noisy pots

Using Arduino General Electronics
#1

we're trying to set up a decoupling capacitor on a noisy analogue pot reading for the purposes of sending midi CC. We've tried using electrolytic caps of 0.1uF, 10uF, and 100uF and we've also tried a whole goody bag of ceramic caps (maximum 0.1uF). All caps we've tried so far have been 63 volts. here is a schematic of what we're trying: -

here it is on a breadboard with a ceramic cap

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This made no difference to the noise. Can anyone please advise us on where we are going wrong with either the circuit or the type of capacitor we are using?

Many thanks

#2

try putting the cap on the wiper.

#3

Right… The cap needs to filter the signal at the Arduino input. :wink:

How much noise are you getting? Is the noise worse with the pot at maximum, or in the middle? I assume the noise is OK at low settings?

MIDI CC is 8-bits, right? You should be able to get a “stable” reading at 8-bits. But, with ANY analog-to-digital conversion you can be on the “hairy edge” and the reading can jump up-and-down by one count. If that’s a problem, you have to deal with it in software (and there a couple of ways to do that, depending on your requirements).

Higher value pots are more susceptible to electromagnetic noise pickup. i.e. 100K is probably too high, but it depends on the “electrical environment”.

Longer wire-runs are more susceptible to noise pickup.

Higher value capacitors will give you more filtering. If the capacitor is too large you’ll filter (slow-down) the pot-adjustments and your adjustments will be delayed. But, that shouldn’t be a problem with 100uF or less.

A higher value pot will also give you more filtering with a given capacitor, but since the noise pick-up is worse with higher resistance, it’s usually best to use a lower value pot and a bigger capacitor.

If you get noise with the pot a maximum (5V out) you are getting noise from the power supply. (And, that might take a BIG capacitor, or a different power supply. (With the pot at maximum, you’ve got the capacitor across the 5V supply, effectively the same as your schematic shows now.)

You can also filter in software (i.e. average the readings), but you may not want to take-away processing power from whatever else your code is doing. (I’d start with a capacitor filter.)

All caps we’ve tried so far have been 63 volts.

That’s not important. The voltage rating on a capacitor is the maximum you can safely apply. You’ve only got 5V and I’ve never seen a capacitor that can’t handle 5V. (They may exist, but I’ve never seen one.)

#4

Dual-layer (super) capacitors are normally 2.7V rated.

#5

Noisy pot?
Buy a new one!

#6

MIDI CC is 8-bits, right?

No it is 7 bits.

#7

LarryD:
Noisy pot?
Buy a new one!

In the olden days, we used to give a short squirt of CRC into the track. Usually worked.

Weedpharma

#8

Better than all.

Learn to use a rotary encoder.

{Seriously - this is digital electronics in the twenty-first century - why use a potentiometer?}

#9

Rotary encoders have no state, often a big issue. And the cheap one's have very low angular resolution.

#10

MarkT:
Rotary encoders have no state, often a big issue.

Often the reverse. A switch-on default or memory of last position may be very valuable where for example, the controls may be interfered with between sessions. Like on a sound mixer.

MarkT:
And the cheap one's have very low angular resolution.

But always better than the average potentiometer.

(Because the step size is whatever you desire it to be!)

#11

Paul__B:
But always better than the average potentiometer.

(Because the step size is whatever you desire it to be!)

That may be true for a low value wire wound pot but carbon or ceramic pots have almost infinite resolution.

Russell.

#12

New pots are inexpensive if you have good shipping access from China.

http://www.ebay.com/itm/10x-B10K-Ohm-Linear-Taper-Rotary-Potentiometer-Panel-Pot-15mm-Shaft-Nuts-Washers-/291010300828?hash=item43c190cb9c:g:BEUAAMXQWzNSeSs6

#13

russellz:
That may be true for a low value wire wound pot but carbon or ceramic pots have almost infinite resolution.

Which is irrelevant. :grinning:

The practicality of this situation is that you cannot sensibly calibrate a single-turn potentiometer of the normal size to more than about 20 steps at most, and even that with difficulty. Nor is that an easy thing to manipulate quickly.

You need digital feedback - a display of some sort which indicates which channel is actually selected at any moment, and once you have that, the rotary encoder is a much more versatile input device. Which is why potentiometers (or tuning capacitors) are no longer used on quality equipment such as car radios. They are becoming obsolete technology. :grinning:

And if you do not need feedback - do not need to know the actual position, then the same thing applies - rotary encoders provide a better "feel". If you need to make rapid changes as well as fine ones, you can use acceleration algorithms to excellent effect.

#14

I am fairly sure i saw somewhere an incremental rotary encoder with on board NV memory of last position, not expensive either , just cannot find it now but ISTR farnell had it.

#15

Of course, you really cannot put a dial on a(n incremental) rotary encoder as it can be moved when the power is off.

#16

Found something similar

http://www.digikey.co.uk/product-detail/en/AD5231BRUZ10-REEL7/AD5231BRUZ10-REEL7CT-ND/3314098

No you need electronic display to indicate position.

But for 1024 positions a 360 deg scale is not very useful anyway.
Multiple turns of the wheel to achieve 1024 re desirable anyway as its much easier to set that way.

EDIT

And with the micro power requirements of lcd it should be possible to leave the display permanently on anyway.

#17

Paul__B:
The practicality of this situation is that you cannot sensibly calibrate a single-turn potentiometer of the normal size to more than about 20 steps at most, and even that with difficulty.

Odd that because I have been doing just that for years and years. Just ask any radio ham.
Ever used an analogue computer?

#18

Grumpy_Mike:
Odd that because I have been doing just that for years and years. Just ask any radio ham.

Note the word I used was "calibrate", not "adjust".

#19

Paul__B:
Note the word I used was "calibrate", not "adjust".

Ever used an analogue computer?

#20

I recall several instruments that had multi turn mechanical dials with a display like a bicycle odometer.

They remembered their position.

Does no one make a micro power lcd equivalent ??

I'm sure they would be very useful.