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I'm working on a custom piece of hardware where a fan is turned on and off automatically. The user can override the decision of the automatic system through a switch. Activating the switch when the fan is on will turn it off, and vice versa.

Is it a valid choice to use a toggle switch (see the image below) for that, if the switch is positioned horizontally (i.e. the handle facing either left or right)? Or should I use a button switch instead?

Those switches, when put vertically, are usually associated with on/off actions, where the handle pointing upwards means something is turned on, and when it points towards the floor, it means the thing controlled by the switch is off—although, in some countries, the opposite may be true. In my case, however, the actual position of the handle doesn't mean anything about the current state of the thing under control; instead, the switch acts only as a way to change the state.

enter image description here

As the question attracted much more attention that I was expecting, here are a few details about the aspects which were originally unclear. I add them here instead of editing the question itself in order to avoid invalidating some of the answers.

While the user can override the decision of the automatic system, such override is limited in time. There are two goals for that. The first one (which should be quite common) is to prevent someone from turning the fan off, and forgetting to turn it back on. Or force it to stay on for a long time, when it may not necessarily be needed. The second one (which should happen rarely) is for the automated system to take the control of the fan back in a situation where appropriate. For instance, if the user decided that he wants a quiet environment, but one minute later the system starts overheating, starting the fan would be an appropriate response from the automated system (the other would be to cut the power to the whole system—this is definitively not something the user would enjoy).

This makes it impossible to use on/auto/off switches, unless they have some sort of a reset feature where the automated system itself can toggle the switch back to “auto” position. A possible option would be to use two push buttons, “on” and “off,” and three LEDs indicating the state of the system, only one LED being on at a time.

The design of this part of the system should not take in account the users who want to actively harm/hack it (by manipulating it to permanently switch the fan off, for instance). In fact, if the system is brought into a risky state, it would cut the power to itself. Turning it back on would require a manual action of a technician.

Right now, the three solutions are:

  • An off/auto/on switch which can reset itself to “auto.” Don't know if such a thing exists.

  • A series of two buttons: “off” and “on,” with three LEDs: “off,” “auto,” and “on.” The active LED shows the state. The user can press a button to activate another state.

  • A single push button, which changes the state (the currently accepted answer), and possibly a RGB LED showing the current state (although the fan is loud enough by itself). I'll start by testing this one, as being simpler, and would revert to the previous one if the single button approach would seem not intuitive enough.

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    I struggle with the logic behind it. If the automatic switches on the fan, it is presumably because a fan is deemed necessary at that moment. If I can safely switch it off, it is obviously not needed after all - why did it turn on then? I find this type of operation unintuitive. A lever with three positions would be more clear: Always on - Automatic - Always off.
    – Johannes
    Aug 13, 2021 at 20:08
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    @Johannes: my explanation was indeed incomplete. The system allows a temporary override, but is still protected against someone turning the fan off completely, and forgetting to turn it back, or keeping the fan on for the whole night when it is not needed.
    – Arseni Mourzenko
    Aug 13, 2021 at 20:47
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    EE here...use labels.
    – DKNguyen
    Aug 14, 2021 at 1:58
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    Even with the additional explanation, the three position system suggested by @Johannes (or something like it) still seems more intuitive to me. I'm also not sure how you're even planning to implement a temporary override with a toggle switch. Does toggling the switch just send a momentary override signal to the controller? If so, that's super unintuitive and you probably should just use a push button (or two, or maybe three: "lock on", "lock off" and "unlock").
    – Ilmari Karonen
    Aug 14, 2021 at 20:10
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    The logic sounds just the same as in most cars, where there's an inside light that by default comes on when you open the door, but you might want to override this behaviour in either direction. As far as I'm aware, car manufacturers always use a three-position switch as described in Izquierdo's answer (but with "auto" as the middle state).
    – AirOfMystery
    Aug 15, 2021 at 5:51

7 Answers 7

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I think your assumption that vertical switches are always up=on and down=off is wrong. In instances where two switches control the same thing then it is very easy for them to get out of sync. For example, consider the switches for the upstairs landing light in a house (at least in the UK).

When using switches like this, you quickly learn that there is no definitive logic to which direction is on or off. Instead, you learn that pressing the switch (in either direction) will toggle the device it is connected to.

So my suggestion is: do not worry about it. Use a vertical switch if that is convenient for you. The user will quickly learn that there is no fixed position for on/off, they will not struggle to understand how to use it. They will simply know: if the fan is on, and they want if off, then flick the switch.

