# What is the difference between the servo track and the line track of a motorized potentiometer?

I have this motorized potentiometer, which I would like to control with an STM32 microcontroller.

As far as I understood there are several parts in this device:

• a potentiometer
• a DC motor
• a conductive knob for touch sense

I need all of these. There are 2 connectors. The 2 pins connector is for the DC motor. Nothing special there. But the 7 pins connector is described this way:

• pins 1 2 3: line track (R1)
• pin T: touch sense
• pins 1' 2' 3': servo track (R2)

The touch sense is straightforward. I guess 1 2 3 is the potentiometer. But what is 1' 2' 3' then? I measured with a ohmmeter, and it seems to be the inverse potentiometer, which doesn't make sense to me.

The question is: what am I missing? I found this related question, but with no answer.

Unfortunately the spec sheet is not very explanatory. It appear obvious to me that the 1'2'3' servo track is to be used by the uControler to know the absolute position of the pot. You could not do that with the 123 pins because those are used for audio signal. Hence, 1'2'3' is repeating the same action as 123 but on a totally different pot, the Servo Pot. I would use them for sending a DC voltage to an ADC input on the uControler. See proposed schematic.

Furthermore, What puzzled me is the pin T. Is it only a one wire touching the slide metal ? One could sense a 60hz buzzing signal at user touch and detect that someone is attempting to slide the pot? A bizarre way of doing it but could work. Any other idea on the T pin ? Cheers :)

• I also had no idea about pin T. But adding it (they sell it without one) and a motor causes the part price to be multiplied by a factor of 10. See here: bourns.com/news/press-releases/pr/2017/01/16/… – jonk Apr 8 '17 at 21:01
• OP even explained it in their question, pin T is for "touch sense" presumably capacitive touch sensing. @jonk I expect its the addition of a motor and all associated hardware that causes the 10x price, not adding an extra wire. – mbrig Apr 9 '17 at 1:42
• @mbrig Yeah. I've just never been involved in high-end audio installations. I've seen them on TV. But never been close enough to touch one. The "touch sense" didn't tell me what kind of sensor it is, though. I think they spec'd less than $2\:\Omega$ resistance (maybe I mis-read that), which may be consistent with what you are saying, but the datasheet wasn't clear to me about its use and how to design for whatever it is. Probably just my own unfamiliarity and nothing more than that. Yeah. The motor, etc., adds a lot of cost. Imagining buying 1000 of these scares me, though!! – jonk Apr 9 '17 at 2:00
• @jonk fair enough :) I can't see anything else you could do with one wire other than capacitive, but there's always weird stuff out there. And yeah, crazy pricey! Motorized pots look so cool in action, but I'm not tossing that kind of money at a hobby project haha – mbrig Apr 9 '17 at 3:14
• Thanks for the answer, it makes a lot of sense. The touch sense is a pin connected to the know (you must use a conductive knob, not a plastic one indeed). The information whether the fader is touched is very useful. These faders can produce force feedback. In this case you may want to stop forces when the finger is not touching the fader. – Tom Apr 9 '17 at 8:00

Servo track has a linear dependence of the resistance on the position and is used to monitor the position.

Line track has a complex dependence of resistance on the position and is used to adjust the analog audio signals (main purpose).

Depending on the last symbols marking three kinds of potentiometers characteristics are shown on the last page of the datasheet.

• Audio pots are usually logarithmic, as that better matches the human perception of volume. ( See resistorguide.com/potentiometer-taper ) The "A Series" and "C Series" tapers in your spec sheet look like this. The "B Series" is sigmoid, changing fastest in the middle. All of these are useful for the thing under control, but not as useful as a linear pot for sense and feedback. Because of that (and so your feedback/sense circuit doesn't interfere with the system being controlled), they mechanically-coupled two different pots. – Rob Starling Apr 9 '17 at 3:07
• Thanks for the answer. It is sad that I can accept only one answer, both are complementary. I upvoted both though. – Tom Apr 9 '17 at 8:01

I would suppose that pins 123 are what you use for the audio or other analog channel you're controlling, and 1'2'3' are what the motor/system uses to figure out where it is on the slide. A similar device on the Sparkfun site explains:

"The slide contains two separate 10k linear taper potentiometers so that you can use one as servo-feedback in order to read the position of the slider and use the other to control whatever your target is"