Diy slider potentiometer?

Question

Hi I need to use a 25cm (10inch) long slider potentiometer for my project. I can't find them anywhere so I'm thinking of getting a PCB made and putting some resistive material on it. My PCB manufacturer supports carbon ink for contacts, can I use it for resistive element?

Edit: I want to get 12 bits of resolution from the pot. I am using a stm32 chip for ADC with it so I will have a 5V potential difference across it. Aiming for a 10K ohm value. Reliability over sustained use is required since it is a part of MIDI device. I have considered belt gear drive with angle sensor but a slider pot is much simpler and cheaper

Edit 2: contact less options - Would serve my purpose better actually, but I don't know if distance sensors like IR or ultrasonic will work for 25cm length and be accurate enough.

Answer 1

Since it's for a musical MIDI application a smooth response may help you justify an analogue potentiometer. The simplest, which you have already considered, is a 10k rotary pot with belt drive.

• Typical 1-turn pot track swept angle is 330°.
• You're looking for 25 cm slide.
• From \$ s = \pi d \frac {330}{360} \$ we can calculate the required belt pulley diameter, $$ d = \frac s {\pi{\frac {330}{360}}} = \frac {250} {\pi{\frac {330}{360}}} = 86.8 \ \text {mm}$$

The advantage is that all your quality and wear issues should be sorted by the manufacturer and would be easily replaced if it broke. You could make this with a string rather than a belt as used in old fashioned variable capacitor radio tuners.

Figure 1. Image source: EDN.

You might be able to simplify further by using a multi-turn pot - a 10-turn, for example. A direct winding on a 6 mm shaft will require 18.8 mm per turn so ten turns would give you a 180 mm slide which might be good enough.

Answer 2

12 bit or 1/4k linearity is impossible to get even in commercial pots let alone DIY pots.

Get a rotary encoder or use a stepper with 200 steps per rev and use gear belts and pulleys to achieve the speed resolution tradeoff you need.

With my servo I had 8mm D pulleys with full steps up to 1m/s speed and 125 um resolution over 1.1m square area or in other words 13 bit resolution and accuracy. Using fraction steps could achieve more but wasn’t necessary.

Using 4:1 pulley reduction you can achieve the same over 25 cm span.

FWIW I used an UNO with CNC code , a CNC shield 12V and Gcode Panel S/W on a laptop set to 2A for 2X + 1Y motors. Awesome open source s/w. $250 for entire system :before mods. Sold to U of T for $4k after 2mos R&D searching for a solution.

Answer 3

One possible solution would be to DIY a digital caliper. The patent has lots of info. It's basically a Vernier capacitive sensor. Here's an article of an implementation with an untethered slider which is what you want.

To be realistic though, you don't need your 25cm long slider to have absolute 12-bit accuracy unless your fingers are also accurate to about 60µm. So I'd drop the accuracy requirement, along with some of the linearity of course. If it is a musical instrument, stuff like reaction time and "it just works" are a lot more important.

A mutual capacitance slider seems like a good fit.

I think the triangles should be filled, though.

Capacitive sensors work best when sensing through thin material. The worst case is a finger on the other side of a thick plastic front panel. But if the finger is replaced with a small conductive plate (on the back of the slider "button") that slides on a thin film of mylar just on top of the PCB electrodes, then you'll get very good coupling.

Unlike an encoder, this has the innate advantage of absolute positioning, ie when the device is powered, it knows where its knobs are.

If the front panel is conductive, the whole thing will be shielded, so it should be robust.

For more accuracy, you can use more electrodes, or a Vernier arrangement.

Answer 4

You can source resistive linear position sensors of the type used in plastic injection molding machines. 250, 275 or 300mm are standard lengths. Typical linearity spec is +/-0.1% or +/-0.05% (photos from Aliexpress and directindustry) when used as a voltage divider. Resolution is better than 12 bit. They're usually 5K\$\Omega\$ with a loose tolerance on the element resistance.

They're used to sense the position of the injection unit, mold and reciprocating screw in machines that operate industrially 24/7 with cycle times measured in seconds, so life is quite good.

Answer 5

Use few (say 10 or 20) 1k normal resistors soldered in a line to make your line pot. Slider in red.