If you want a switch that doesn't show any "state" at all, then use a simple button, as it always looks the same regardless.

enter image description here

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    And if this switch has an internal light to indicate "now on" then pressing it not only toggles the state but provides an unambiguous indication as well.
    – Andrew Leach
    Aug 13, 2021 at 21:30
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    @AndrewLeach, this could also be a useful indication when the automatic system retakes control of the fan, as the OP says it's a temporary override anyway.
    – computercarguy
    Aug 13, 2021 at 22:20
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    But STILL use labels!
    – jamesqf
    Aug 14, 2021 at 5:50
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    +1 for using a simple button. It is more obviously just on or off. The toggle switches aren't as obvious.
    – PhillipW
    Aug 14, 2021 at 8:39
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    Minor nitpick, but what you've illustrated is not a "simple button" - it's a capacitive sensor which requires power and provides an open collector output contact. It is a momentary button, so for any application that is using it for a state change it requires a logic controller (or logic circuitry) of some sort to process the input and persist the state change in the system. That one also has an LED for indication, so it further requires a separate signal as an input to show the state of the system.
    – J...
    Aug 14, 2021 at 11:20
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You're possibly using a two-state switch to control three states:

Off | Auto | On

"Activating the switch when the fan is on will turn it off, and vice versa."

  1. Like a lightswitch in a room with two lightswitches, where if it's on, flipping the switch in either direction turns it off? Then left or right could both mean "off."

  2. Left is manual off, neutral (center position) is auto, right is manual on? The user might flip the switch to off and later wonder why it's not working automatically.

The switch itself seems ambiguous. It might be better to use a control with more obvious signifiers.

On auto off switch

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  • It's possible to get toggle and slide switches that have 3 states as well. And yes, labelling them would definitely be a good recommendation.
    – computercarguy
    Aug 13, 2021 at 22:17
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    On rereading the comments below the Q, the OP says it's a temporary override, so the switch might need to have an automatic reset or be a momentary switch to be what they need.
    – computercarguy
    Aug 13, 2021 at 22:31
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    +1 This is absolutely the correct solution. I have countless systems with controls like this. I would usually prefer the centre position for "auto" on a 3-way like this, with ON at the top and OFF at the bottom.
    – J...
    Aug 14, 2021 at 11:14
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Having a single control introduces the risk that the automated system commanded a change of state immediatley before the user did, but before the user could notice. the user would then be required to deterime if the fan is slowing or not to decide whether to revisit the control.

I think two buttons "pause fan" and "run fan", is the best match for your use case. as the user can be confident that they have not just countermanded an automated choice that already did what they desired.

Alternatively a self-centering (momentary) toggle sqitch could be used with "run" in one direction and "pause" in the opposite.

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  • While unlikely to happen too often, this is indeed the case I haven't considered, and when happening, it would look like the device is simply malfunctioning. Good point.
    – Arseni Mourzenko
    Aug 15, 2021 at 22:29
  • @ArseniMourzenko Or the user wants to preemptively override the fan, e.g. "the fan is about to stop but I want to make it stay on" -- but the only thing the switch can do at that point is stop the fan. Should they flick the switch twice in that case, to get to always on?
    – user3067860
    Aug 16, 2021 at 18:49
  • @user3067860: “the fan is about to stop” situation is very unlikely: with multiple factors that the automatic system uses, I hardly doubt a user can predict whether in a few seconds, the fan status would change or not. The case where the user doesn't want the fan to start in the next minutes is likely, however. This being said, having to handle this case as well would make the overall system too complex (even with an on/auto/off switch with auto-reset feature).
    – Arseni Mourzenko
    Aug 16, 2021 at 19:26
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Horizontal or vertical is immaterial. There is no "standard" or "expected" behavior. At no point should the user ever have to assume what the switch does in one position or another. The device should always have clear labels indicating which direction does what.

In your case, there is no clear, simple set of labels that you could use that describe what each direction does since it depends on the current state of the fan. That's a hint that this isn't the right type of switch. Based on your description, you need either a pushbutton or a momentary toggle switch (after you flip it, it flips itself back). You would label this switch something like "override" or "fan toggle". When activated, it would override the current state of the fan.

