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Old 30th Mar 2020, 6:04 pm   #1
Al (astral highway)
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Default Potentiometer calibration dials - what to use?

Hi there,

I'm using an old WWII era US Signal Corps multimeter case to house a digital circuit with variable pulse width and frequency.

These variables are controlled within internally pre-set limits, by two external 100K pots. They will have knobs fitted and I will calibrate the circuit repetition rate and pulse widths using a 'scope.

It will be helpful to mark some intermediate settings between min and max, using some kind of home-made dials. Likewise, to label the whole thing. I'd be interested if anyone knows what typefaces resemble those used in WWII avionics* This would be ideal for my application.

I've seen lots of great finishing on various projects here and would love to make something that looks very neat and possibly of the same era as the housing.

I think yellow type would pop out nicely as I've re-sprayed the outside (black) first with acrylic primer and then a spray paint for automotive use.

Picture is the case with drill holes before clearing the burrs. The grey inside base won't be visible as the circuit board will be mounted on stanchions over it.

You'll see if you look closely that I didn't strop off the original paint (texture is still visible) but this was a deliberate decision.

It's in the 'wabi sabi' aesthetic, in which the history of the object is not concealed by completely removing all traces of its age. Instead, something of its journey is preserved and ideally, even foregrounded. If I hadn't done this, the case would look like any other alu box.

Any tips or examples much appreciated.
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Old 30th Mar 2020, 7:44 pm   #2
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Default Re: Potentiometer calibration dials - what to use?

Hi Al, slightly off track, but have you considered a 10turn pot with a turns counting dial ?

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Old 30th Mar 2020, 8:05 pm   #3
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Default Re: Potentiometer calibration dials - what to use?

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Originally Posted by Ed_Dinning View Post
... have you considered a 10turn pot with a turns counting dial ?
Hey Ed, that's a great idea from the viewpoint of precision and accuracy.

It may be too much for this purpose as I'm likely to land on only a couple of ideal pulse repetition rates and pulse widths for the application in mind.

But if you can point to a good source, I'll take a look at them and keep it in mind!

Thank you! I hope you're keeping well and safe.
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Old 30th Mar 2020, 8:23 pm   #4
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Default Re: Potentiometer calibration dials - what to use?

I suppose you could use 0..10 dials and have calibration curves on graph paper (like the HRO used). That would make use of the lids inside. And make re-calibration easy. https://www.vintage-radio.net/forum/...d.php?p=721475
 
Old 31st Mar 2020, 12:43 am   #5
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Default Re: Potentiometer calibration dials - what to use?

Hi MerlinM,

I hadn’t actually heard of the HRO, so that’s interesting to investigate in its own right.

The 0-10 dials is a good idea, although actual pulse-widths in uS will be more useful. The range is quite small, and so is the frequency range.

I should have put the question more clearly : how do people make dials on general ? I don’t have any particular skills or experience in this and want to make them look great.

Same with the lettering - do people use stencils and if so , where from and how do we make them up?

Thank you
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Old 31st Mar 2020, 12:54 am   #6
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Default Re: Potentiometer calibration dials - what to use?

When I need dials I stick a piece of paper to the front panel and pencil in my calibrations by rotating the dial until I get relevant numbers, for instance 10 uS, 15 uS, 20 uS, etc.
I try to get these as accurate as I can then use Librecad to draw up a professional looking dial. Somebody posted quite recently a set of .ttf fonts that will/should be exactly what you need. The file is called UNO Stencil Alphabets.zip ( apologies to the poster).

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Old 31st Mar 2020, 11:43 am   #7
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Default Re: Potentiometer calibration dials - what to use?

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I try to get these as accurate as I can then use Librecad to draw up a professional looking dial. Somebody posted quite recently a set of .ttf fonts that will/should be exactly what you need. The file is called UNO Stencil Alphabets.zip ( apologies to the poster).
Hi Joe,

That sounds like a good process to follow. I'm comfortable with measuring and calibration; as for the Librecad or navigating with .ttf fonts (uploading and using) - we'll have to see.

