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The title is probably already enough info. Just an example:

Imagine a power button on the front of a computer that is always connected except when pressed, it disconects for a brief moment.

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The power button of a PC isn't a toggle switch. It's a "soft switch", which means that its just a momentary normally open switch that instructs the motherboard to start the computer. Replacing it with a normally closed switch just means that you would have to keep the button pressed at all times you want to use the computer. Practically all desktop PCs have a reset switch somewhere that does exactly what you asked. If it's a laptopt then you are out of luck, modifying it to have a reset switch would take a lot of work. – jms Feb 25 at 12:53
    
Not on a computer, Just a example of the Type of button, It will be connected not even to a button, but to a IR Reciver, It will connect when it is disconected. – epicdig07 Feb 25 at 12:58
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Note that I just edited the question. Seems like it's mostly about terminology and asked by a non-native English speaker. @epicdig07 Please be careful about the tags you choose. – zebonaut Feb 25 at 13:04
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Normally closed momentary push-button – Chu Feb 25 at 13:25
    
Depending on your circuit, (ie: if you just want to pull a pin high or low), you could use either NO or NC switches with pull-up/pull-down resistors. If your design requires opening a circuit or leaving a pin at a floating voltage, then the switch type will matter. – DevNull Feb 26 at 2:22
up vote 16 down vote accepted

That's called a normally closed pushbutton.

Most pushbuttons are open when left alone, and pushing them closes the electrical connection. These are called normally open. Normally closed pushbuttons do exist, but will be harder to find and more expensive when you do.

Nowadays, pushbuttons are usually just inputs to microcontrollers, so their polarity doesn't matter to their function. The micro can tell whether the button is pressed or released at any time, and then take the appropriate action.

For example, you could use a normally open pushbutton into a micro, which can then interrupt power to something else when the button is pressed. Even without a micro, you could use a relay that allows power to flow when not energized. Pushing a normally open pushbutton would energize the relay, which would interrupt power. This has the advantage of the power and possibly high voltage not being near the pushbutton where you have to worry about proper isolation. The pushbutton also need not be rated for the full power current. It only needs to handle enough voltage and current to energize the relay.

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So basicly I can acomplish this by shorting out the circut and therefor disconecting it when it has power? That will work fine if so! – epicdig07 Feb 25 at 13:36
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@epi: I don't understand what you are asking. Shorting something to turn it off is not a good idea. It can work for emergency overvoltage condition and the like, but deliberately causing a short to pop the breaker to turn things off should not be normal operation. – Olin Lathrop Feb 25 at 15:29
    
you can also just put a NOT gate on the signal from a normally-open pushbutton to get the same kind of signal as a normally-closed one. – S E Feb 25 at 18:12

schematic

simulate this circuit – Schematic created using CircuitLab

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There are also single-pole, double-throw (SPDT) pushbuttons that can be used as either normally-open or normally-closed, depending on which terminals you connect. – Peter Bennett Feb 25 at 17:09

Such a switch is called normally closed. Also, pushbutton is a better word than switch, because a switch will remain in its last positions whereas a pushbutton will return to its original position once you release it.

This normally closed pushbutton opens its contacts once you push it. It makes a contact again once you release it.

The switches you consider "normal" are called normally open and make a contact once you push them.

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