Problems with direction of potentiometer


#1

Hi!

I am a beginner with this, have only completed two PCBs so far, two VCO modules for a modular synth. One problem I am struggling with is how to get the “rotation direction” I want on the potentiometers. I haven’t found a way to “mirror” the footprint in PcbNew (except putting it on the backside, but then I’d get its graphics there as well (the “outline” and reference number), right?

The only other solution I’ve found (but not tested, so maybe it’s not a solution) is to rotate the pot in the schematic, but that feels just plain wrong to me.

Also, if that line of thought is correct, it seems like one has to be extra careful with potentiometers in a schematic. When I place e.g. a volume a pot in a schematic, the way it’s usually placed, it will have max volume when the middle lug is at the top, and when the PCB is mounted I expect that to be fully clockwise. Rotating it in the schematic, to better fit in how the schematic looks, would (I assume) swap pin numbers for the outer lugs, and affect rotation direction, while mirroring it won’t. Right?
I suspect I am missing something really simple (and important), because it feels like it just cannot be this fragile.

Could you help out, please? What am I getting wrong here? :slight_smile:

Håkan


#2

The footprint, unlike the symbol, is a physical entity. You can’t just mirror it.

If the lugs aren’t where they are on the physical entity then you need to edit the footprint to match the physical reality.

What footprint are you using? There may be some alternate versions with differing pad numbers. Or, maybe not since it is pretty simple to edit.


#3

Why do you assume mirroring won’t swap the pins? Also, you don’t need to assume; just try it. The pins on the symbol are numbered, so you can easily see where they go when you rotate or mirror the symbol.


#4

It shouldn’t, because that is the correct solution.

This is true. I would say one has to be careful with all components, pots included.


#5

Can you give some example of a potentiometer (datasheet), symbol and footprint which feel problematic to you? Things may be easier to explain with concrete examples.


#6

Thank you for all the help. I have a better grip on it now, I think making my own footprint (and possibly a clearer schematic symbol) is the way.

My main gripe remains, though, but at least now I know that is the way it is supposed to be.

What I think is ”plain wrong” (in my original post) is that the schematic, and how symbols are treated there, is connected to the PCB in an opaque and non-intuitive way. For me, a schematic is a totally symbolic representation of a circuit. I can rotate or mirror a transistor symbol to better fit into the schematic without having to worry how it will end up on the PCB, because that is a representation on a different abstraction level that deals with other things, like the actual board and components. There I can rotate and move the transistor to fit on the physical board.
If I on the other hand rotate or mirror a pot it gets physically different results on the PCB. If I rotate it, as opposed to mirroring it, the outer pins get swapped, which has an effect on the physical world – how a pot is turned.

I cannot fully understand where the difference lies, but one thing is that a pot has a mechanical connection to the user, it kind of reaches across abstraction layers.

Also, the pot symbol has no visible pin numbers in the schematic, and no pin notation in the PCB editor either.

So, I guess I’ll make my own PCB footprint, and possibly also schematic symbol (with pin numbers).

Thank you all!


#7

I still don’t understand. KiCad v5 has symbols R_POT etc. which have pin numbers. The corresponding footprint must also have numbered pads. That’s how all components which are sensitive to pin order are made in KiCad. The symbols can be rotated and mirrored freely because the pin numbering isn’t changed (you just have to draw the wires correctly, of course). Footprints can’t be mirrored. This all is just plain logic, it’s not KiCad specific in any way. So, again, if you can point to the datasheet, symbol and footprint we can explain better if needed.


#8

Edit the symbol with the library editor and, under properties (the cogwheel icon), check show pin number.
I’m sure the pin numbers are there, but not shown.

This is unlikely: every pad of a footprint should have a pin number. There could be exceptions but not for signal pads, for chassis soldered pads in some cases which are not the potentiometer pads.

As eelik says, the link between symbol and footprint are the pin numbers.


#9

As far as I know, all round-knob pots are like this:
image

Top view:
Pin 1, down left, counter-clockwise
Pin 2, middle, swipe
Pin 3, down right, clockwise

One footprint from KiCad library:
image

And the symbol has corresponding pins 1-3 where 2 is the middle one.

All you have to do is draw the wires in the schematic differently according to your application.


#10

I agree. This is explained in detail here: How does KiCad know which symbol pin is connected to which pad of the footprint?


@heriks
Footprints can not be mirrored as mirroring a hard body is impossible. (A physical thing can only be rotated and translated. Every other operation can only be done on mathematical or virtual things. A footprint is a representation of a physical hard body and therefore limited to the operation possible on them.)

Might it be that your problem is related with this? https://github.com/KiCad/kicad-symbols/issues/165


#11

Here is the R_POT from v5, it should be unambiguous:
image


#12

I am on KiCad 4. There are no visible pin numbers on the pot in the schematic – if that had been the case my initial confusion would not have been so big.

image

I see now that there is pin numbering in Pcbnew, but not that easy to spot (and also, without reference to pin numbers in the schematic, not that useful).

But now I know more, thank you.


#13

The schematic is connected to the PCB by pin numbers. It is predictable and unambiguous (otherwise you would have a hard time designing a PCB). The official libraries provide suggested symbols, you don’t have to use them.

Ah, that may be where you understanding is going wrong. Clearly you can’t rotate or mirror a transistor symbol without it affecting the PCB. Unless you also rotate or mirror the connected wires but that is exactly the same for the pot symbol.

I think all you need is the pot symbol with pin numbers, this was a deficiency that has been rectified in v5.


#14

I’ll try to be clear.

This is a picture of the potentiometer Kicad4 and the settings to show the pin numbers.

As the example with a potentiometer may be confusing, I will use a transistor.

This is the same transistor symbol, rotated and mirrored. I have used letters for the pin numbers. A pin number is actually a 4 character alfanumeric string.pot2

We can see that gate, drain and source are in the same positions.

This is a datasheet of a transistorpot3

See gate is on the left.

Here are 2 foorprints: the real one on the left and a fake one “mirrored” on the right!!

Now you want to solder the chip onto its footprint. So to match the real part to its footprint you must solder the transistor this way:pot5

I hope you now understand why a symbol can be mirrored but a footprint can’t.


#15

It would be not only a bug, but a dead bug. :slight_smile:


#16

I hate to nitpick an excellent post, but I think the 4 character restriction has been lifted in v5. :slight_smile:


#17

heriks said he is using kicad4…

-You are very finicky
-Well, finicky is not the exact word…


#18

Sorry, I missed that, my bad :slight_smile:


#19

Excellent post.

In my opinion, with a little extra information, would make a great addition to the FAQs.

One issue that I would like to see added is how the Footprint of the device package affects the pin numbers on the schematic. The physical part defines the pin numbers, and not all transistor packages assign the same junction to the same pin number.


#20

Yes, it can be that fragile, you have many degrees of freedom here.
It is quite easy to get something ‘visual’ on a SCH that is not related to the physical final rotation.

I think the better way to symbolically define a pot, is like Bourns etc do

image

Notice this symbol has pin numbers and a very clear CW/CCW direction tag too.