In any case it will now be a good while before I can go anywhere to get them printed, even if I do manage to rustle up something good-looking!

Many thanks again!
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Old 31st Mar 2020, 11:52 am   #8
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Default Re: Potentiometer calibration dials - what to use?

I've some Helipot 10 turn pots and dials if needed.
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Old 31st Mar 2020, 1:02 pm   #9
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Default Re: Potentiometer calibration dials - what to use?

Helipots with their ten turn dials look "in place" almost anywhere, and a printed background with hand drawn (by fountain pen or pencil, nothing else) graph would complement them well.
 
Old 31st Mar 2020, 1:16 pm   #10
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Default Re: Potentiometer calibration dials - what to use?

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Ive some helipot 10 turn pots and dials if needed
Wow that would be great ! Thank you , I’ll PM you !
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Old 31st Mar 2020, 1:41 pm   #11
Steve G4WCS
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Default Re: Potentiometer calibration dials - what to use?

Al, more details
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Old 31st Mar 2020, 2:23 pm   #12
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Default Re: Potentiometer calibration dials - what to use?

Bourns 'Knobpot' A ten turn pot built inside a knob, with clock hands turns counter under a perspex domed end.

Gorgeous period items.

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Old 31st Mar 2020, 3:57 pm   #13
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Default Re: Potentiometer calibration dials - what to use?

Quote:
Originally Posted by Steve G4WCS View Post
...more details

Thanks, Steve!

They do look amazing. Where are the connections - is that what we're seeing on the body?

I would want to mount them axially with the connections screened by the lid.

I'll have a think, as this level of precision and accuracy is wonderful where needed but I'm not sure that I do actually need it. Also, I'd have to have 'course' controls with 100K pots and 'fine' with 10K pots in series, so complicating the design.

Can you leave it with me to mull for a few days?

Thank you
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Old 31st Mar 2020, 3:58 pm   #14
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Default Re: Potentiometer calibration dials - what to use?

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Gorgeous period items.

True, that! They're amazing!
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Old 31st Mar 2020, 4:05 pm   #15
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Default Re: Potentiometer calibration dials - what to use?

With a 10 turn job you won't need fine and coarse, 10 turns 100 divisions per turn 1000 divisions. Your coarse control would set the "accuracy" as that sets the overall figure, it would be different if the coarse was a 10 way switch.
 
Old 1st Apr 2020, 9:26 am   #16
Al (astral highway)
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Default Re: Potentiometer calibration dials - what to use?

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With a 10 turn job you won't need fine and coarse, 10 turns 100 divisions per turn 1000 divisions.

Ahah, I see the source of confusion. I need an 100K range and Steve doesn't have 100K pots. So I'd be using one of his 10K's in series with my 100K to meet the demands of my circuit.

I see exactly your reasoning in terms of the divisions etc...
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Old 1st Apr 2020, 12:36 pm   #17
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Default Re: Potentiometer calibration dials - what to use?

What is your circuit? Could you increase a capacitor by a factor of 10? Also for best accuracy, use it as a potentiometer not a variable resistor, the contact resistance disappears then.
 
Old 9th Apr 2020, 8:04 pm   #18
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Default Re: Potentiometer calibration dials - what to use?

Hi folks,

Here's the completed modulator, variable pulse width and frequency. In the end I used this wonderful old dial (the white one) that I had lying around for the pulse width. I calibrated it so the 40 mark on the dial is 40uS pulse width. The large bakelite one controls frequency

The rest of the markers I graduated against pulse width and will graph them.

Also convenient is that 270 degrees is a significant pulse width. I haven't marked it yet, but simple enough.

The whole thing is encased in this wonderful old US signal corps meter case.

All up and ready to go, with optical fibre output.
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