NC or NO?

[jrfuda]

I'm confused about normally open (NO) and normally closed (NC) sensors. I understand the open and closed parts (The circuit is open or closed) but do not understand the normally part. What's normal? Is normal when the two halves of the sensor are together, or is normal when the two halfs are apart?

 

[BraveSirRobbin]

I "believe" that NORMAL is the state the device is in without any external influences or forces.

For instance say you have a magnetic door sensor that is OPEN (i.e. there is no closure between the two leads) when the magnet is AWAY from the sensor. That device would be labeled as "Normally Opened".

Can someone check me on this?

BSR

 

[smee]

I thought it worked the other way around for these magnetic switches.

"Normally" indicates the state when the door or window is closed. So, a "normally closed" magnetic switch allows current to flow when the magnet is near the switch (the door or window is closed). "Normally open" does not allow current to flow when the magnet is near the switch.

This still fits BSR's definition, if you consider the "state the device is in without any external influences or forces" to be when the door or window is closed (uninfluenced by being open).

 

[DavidL]

Yup, normal is the state the device will be in most of the time. In the case of a window or door, usually interpreted as window / door closed and the magnetic contacts or physical contacts will then be connected, and electrically the device might then be either Open circuit or Closed circuit. Most of the time it will be Closed circuit for majority of window / door magnetic style contacts. Don't forget about the End of Line resistor which is put in serial to thwart bypass attempts of the sensor.

 

[BraveSirRobbin]

hmmm, I tried to look at some of my magnetic sensors here but the stupid thing says "CC" or "Closed Circuit". Not much help for this discussion.

I was comparing the magnet operation to that say of a relay. If a relay has "Normally Closed" contacts then those contacts are closed without the influence of voltage/current on its solenoid. Whenever you apply voltage/current to the solenoid this "force" influences the relay and "opens" the contacts.

I guess you could also use this example and say the state the relay is in most of the time would be OFF or "normally" closed.

Of course I have been known to be wrong at least one other time that I can remember (but then again I am getting older and the memory just isn't there)!

 

[BraveSirRobbin]

OK, I have THIS Semco-Larm sensor sitting here on my desk. When the magnet is AWAY from the sensor the contacts are OPEN. When the magnet is NEXT to the sensor the contacts are CLOSED.

If you look at the SM-240 specifications (screen shot below) you will note that they are described as "N.O. contact for N.C. circuits"

I take this as the "sensor" itself is N.O. or normally opened. Thus my first argument would be correct in that this is the state the device is in WITHOUT any external (magnet) influence.

Hey, this may turn out to be a better debate than which processor is better, Intel or AMD!

 

[electron]

There is no debate as to which processor is better j/k Interesting thread for sure, I always thought that 'normally' referred to the state the device is in without any external influences as you described, but the other reasoning make sense too lol. I will have to add this to the Glossary.

 

[smee]

The glossary note should probably avoid calling the alarm circuit normally closed or open.

The definition for NC or NO magnetic switches is good (i.e., a NO magnetic switch will conduct current when the magnet is near the switch and a NC magnetic switch will conduct current when the magnet is removed).

The alarm circuits should probably be referred to as "closed circuit" or "open circuit" instead of NC and NO. I think using NC and NO for the alarm circuit itself may help to continue confusion. However, the glossary should note that these terms (NC and NO) may be used for the alarm circuit itself.

So a normally open (NO) switch will be used with a closed circuit (CC?) alarm system. A normally closed (NC) switch will be used with an open circuit (OC?) alarm system.

There is no debate as to which processor is better.

Absolutely correct. There is no debate. Atmel AVRs are the best.

 

[jrfuda]

[sarcasm] Oh yes, this cleared everything right up! [/sarcasm]

Let me see if I understand correctly, in layman's terms: If I want the circuit to be closed when the sensors are together, I should get a normally closed sensor?

I want to monitor my garage door with one of them new-fangled garage door sensors (available at Both AO and RatShack) but wasn't sure which to get. I want the circuit to be complete when the garage door is closed (and the 2-halves of the sensor are together). So which is it for this scenario NC or NO (since, correct or not, that's how they're labeled).

Option 2 is to do the same as above using a DS10 instead of the Ocelot (but with the garage door sensor rather than the sensor included with the DS10). The sensors that are included with DS10's are "closed" when the 2-halves are together.

 

[smee]

If I want the circuit to be closed when the sensors are together, I should get a normally closed sensor?

If you want current to flow when when the magnet and switch are together, you want a normally open (NO) switch.

I want to monitor my garage door with one of them new-fangled garage door sensors (available at Both AO and RatShack) but wasn't sure which to get. I want the circuit to be complete when the garage door is closed (and the 2-halves of the sensor are together). So which is it for this scenario NC or NO (since, correct or not, that's how they're labeled).

According to the above discussion, you want the NO switch. It is the correct designation for the switch. It's just an incorrect or confusing designation for the circuit.

Option 2 is to do the same as above using a DS10 instead of the Ocelot (but with the garage door sensor rather than the sensor included with the DS10). The sensors that are included with DS10's are "closed" when the 2-halves are together.

If this is the case, then the DS10 switches are NO and should be replace with the NO garage door sensor.

 

[jrfuda]

Thanks smee

 

[jlehnert]

Electron said

I will have to add this to the Glossary

It was already in the glossary BC (Before Crash). It will be back in the glossary if I ever get a chance to sit down and do some typing again.

 

[BraveSirRobbin]

[sarcasm] Oh yes, this cleared everything right up! [/sarcasm]

[sarcastic reply to sarcasm] Next time ask the correct question FIRST, in this case it might have been "What sensor do I need to monitor my garage door with my Ocelot?"[/sarcastic reply to sarcasm]
Actually I'm just kidding around! With the Ocelot and SECU16 it doesn't matter what you do (open/close contacts for your garage door) because you just program the CMax logic appropriately!

 

[jlehnert]

The missing word that is probably causing the confusion when talking about alarm systems is "loop". Normally closed loop, and normall open loop. You need a NO switch (open if the magnet is not present) when creating a NC loop. Therefore the switch is closed when the magnet is present (door closed), creating a circuit.

You have to be very carefull when ordering alarm contacts, as some companies will list a contact as NC, omitting the "loop". It works for the alarm techs, as they think in terms of loops, not switches.

 

[JohnBullard]

I agree with BSR's first post, thinking in terms of relays or mometary (push button type) switches.


Then you guys go and change the subject to alarm system type contacts, turns the world upside down! Alarm tech people think differently!