Motorised Volume Pot - Controller isolation - diyAudio
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Old 12th July 2013, 01:17 AM   #1
frogboy is offline frogboy  Australia
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Default Motorised Volume Pot - Controller isolation

Hi everyone,

I am planning on embarking on a new amplifier project starting with a preamp I built many years ago, but I am very much a dabbler when it comes to electronics.

I would like to use a remote control for the preamp and thought I would use an arduino to receive the remote signals and also drive a motorised pot for the preamp volume.

The question I have is, I would like to send feedback on the pot position to the arduino to allow some different decision making and possibly an LCD display relating to the volume level. Is it possible to provide this feedback information to the arduino but also keep a suitable level of isolation from the audio circuit so that noise isn't introduced.

Any thoughts / suggestions?

Thanks.
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Old 12th July 2013, 02:34 AM   #2
DUG is offline DUG  Canada
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Put 1VDC on the top of the pot and take off the DC with a low pass filter / buffer to the Arduino.
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Old 12th July 2013, 02:35 AM   #3
gootee is offline gootee  United States
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I love my motorized volume pot with remote control and would not want to live without it. The only feedback mine has is an LED on the volume knob, so I can see it move and see what the setting is.

What feedback does the motorized pot provide?

What kind of motor does it have? Is it a stepper motor?

Is there a separate rotary position encoder, or tachometer?

Where would the display be? On the remote?

One nice feature would be a settable stop, above which the knob would not normally go.

Is this a package you're thinking of purchasing, that includes the handheld and the receiver and the motorized pot?

Separating the power supply would probably be the main way to isolate it. And don't make any antennas with the wiring, i.e. keep all pairs tightly twisted.
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Old 12th July 2013, 03:08 AM   #4
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A combination of drag and inertia can make a motorised pot very difficult to control precisely. I think it would be hard to use the feedback in a control loop, if that's what you had in mind.
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Old 12th July 2013, 03:32 AM   #5
frogboy is offline frogboy  Australia
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DUG,

Thanks for the suggestion, but I am not 100% sure of what you are proposing and how I would achieve this.

gootee, The motorized pot is a fairly straightforward ALPS one. It doesn't provide any feedback as such but I thought I could use the POT in some sort of feedback loop.

The more I think about this the more I think the better thing is just to go for the old KISS principle and Keep It Simple.

I will get everything working first and then if I want to expand on the functionality that is the time to do it.


Thanks so much for the feedback guys.
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Old 12th July 2013, 09:26 PM   #6
DUG is offline DUG  Canada
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Quote:
Originally Posted by frogboy View Post
DUG,

Thanks for the suggestion, but I am not 100% sure of what you are proposing and how I would achieve this.

...
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Old 23rd August 2013, 09:53 AM   #7
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Smile newbie question

I'm trying to solve the same issue, reading the values of the pot when the signal is going through out it.

Please could you provide values for capacitors and resistors when using 100K motorized pot ?
Thanks
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Old 23rd August 2013, 10:50 AM   #8
gootee is offline gootee  United States
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Maybe a low-value current-sense resistor could be added in series with the audio signal, downstream of the pot. Then use two instrumentation amplifiers, one to measure the voltage across the current-sense R and one to measure the voltage across the input and output of the pot. Then calculate the current (using the known value of the current-sense R) and divide the voltage across the pot by the current, to get the pot's resistance. You could also calculate the voltage-divider ratio of the pot and it's -dB gain factor.

You would want to sample the voltages at about the same time. And you might want to low-pass filter the outputs of the instrumentation amps, and/or average the final results over the last n samples (or low-pass the final result, if calculating it with analog only).

If you hand-matched resistors, you could make the instrumentation amps using two or three op amps each, instead of buying them.

If no digital processor was desired, an analog multiplier/divider chip could be used (or even a discrete version made with transistors).

Application Notes AN-20 and AN-31 from ti.com should have everything needed. And analog.com has good analog multiplier/divider chips.

Last edited by gootee; 23rd August 2013 at 10:52 AM.
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Old 23rd August 2013, 11:12 AM   #9
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Putting DC across the pot is not such a good idea. For a start you might well introduce severe crackling noise into the signal path.

If you have the space, I would put a gear onto the pot shaft and use the gear to drive another positional sensor, be-it a pot or an optical encoder. Then use the output of that sensor to determine the position of the audio pot.

Last edited by KatieandDad; 23rd August 2013 at 11:16 AM.
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Old 23rd August 2013, 11:15 AM   #10
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IME it is bad practice to allow DC on a volume pot....results in crackly throughput/operation in time.
Electrolysis is the culprit I think/suspect.

Dan.
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