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Motorized potentiometers are easy to find and often used e.g. in music studio equipment, but I could need a similarly motorized rotary switch for a project. I am either searching for the wrong term (are such switches known under a specific name?) or they are very hard to find. The rotary switch will be used as an input device to a micro controller, but I need the micro controller to be able under specific circumstances to kind of 'override' the user input and choose one of the other selections. The microcontroller can of course simply ignore the user input and do whatever it likes, but it would make operating the device much easies and intuitive, if the micro controller was able to actually move the switch, so that the knob position could also be used as a visual feedback to the operator.

The switch should ideally have a 15° turn between each position. I will likely only need 7 positions, but due to panel design, these should be in reach within a ¼ rotation or so. As long as I somehow can drive the motor from a micro controller, the type of control interface is of less importance. If it is necessary, I could also drive a 4-wire stepper motor directly.

I do not have any specific requirements for the switch regarding voltage, current or behaviour (bouncing, break before make etc). It should only provide control signals to the micro controller and any behaviour sanitation can be made in software.

The only ready-to-use product I have found is from the Swiss company Elma. They have several different rotary switches which are compatible with a motor driver they call 'Remote Audio Plus'. The motor driver comes complete with a micro controller, IR receiver and can be operated with a standard IR remote control, as it is primarily targeted for use in audio amplifiers. I don't need the remote control functionality, but the controller also has an input interface and can be controlled independent of the IR receiver using a simple protocol. This product would cover all my requirements, but it is not exactly cheap. The motor drive alone retails at around €350 and that is not just slightly outside my budget.

I realize that I with some effort probably could adapt any stepper motor to operate a rotary switch, but I don't have the tools or knowledge to make the necessary fittings, brackets or whatever is needed to combine a motor and a switch, so a ready-to-use product will very much be preferred.

Do anyone of you know of a product, which at least comes close to my requirements?

Answers to comments:

Are alternatives viable? If not a must-have, this is at least a want-very-much. I have tried to consider other options as well, but not found anything making sense. The motorized switch is indeed not crucial for the thing to work, but there will be several rotary switches and potentiometers on the same panel and only one of the switches and one of the potentiometers should be motorized/automated. If I replace just the switch with e.g. a rotary encoder and display combination (or some other type of input and feedback mechanism), I ought to do that with all the other inputs as well for consistency and that would compicate things a lot and also IMHO mess up the visual design. Having knobs with a notch indicating the position would be preferrable, because all switches and potentiometers then would look the same.

Have I considered to have anything custom made? Not really. I can't believe that a custom made device is any cheaper than the Elma system I already mentioned and the price is the reason why I am searching for alternatives.

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  • \$\begingroup\$ How about a strowger switch \$\endgroup\$
    – Andy aka
    Commented May 13, 2020 at 14:03
  • \$\begingroup\$ @Andyaka I know what a strowger switch is, but I don't see how I can cover the requirements with one. They can usually only step in one direction, I doubt that they are manually operatable and I have never seen one with a reasonable size. I know that I didn't mention anything about size, but all strowger switches I have seen are quite large, bulky devices. \$\endgroup\$
    – jarnbjo
    Commented May 13, 2020 at 14:16
  • \$\begingroup\$ Yeah but you could use grandad's rotary phone as a remote control. \$\endgroup\$
    – user16324
    Commented May 13, 2020 at 14:23
  • \$\begingroup\$ have you considered using LEDs for visual feedback instead of a knob pointer? \$\endgroup\$
    – jsotola
    Commented May 13, 2020 at 15:00
  • \$\begingroup\$ Would it be possible to use an incremental rotary encoder in conjunction with a radial discrete LED display? Or does the knob actually have to move? If the latter you could ask the makers who do a lot of custom $$ work like Grayhill if they can offer anything. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented May 13, 2020 at 15:58

1 Answer 1

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Consider using a motorised rotary pot, connected to an ADC read by the MCU that controls it, which also controls solid state analog switches (or relays as appropriate). (If it's merely input to a micro, these switches are unnecessary; I mention them in case you need real switching too)

You won't get the click detent of a rotary switch, but you can simulate that by driving the motor.

As you turn it, the ADC reading increases : when it crosses 50 increasing, drive the motor (at low torque = low current) to 100. If the user lets go at 125 it springs back to 100; if at 175 it clicks forward to 200. Experiment with torque and deadbands (90 to 110 count as 100) to get the feel you want without motors chattering.

Number of switch positions is obviously software configurable.

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  • \$\begingroup\$ That might actually be a good idea. I do need 'real' switching as well, but I only intended the device to directly interface with the MCU and then do some sanity checks in software before actually operating relays. I am more than just a bit worried that the haptic feedback when the motor decides to 'take control' of the knob can be very confusing, but it might be worth a try. I am also not sure if I can reasonably restrict the rotating range of the potentionmeter. Even with motor control, I ought to be able to physically prevent rotation outside the intended 90° or so range. \$\endgroup\$
    – jarnbjo
    Commented May 13, 2020 at 14:29
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    \$\begingroup\$ It took some days to get a motorized potentiometer to try your suggestion. Unfortunately, it does not work very well. The potentiometer is moving too slowly to emulate the feel of a real rotary switch, when it snaps into position, so the 'tactile feeling' of operating the 'switch' is actually very odd. The best description I can give is that it kind of feels like holding a door handle or knob, while someone else on the other side of the door is also playing with the handle. Motorized pot.-meteres do not seem to come in a great selection of speeds. \$\endgroup\$
    – jarnbjo
    Commented May 24, 2020 at 17:05

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