Does anybody know of a Potentiometer that can be set digitaly but operate passively?

I wonder if this technology exists.
Imagine the following situation:
During setup/calibration I can power the circuit and "tune" my potentiometer using some type of digital interface. (e.g. I2C)
However, during operation the device is fully passive and will not have a source of power. Thus, the potentiometer would need to "remember" the settings and operate passively.
Currently I use standard potentiometers, but tuning them manually is way too cumbersome and not practical.
Ideas?

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seandogue2 years ago

If you don't need the tactile feel of a hand adjusted pot, digital pots will do the trick, many are even burnable, ie, set and forget forever. Some allow reset to a set val on powerup but allow user adjustability via a control voltage or other interface, some locked to a given value (a bench-set cal value for instance). Some can be written once, some written on power down, etc. etc.

The automotive industry has encouraged the development of a wide variety of set and forget digital pots, as has instrumentation, for bias, frequency adjustment, volume control, etc, so the main thing will be determining and assembling your specific tech requirements (signal type, control type, range, power requirements, S/N, etc.) and then comparing them against published specs at either the IC mfg sites (and/) or at distributor sites like Digikey

try this. go to digikey.com and type "digital potentiometer" into the search bar...ya see? Yikes! a whole lotta options.

Kapiau (author)  seandogue2 years ago

Thanks for the response, however, I'm not sure if it applies. (event though I'd love for it to be true)

The digital potentiometers you mention are a great option if you have a stable supply of power. In my case I don't. For the sake of my argument, imagine you were to build a voltage divider and you want to increase it's accuracy.
During calibration I have access to this "stable power supply". Then I wish to set the value and have the potentiometer stay that way if powered or not.

Thus, the digital potentiometers available at Digikey won't work as they need to be powered up to "remeber" were I set them.

You did mention some "set and forget" pots. Would they fit the requirements I just presented?

Thanks

Yes, there are many set and forget (burnable) digital pots. One sets them while power is applied and then writes the settings to the device (burn it).

Once programmed, when the device powers up, it is preset to to the desired value.

I'm afraid I don't have a specific part number to provide for you, although you can find suitable candidates easily by comparing and contrasting the selections offered by major distributors like Digikey, Mouser, Farnell, etc.

Enter "digital potentiometer" into the search field at one of the major electronic parts distributors and I think you'll find a whole lot of choices at your disposal.

iceng2 years ago

I have built intelligent instruments that use digital controlled pots.

These pots assume 50% at Power_On and immediately after that the uP directs them to the last user setting.

There are linear slide pots that are servo controlled, which is about as close as you can come to passive operation !

Kapiau (author)  iceng2 years ago

You are getting somewhere. However, these are HUGE!
I hope to find something much much smaller but with that functionality.

Thanks for the idea...

iceng Kapiau2 years ago
motorPOT.jpg
-max-2 years ago

There are a such thing as digital potentiometers, they take digital data and the 'output' is essentially a pot. However from what I understand, they are difficult to use because they have so many requirements that have to be met for them to work properly. If the pot's outer conections go to a fixed voltage rail, it may be a lot simpler to use a DAC.

Also, you could use a rotary encoder instead. Essentially, a rotary encoder give rotational speed data. That data can be integrated from power-on to present to figure out "position" (because speed is the derivative of position... 2nd fundamental theorem of calc can be used.) That is how basicly all modern volume knobs on digital equipment work. It requires some basic knowledge of programing though. The advantage of encoders is that they can rotate forever, so no need to worry about reaching an endpoint when volume is controlled remotely or digitally.

OR, you could use a servo motor to actually physically turn the knob, but that would probably be mechanically challenging as you would need to figure out how to only engage the servo when turning the knob remotely, and disengage when turning the knob by hand.

Kapiau (author)  -max-2 years ago

What you described is close to the functionality and work flow I'm looking for:

I have lots of resources, to set the potentiometer, power, digital communication, high end instrumentation. But, as soon as the pot is set, it should operate as a resistor and not have any other requirements. (not even power)

-max- Kapiau2 years ago

Rereading the original comment, here is what I understand you need:

You need a device with 3 pins on the output which are like variable resistors exactly like a real potentiometer, but you need to control this hypothetical POT with a digital serial interface and a physical knob, and a DAC does not fit the bill because the ends of the pot do not go directly to the voltage rails. (like +5V and ground, where the center tap is possibly buffered and used as a variable voltage, such that the output voltage of the adjust is a adjustable fixed level in some range.)

If that is the case, you may NEED to use a digital pot as your only option. Preferably, you ma need to reconsider how you are approaching the problem you are trying to solve, like redesign the instrument to make use of a DAC instead. If you are doing some hack on some audio equipment where the design is not easy to change, then that is one of the few times you would use a digital POT.

If you take care to pay attention to their ratings and not exceed the limits on it based on the datasheets of what part you choose.

So, then, you can use a rotary encoder and some microcontroller magic to be a knob that you can tube by hand, and also bypass all the rotary encoder processing stuff in code with a prioritized direct data to the digital pot or DAC.

If I recall correctly, many of those digital pots will have built in memory to remember the stored value. If they do not remember the output value and reset with loss in power, you may be able to add a coin cell backup battery to it to keep data stored in some volatile cache memory build internally. Thats how my old devices stored information, and always needed a small 3V coil cell backup.

Some amps use motorised potentiometers.

But basically any potentiometer can be modded by adding a servo or similar.

Tricky bit is to keep track of the actual position.

If the memory is not too important you can just add limit switches on both ends.

For remembering the setting you would need an index disk of some sort, like in the old mechanical computer mouse for example. An arduino could "remember" the last setting and operate from there.

Kapiau (author)  Downunder35m2 years ago

Your description if of my plan C, not even B...
The hope is to have some type of POT that can be digitally set, and not require power to "remember" it's resistance but just operate as a resistor.

If nothing is available, I might consider building a robot to turn the knob/screw of an off the shelf potentiometer until it reaches the proper settings.

But as you can imagine this is a huge undertaking and I really don't want to go there unless we absolutely positively must. Thus, I urge the brightest minds of this forum to come up with a kick butt solution. :)

-max-2 years ago

The disadvantage to rotary encoders (the popular relative types) is that the output of them is not an absolute position, so that has to be assumed on power up. In other words, if there is no permanent memory data stored to keep track of the position of the encoder since the beginning of time and the universe, the processor cannot know what the set volume was on last power up. (after all, encoders only give tell a microcontroller how fast it is turning.) So some memory may need to be used just to remember the last position on power up.

Also, rotary encoders are sort of useless without digital processing circuitry. That can be a limitation of their use in really cheap things.

-max-2 years ago

multi turn POTs are what you need for precise control. Either that or some encoders with good software.

iceng2 years ago

What are you ?

Do you mix audio ?

What is your skill ?