panel mount knob with indicator

[VJ]

Hello,

Strange question for a friend of mine. He is looking for a rotary knob (rheostat) that you can mount on a panel. That is easy to find...

But... he would also like it to have a needle behind the panel, a bit like on old school devices. All the rotary knobs we can find that mount on a panel have no rotating part on the backside of the panel. However, mounting them on a panel that is behind the "decorative" panel makes the axis too short to reliably put a knob.

Any suggestions on where we might find such a rotary knob? Or any suggestions on how to best lengthen the axis?

Thanks!

[Umfriend]

I have no clue but...would a 3D printer not help with that?

[VJ]

Maybe... do you need photos of the 3D printer?

The issue is that either the attachment point is too short (if you mount the button further behind the panel), or you lack rotating things on the backside of the panel.
An option could be to move it FAR back, so that the entire rotary switch is behind the panel: that would give you ample space to attach both a needle and an extension to the axis to mount a knob on the outside... Didn't think of that one... although I'm not sure there is that much space on the back of the panel...

[UtwigMU]

I think you need particular panel and particular knob to see all aspects. You could have the part machined if you know some CNC shop where they might do you a favour. Commercially it would cost several 100€ just for one part. It's probably just some metal axle, which can be turned on a lathe with extended length to original and have the threads cut. If you have a caliper and a CAD app, you could probably draw it yourself.

[VJ]

The problem is not really the panel... Yes, it will have to be custom made, but that is not an issue. Panel mount rotaty knobs usually look like this (viewed from the side):

[]=|--

[] is the hardware of the button
= is a static piece in which to rotary part rotates
| is the panel (attached with a nut)
-- is the part that rotates

What would solve the issue, is a button that would look like this:

[]=-|--

But of course that would complicate the attachment so it does not exist. The idea I had in the above post was to do this:

[]=|--~|~~

where the first | is the panel on which it is attached and ~ is an extension of the axis to stick through our front panel. I have found rotary potentiometers that have quite a long axis, so the ~ part is something that we would not have to attach ourselves. However, as Umfriend points out, making an attachment would be easy with a 3D printer.

The issue now becomes that the construction becomes quite big on the back. It mainly was to give a retro look, but maybe the design has to be reconsidered.

[VJ]

Just got an idea... What if the potentiometer would be hidden in the knob?

x--|=[]

x would be a fixing point of the part that normally holds the knob, the entire potentiometer could be fixed in the button. Only thing is then that it has to be mounted so it can rotate in the panel, and the cables have to also go through the hole...

[Marshmallowman]

You can get extensions for standard potentiometers...

https://www.ebay.com/sch/i.html?LH_CAds=&_ex_kw=&_fpos=&_fspt=1&_mPrRngCbx=1&_nkw=potentiometer+extension&_sacat=&_sadis=&_sop=12&_udhi=&_udlo=&_fosrp=1

the above seem overly elaborate, I have seen small aluminium ones that simply slide onto the shaft allowing you to mount any standard knob you want

[VJ]

Thanks!
It seems he decided to go back to the drawing board. There is not that much space, so either the design will change to make more space (and then some extender on top of the potentiometer would be needed), or the design will change that there is some alternative to the needle.

[UtwigMU]

Usually you have a needle in front of the panel with translucent plastic cover over needle and panel.

___________________ ----plastic cover
__|______I======__|__ ----needle
----panel

[VJ]

Then it is difficult to put the needle behind the plastic but have the knob accessible. But, if there would not be a plastic cover, and just a round knob with an arrow like extension, there is not even a need for a translucent panel that hides the needle... A bit a less elegant design but possible...

But the general idea is already changing, perhaps moving away from the mechanical needle to an eink display and a rotary encoder instead of a potentiometer. Could still look old fashioned but allow for much more things...

[Marshmallowman]

Well it does depend on the precision required, but you can go for large marked knob and mark the face plate accordingly. If you want precision you probably going to need a separate readout.

Try browsing here , they even have knobs that go all the way up to eleven

https://www.allparts.com/knobs

maybe a chicken head knob

https://www.ebay.com/bhp/amp-knobs

Also Slider pots can you good visual indicator

[VJ]

Yes, the chicken head was along the lines of what I wanted to say!
Nice idea on the slider pots!

Plan has evolved however from "potentiometer with needle" to "rotary encoder with display (perhaps eink)". The potentiometer would be an input to an Arduino, moving from a pot to a rotary encoder and a display is not that difficult but gives much more options. The display can possibly can be a waveshare eink as it just serves to show labels/static (I know the eink is too slow to a moving needle). Other alternative is just to go with a regular display and integrate it nicely.
The slider pot idea may be useful in another location, we didn't think of that...

[VJ]

After seeing these things, I got fascinated by this basic programming stuff, so I decided to get me an Arduino with some accessories. At the moment, there are huge discounts on Amazon for Elegoo Arduino stuff, so I actually went ahead an ordered it: I got the Uno R3 starterpack, sensor pack and a Mega2560. (extra discount for buying 3 items made the last one free).

I remembered how much fun I had with a small electronics kit as a child, how you could make nice things with very basic building blocks. Then I saw my code for work, which is the hugely complicated class structure; stepping through it to uncover the behaviour is hell and compared it to the way an Arduino is programmed (main loop, pin commands and interrupts) and thought that playing with the basics could be fun. I'm still am a few months in Spain, so I thought that could be something to have fun with, perhaps with some uses for the 3d printer I will get on my return.