain. Their NC switches turn things OFF when the two halves are apart, while NO does the opposite. This is why even some salespersons are confused as well. Therefore, before purchasing any magnetic switches, read the product descriptions carefully."

The OP can make up his own mind. Listen to two electricians and a switch company that says confusion does exist and the only way to know is to carefully read the description, if it's even contained in the description. Or you, who says the terms are only used one way. Clearly in the case I just showed you, if you went by the one line title of the switch, you'd be getting the opposite of what you expect. And in most cases, that one line is about all they say about the switch, ie there is no further explanation.

[salty]

A doorbell button is a clear and easy to understand example of a normally open switch.

[Existential Angst]

Indeed, it must be.... Assuming you didn't get some behind-the-scenes tutoring.

[clare]

From the site referenced :

Surface Mounted Normally Open Magnetic Reed Switch Philmore # 30-17050

Use this switch if you'd like to turn something OFF when the two halves of your switch are apart. Commonly used with security systems.

Specifications

Action: Closed Loop / Normally Open (when magnet is beyond the actuations range) Maximum Gap (actuation range): 1-1/4" Max Mounting Hole Centers: 2-3/32" Terminal Type: Screw Type: SPST Current Rating: 0.5A @ 100VDC

There is NOTHING contrary in this description. With the magnet beyond actuation range, the NO switch is turned off - exactly as it should be.

Radio Shack's is normally closed when contacts are apart (poor description - but basically when magnet is beyond actuation range)

Digi-Key gives no description other than "normally closed"