Ring style encoders and pots function electrically the same as rotary pots and rotary encoders. Mechanically, they are very different because they do not have a shaft in the center of their rotation. Instead they are a hollow ring with the rotational mechanism around it. This allows for designers to place the rotational knob on the outside of something that doesn’t rotate on the inside. This can be very useful if you are looking to place graphics or visual feedback inside the knob instead of on the outside border. A clock face is a good example of this.
Ring style encoders / pots are most common in newer cars. They are mainly used for climate control, and will have temperature readouts or positions on the ring displayed in the center:
Until recently, Ring encoders / potentiometers were predominately custom engineered for specific products or cars. Now there are mass produced version appearing. The larger diameter ones are still tricky to find. Also difficult to find are knobs to fit them, however with a laser cutter you can make your own pretty easily.
Currently ring potentiometers are easier to get a hold of than ring encoders. Here are a few of both that I found on mouser:
For now we will stick with this ALPS RK45C1A00003 ring pot which has an LED that moves around with the position of the pot’s wiper :
Finding out how to wire this sensor up is a little tricky, since the best ALPS has to offer for identifying the pins in their data sheet is this:
Assuming the pins labeled 1,2,3 are pot right, pot wiper, pot left, and the A pin is LED Anode, and the K pin is . . . .Kathode? Not sure about the K pin. This is all verified with a multimeter, and here is a simple schematic of how you could attach it to an arduino and light the LED up. This is the order of the pins when viewing the potentiometer from the TOP:
In the photo below (same POT as in the diagram) The black wire is LED ground (Cathode), the orange wire is the left side of the pot, the blue wire is the POT’s wiper, the yellow wire is the right side of the POT, and the red wire is LED positive (Anode).
All of the ring potentiometers I could find (made by ALPS or Panasonic) are only available with detents – meaning they click into specific spaces when you rotate the POT as opposed to traveling smoothly. But what if you want your pot to rotate smoothly and need more resolution than 32 steps or 16 steps? Here is a video showing the difference between a 32 detent ring pot and one without detents:
You can carefully remove the detent mechanism from most of these pots fairly easily by taking them apart. This does involve prying small metal parts, so here are some safety recommendations:
- Put an old towel or thick shop rag on your workbench (or in a vice clamp) and then the ring pot on top of that to help you work on the back of it. Use something that has a lot of friction between table. The more friction the better! The ring pot is a weird shape, and the towel will help keep the part from slipping all over the table.
- ALWAYS pry away from yourself! Never point prying tools towards yourself or towards any fingers you’re using to hold down the part!
- I don’t recommend using a knife blade for any of this as they are very sharp and can chip. Use a small screwdriver or better yet a plastic spudger tool.
- Use the right sized tools – try to find some small pliers.
- Be patient and ask for help if you get stuck
Step one is opening the ring pot. There are four metal tabs and two metal latches on the ALPS pot:
Be careful not to loose anything!
On the inside of the metal cage you just pried off, you will see the detent mechanism. It is a small copper semicircle piece with the plastic detent in the middle:
Cut it out with some diagonal cutters near the base on both sides and then remove it completely:
Assemble everything in reverse, and bend the metal tabs on the back back into place to hold the whole thing together:
As a final step, I like to use a pair of slip-joint pliers or a small vice to clamp each of the four metal tabs back into place:
You now have a smooth action ring pot!
Another hard part of using these ring pots is finding knobs for them. For one of my projects I simply traced the outline of the outside ring, scanned it into a pdf, and scaled it in illustrator. Then I laser cut them and glued them to the ring: