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So, I can get push-to-break switches that will break my circuit when pushed in. I assume this is how the light in my fridge works.

But how do these works? I would like to make something like this, but I have no room for a switch. My door closes very tight and I cannot drill any holes or mount any switches where it closes. I can, however, run a wire on the door and on the flange it closes against, and have them touch to complete my circuit.

So I imagine, that using the push-to-break functionality, I can break when the wires touch, and un-break when they no longer touch.

I have no clue how to achieve this though. Maybe I am even thinking about this wrong.

How can I turn my lights on, when the door is open?

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  • \$\begingroup\$ There a quite a few ways to do what you're looking for. How big of a project do you want to do? Are you inclined to use small components (like transistors), or would you prefer relays? \$\endgroup\$ – bitsmack Apr 3 '14 at 16:47
  • \$\begingroup\$ @bitsmack I actually don't really have any preference. As simple as possible, definitely. \$\endgroup\$ – Nicolai Apr 3 '14 at 17:40
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A simple way to do this, especially if you're switching 120V, would be to use a relay. Look for one that is either normally-closed ("NC") or has Form-C contacts. Form-C combines NO and NC contacts. Sometimes, in relays, Form-C is called "CO" or "SPDT":

FormC

You'll need some power source for the relay coil. Since this power is getting interrupted by your door contact, make sure it's not hazardous! Don't have a live 120V contact on the door jamb :)

Then, this is how you would hook it up:

Circuit

(Sorry for my cheesy drawing!)

This idea could also be used with a solid-state relay, or even a MOSFET, if those would suit your applicaion better.

Good luck!

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Does the schematic symbol give you any ideas?

enter image description here

When you push it down, it breaks the connection.

There are a zillion types of switches, I doubt very much that you need to re-invent the wheel (or N.C. switch in this case).

Edit:

If I have misunderstood and your intent is simply to reverse the action of a N.O. switch to normally closed, one method is to use a relay such as the OMI-SH-112D,394 12VDC relay linked above. It draws 60mA on the coil, so any small wall wart with a 12VDC output will work to supply the coil current. The contacts are capable of switching 10A at 250VAC, so they should more than suffice. I like this style more than some slightly cheaper models because the coil pins are well separated from the (mains voltage) contact pins, which helps for safety.

enter image description here

The relay coil will draw current continuously to hold the contact off. That, I think, is the simplest. A low voltage coil relay could be used which could mean that your switch would be touch-safe (assuming a wall wart supply or other isolated supply was used).

That may not be acceptable due to the current draw. The light could be driven with a solid state switch that would be turned on when the switch is opened. For example a small SSR with a pullup resistor to (say) 12V that is shunted or by the switch closing.

Having at least a few mA through a switch can help improve the reliability. If the above does not adequately answer your question for some reason, please explain why and I'll attempt to adapt the answer.

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    \$\begingroup\$ No, the OP 'has' a NC switch and wants to 'convert' (electrcally) it to NO (or vice versa). \$\endgroup\$ – Wouter van Ooijen Apr 3 '14 at 16:22
  • \$\begingroup\$ @WoutervanOoijen, you're right now I re-read, OP already has a NC switch. But my reading is he doesn't want to convert it to something else; just wants to know how it works. \$\endgroup\$ – The Photon Apr 3 '14 at 16:25
  • \$\begingroup\$ He wants to switch a light on when his NC contact breaks. \$\endgroup\$ – Wouter van Ooijen Apr 3 '14 at 17:19
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    \$\begingroup\$ I think it's pretty clear that the OP fully understands the concept of a normally-closed (push-to-break) switch. The question is about getting the same functionality when a physical switch can't be mounted. The giant graphic was just rude. \$\endgroup\$ – Joe Hass Apr 3 '14 at 17:26
  • \$\begingroup\$ @JoeHass Yes. That is exactly what I am looking for. There is no room for the switch. \$\endgroup\$ – Nicolai Apr 3 '14 at 17:42

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