Normally Open vs Normally Closed - does it matter?

Posted on
Thu Jan 28, 2010 10:46 pm
hamw offline
Posts: 893
Joined: Mar 31, 2008

Normally Open vs Normally Closed - does it matter?

I got one of the garage door I/O Linc setups. Works fine. However, the readout on IndigoTouch is counterintuitive: because it is a normally closed switch (I think this is right) when the door is in the closed position, e.g. the magnet is next to the sensor, the sensor is held open (maybe that means it is normally open?). In any event, Indigo Touch tells me the garage door is OPEN whereas it is actually closed. Gives me a headache to think about it.

I found some 2 -1/2 inch active sensors at SmartHome that are normally open, so when the door would close, they would read closed. But, that means that 5v is always running thru the sensors. Does that matter?

http://www.smarthome.com/7455B/Garage-D ... -3-/p.aspx

Alternatively, could I move a wire or something on the Garage Door sensor to make it a normally open sensor?

Thanks!

BTW if I just got all this backwards, read it as I mean it.... ;-)

Posted on
Thu Jan 28, 2010 11:07 pm
DPattee offline
Posts: 400
Joined: Jan 14, 2004
Location: Redmond, WA

Re: Normally Open vs Normally Closed - does it matter?

Scroll down to the "screw terminals" picture in the manual: http://wiki.smarthome.com/index.php?title=2450_Manual

You'll see that there are 'common', 'n/o', and 'n/c' connectors. Leave the 'common' wire where it is, and then just swap the n/o / n/c wire (you should only have a single wire in one of those two).

Proud owner of an Indigo-based, Phlink, EvoCam, & custom app-supported super house!
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Posted on
Sat Jan 30, 2010 4:30 pm
berkinet offline
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Re: Normally Open vs Normally Closed - does it matter?

...could I move a wire or something on the Garage Door sensor to make it a normally open sensor?

The terms normally-open (NO) and normally-closed (NC) refer to the state of the switch contacts in a relay/magnetic switch, etc when it is in the normal (i.e. not operated) condition. So, with a magnetic sensor, when it is not near the magnet (the "normal" case) a set of NO contacts would be open (not in contact). Then, when the switch came near the magnet the switch would close.

It is very common for relays, etc. to offer both NO and NC contacts. This lets you choose the way you wish the output to appear. This diagram of a NO magnetic switch provides a good illustration. Since it is being operated by the magnet, the NC lead is connected to the common. But, when the magnet is moved away, the NO contact will once again be connected to the common:Image

In your case, you would probably want to use the NO contacts of your garage door sensor. That way the I/O-Linc would report open when the door was in fact open and closed when the door was closed.

Hope this helps.

Posted on
Sun Jan 31, 2010 7:43 pm
hamw offline
Posts: 893
Joined: Mar 31, 2008

Re: Normally Open vs Normally Closed - does it matter?

Very helpful, thank you both. The fixed door sensor has 3 wires, black, red and green. Not having played with these before, I just hooked up black and red, figuring the green was a ground - for what I didn't know. Anyway, I changed to the black and green, and it works like a charm. When the garage door is closed, the IO linc reports closed.

Also, I now understand what they mean by "normally" open or closed, e.g. the state of the sensor when the magnet is not near it.

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