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    There is absolutely a standard, expected behavior for most switch types. A user should always be able to assume the engineer did their homework for simple cases like up is on, down is off. Having switches that randomly deviate from standard just wastes time and effort, and is annoying. I agree labels are nice to confirm operation in case it doesn't seem to be working (or for noobs), and this is a somewhat weird case. But your first two sentences are very much incorrect in general. At the very least, all switches on a single device should behave the same way.
    – MichaelS
    Aug 15, 2021 at 7:34
  • @MichaelS Stair switches don't have on or off direction. Almost everyone have used stair switches unless they live somewhere where people don't put lights at stairwells or somewhere that have no stairs.
    – slebetman
    Aug 16, 2021 at 4:13
  • @slebetman: Yes, 3-way switches are an exception to the general rule due to it being far more economical to do it that way. And those are really fairly useless. I'd rather walk 10 extra feet to keep the switches the right direction than have a couple of oddball switches. I'm not sure I've ever had a place where I didn't just ignore one side of the 3-way. And it's possible to make them momentary so they activate a latching relay then return to center, meaning you maintain directional continuity (and you can easily add 8 switches to one device if you want).
    – MichaelS
    Aug 16, 2021 at 9:14
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    But the point is that there are, in fact, standard orientations. Up and right are on, down and left are off where I'm at. The UK reverses up and down (not sure about left and right). So if I went to the UK, I'd make sure to use the UK standard, instead of just willy-nilly throwing the US standard in there to confuse people. If there's a reason it needs to be reversed, then you reverse it. If (like 3-ways) you can't have a consistent direction, so be it. But don't go against the grain just to be stubborn.
    – MichaelS
    Aug 16, 2021 at 9:16
  • Unfortunately switch directions in the UK are a mess, the traditional UK norm was up off down on and this is still used on lightswitches, sockets etc, but switchgear nowadays is usually designed to IEC standards which use up on down off.
    – Peter Green
    Aug 16, 2021 at 20:24
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If the automated system puts the fan on, it's presumably because some condition has warranted it. Temperature, say, for example, has hit a threshold. So if the user then turns the fan off, how long does it stay off? Forever? For some length of time? Until conditions change by some amount (temperature increase to a secondary threshold?)? Conditions exit the threshold state, and re-enter? If the user turns the fan off, and then on again, is it now under automated control, or does it stay on forever?

The workflow that you describe makes things really, really ambiguous. The "on/off/auto" switches are the way to go, if you really want user override of an automated process. Basically the fan is unambiguously under automated control, or manual control, at any given moment.

Or maybe what you want is a variable control that changes the threshold, so that the user can set the temperature (or humidity, or gas concentration, or whatever) that the automated system uses.

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I would suggest a modification of two existing answers is best here. A 3-position, self-returning switch. Not sure the technical term, but it can be called a momentary switch or a spring-loaded switch.

Image from Amazon product showing 3-position switch.
Image taken from Amazon and modified for labels.

Use

It's electronically only a 2-position switch. Up activates one output, down activates another output, and center is just a float position where neither output is active.

As long as the user does nothing, the circuit is disconnected and the automatic system can ignore it. When the user presses up, it activates the temporary Fan On circuit. When the user presses down, it activates the temporary Fan Off circuit. When the user releases the button from either up or down, it immediately returns to the central, disconnected position.

Clarity

Unlike the 3-position toggle switch, it's immediately clear that On and Off are momentary inputs. Also, repeated attempts to manually turn the fan off don't require manually resetting to the middle position.

Consistency

Unlike the 1-position momentary switch, the user doesn't need to know the current state of the fan. Pressing up always attempts to turn the fan on. Pressing down always attempts to turn the fan off.

Improvement

If the fan is distant or quiet, some type of indicator would make it clear to the user whether the fan is currently on or off. A malfunction indicator could also be placed here.

Same switch image, with 3 LED bulbs on the side.
LED images borrowed from lifewire

Here, we have green for "fan on", yellow for "fan off", and red for "fan error". If you have another location for your error lights, or no error-sensing circuits, the red light could be omitted. I would also explicitly label the indicators.

I would personally avoid red for "fan off" because I associate that color with a bad condition. Yellow tells us the fan is working, but idle. It also lets the user know the system could come on without warning, instead of indicating it's completely shut down.

You could omit the yellow light entirely, but it might be less clear what's happening, and having an indicator for each state means a loss of one bulb still lets the user figure out the state.

Horizontal vs. Vertical

There's no reason you can't put the switch horizontally. I think it's less standard that right is on, left is off compared to vertical positions, but it doesn't need to be vertical

I would suggest it's more typical for right to be on, left to be off, but I can't find any good studies on the subject. The trucks I've seen use switches that way. Android interfaces do that. I believe iOS does as well. Most video games I've seen have it that way. Maybe non-Americans do it differently, so if you're from somewhere else I'd look up local conventions.

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No, a switch should not invert an automatic system if the switch is expected to only turn a fan off. If the direct intent of having the switch is it to invert a specific state then it should be labeled as a invert function.

If the question is literally the title then my answer is: In my experience the commonly expected orientation for a on/off switch is vertical though not exclusively the case.

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