I would like to create an inverter because I would like to have an inverse switch, that is: my circuit should only work when some signal current does NOT flow. This is even mechanic: When a certain switch is NOT set, I want no current to flow at all.

The only idea that I have right now is using an NPN transistor in parallel with my circuit, while the circuit has a higher resistance than the collector-emitter part of the transistor. I could then connect the switch to the base of the transistor. If closed, the main current flows through the emitter-collector. But the down sides clearly are that:

1) current flows, nonetheless

2) a certain current will flow, according to the resistance, through my circuit anyways

My circuit is battery-powered and I would like to not have the battery be consumed at all. I have a 3V coin cell and, without use, it should last weeks or months.

I do not need an electronic inverter, it could also be mechanically triggered (my application is that I have a box and I want to have LEDs glow when it is opened... when the box is closed, I can have contacts closed, i.e., a closed switch; but I cannot close a switch when the box is opened)

Any ideas welcome!

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    \$\begingroup\$ I don't quite understand what about your application the switch itself wouldn't solve? \$\endgroup\$ – Marcus Müller Aug 29 '19 at 8:40
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    \$\begingroup\$ but switches come in both, normally closed and normally open configuration? I really don't see the problem. \$\endgroup\$ – Marcus Müller Aug 29 '19 at 8:42
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    \$\begingroup\$ ... exactly the kind of switch that closes when you open your fridge door. \$\endgroup\$ – Marcus Müller Aug 29 '19 at 8:43
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    \$\begingroup\$ You can make your own push-to-break switch. Think "car door light switch". \$\endgroup\$ – Steve G Aug 29 '19 at 8:58
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    \$\begingroup\$ @IceFire A switch, which opens a circuit when pressed is as normal as the type which closes at the same event. Both variants are so commonly needed that a lot of MF decided to produce only combined NC/NO devices (SPDT in English). It is such a common pronciple that I don't even remember when I learned about it. That's what I'm wondering about. \$\endgroup\$ – Ariser Aug 29 '19 at 11:10

You just have to buy a NC-switch. There are a lot of these devices.

If you don't want to change the type of the switch (I'm assuming you have some sort of NO-switch), you can flip the switch around, add a spring which keeps the NO-switch in the closed position and add some sort of a lever which acts against the spring. The lever then can be moved by closing the box.


After having understood OP's self made switch's principle, I feel urged to deliver another self made NC-solution.

Fix a clothes-peg inside the box with its mouth facing towards the box' cover. Do it by e.g. glueing or screwing one lever to the wall of the box. Put tin foil onto the other lever and contact it with another tin-foil wrapped plastic or wood piece glued to the bottom of the box. Then apply a nail to the box cover so that it opposes the clothes-peg's mouth. When the cover is put in place the nail should pry the mouth of the peg open lifting the moving lever away from the foil wrapped piece. That's your NC-switch.

  • \$\begingroup\$ any recommendation for a NC switch or a synonym? I'm searching for "push to break" switch or "nc switch" in some shops and do not find anything \$\endgroup\$ – IceFire Aug 29 '19 at 9:13
  • \$\begingroup\$ produktinfo.conrad.com/datenblaetter/1400000-1499999/… there are versions of this switch at conrad with NC. But you can always use any switch with both contacts and ignore the NO contact: conrad.de/de/… \$\endgroup\$ – Ariser Aug 29 '19 at 9:19
  • \$\begingroup\$ @IceFire you might be looking in the wrong shops. My local Baumarkt (construction material shop) has such switches for the purpose of turning on lights when one opens a door. I also can't believe this 150 000 switch collection doesn't have what you're looking for. \$\endgroup\$ – Marcus Müller Aug 29 '19 at 9:22
  • \$\begingroup\$ @Ariser ah, nice, thank you! What gives away that this is normally turned on? Is it only because of the "on/(off)" label? How did you find this one? \$\endgroup\$ – IceFire Aug 29 '19 at 9:25
  • \$\begingroup\$ @MarcusMüller yeah... question is: how do I find push-to-break switches? What is the search term for them? (I am German, too, and cannot find a translation, either) \$\endgroup\$ – IceFire Aug 29 '19 at 9:25

I have a box and I want to have LEDs glow when it is opened

I can think of several options...

  • A switch designed to light the inside of a cupboard when the door is opened. Available at home improvement stores, online, etc. Keywords: Door light switch. Caveat: make sure the switch closes when the door opens!

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  • Magnetic switches for alarm systems. Beware: these will usually be closed when the door is closed, you want the opposite. The one on the picture below has both options:

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  • A microswitch, possibly with a lever. Get a 3 pin one, so it has NO and NC contacts. Cheap and readily available, but it will make a "click" sound.

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  • Reed switch and magnet. Easy to find and silent.

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I can't remember whether the "Normally" in "NO" means "there is a magnet nearby" or not, so I'd just pick a 3 wire switch...

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  • DIY option

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You can buy these leaf switches, but you can also DIY. When the door presses on the plastic knob on the right of the picture, the two contacts separate and the circuit is opened. When the door is opened, the springy contacts come back together and the circuit is closed.

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    \$\begingroup\$ Very nice summary of options (and words for the particular switches), thank you very much! \$\endgroup\$ – IceFire Aug 29 '19 at 12:11

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