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I have a 4 switch (I think it's called "4-gang"?) light switch on the wall.

It's not connecting to lights though, but to AC vent doors. Now, these AC vents must never all be closed at once. Meaning, minimum one switch must be on at all times.

But, it's just an ordinary 4-switch wall panel, so you can actually turn them all off, potentially causing damage to the vents or AC unit.

But I imagine there might be switches that actually enforce this requirement (a switch won't turn off if it's the only one on). Perhaps physical switches, perhaps electronic buttons.

Does anyone know if these exist? Or what they would be called?

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Is it "exactly one" or "at least one" switch at a time? –  Niall C. Apr 9 '13 at 4:40
    
rotary switches can do it, but that would be 1 of 4 –  HerrBag Apr 9 '13 at 11:54
    
Should be possible since a typical three-way switch is effectively an XOR gate, as explained here. I'll have to think about this more later. –  Brad Mace Apr 9 '13 at 13:24
    
Is there one vent that you want to leave always open? Pull the switch out and hard wire it in the correct manner. –  Chris Cudmore Apr 9 '13 at 17:49
    
@NiallC. At least one. –  MGOwen May 9 '13 at 12:07
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3 Answers

The simplest/cheapest way I can think of is to use a 4-position rotary (selector) switch in parallel with 4 single-pole switches.

enter image description here

The rotary switch forces one to be on all times, the other 4 switches allow selectively turning on any of the additional vents on in any combination. I'd install the 4 single-pole switches in a 4-gang box, then use a single-gang with blank faceplate, drilled out to fit a panel-mounted selector switch like the one below. Ideally install a short metal conduit between the gang boxes to run the 5 individual wires (you can buy "primary wire" for this purpose, or just strip stuff out of regular NMD wire), but having a 5-conductor wire between the two (or eg, a 14/3 and a 14/2) would work also. Make sure you label everything appropriately.

enter image description here

You may also be able to get away with a 5-gang box and modify the faceplate to fit (drill out one of the switch positions to fit the selector switch).

enter image description here

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You really need to align those screw heads on that faceplate. –  Chris Cudmore Apr 10 '13 at 14:32
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This is possible with a simple guard. The guard should have a notch for each switch that is approximately 4x the width of the switch, except the plastic should not be notched in one of those four positions for each switch. For example, switch 1 should not be notched in position 1, but should be notched in positions 2, 3 and 4.

The guard also needs 4 horizontal slots that correspond to the switchplate screws. The screws hold the guard against the switchplate and the horizontal slots allow the guard to move left and right.

Here's a crude drawing. The red box represents that footprint of the entire switch, the tall black rectangle is the switch lever, and the large black thing with the 4 horizontal slots is the guard. The guard is shown in each of the 4 possible positions.

enter image description here

As you can see, each switch has exactly one position where it can not be turned off. In the first drawing, switch 1 is blocked from being turned off. In the second drawing, switch 2 is blocked from being turned off. And so on.

Let's set up an example:

  • the guard is in the first position
  • switch 1 is on
  • switches 2-4 are off
  • you want only switch 2 to be on

To move the guard to allow switch 1 to turn off, you must first turn on switch 2 to allow the guard to move. Then you slide the guard one position to the left. Finally, you turn off switch 1.

This could easily be made using snips and a scrap piece of sheet metal or plastic.

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Wow, you have gone to a lot of effort. Thank you. –  MGOwen May 9 '13 at 12:08
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You could use some relays.

Take 3 relays

Connect the coil of each relay parallel to one of the first 3 vents.

Connect the “normally closed” (the output that is switch on, when no power is going to the coil) of all the relays in series, so the 4th vent is turned on when all other vents are turned off.

The 4th vent is also connected to its own switch so it can be turned on when other vents are on.

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