Ring-shaped rotary encoder?

[komet]

I want to make a circular PCB with a knurled knob around the outside that works as a rotary encoder, somewhat like the Nest thermostat (I think - never actually seen one live). Are there parts that do this or do I have to 3D print some sort of gear mechanism?

[Jarrod Roberson]

why not just use a regular rotary encoder with a "cap" glued onto it?

imagine a Paint Can cap attached to a normal rotary encoder post at the center.

[jeremy]

Aren't all rotary encoders ring shaped?

this is the one I always use: http://www.digikey.com.au/product-detail/en/AMT102-V%20KIT/102-1307-ND/827015

[komet]

I want it to be a hollow ring so that there can be a display inside.

[PedroDaGr8]

I want it to be a hollow ring so that there can be a display inside.

Use a gear or some gears plus a standard rotary encorder, you can get the same effect.

[ajb]

If you're fabricating the outer ring yourself you could just incorporate some tabs/notches/holes/magnets to be detected by opto/hall sensors on your PCB.  Difficulty of the fabrication will depend on the resolution you require, but as a bonus you could easily achieve absolute position with a couple of extra sensors and the right pattern of detectable features on the ring.

[ludzinc]

I'd be tempted to mount the optical sensor from a mouse underneath the ring.

As you turn the ring, the 'mouse' will detect the movement, and at a heck of a finer resolution that any other DIY attempt that I can think of.

[free_electron]

use an optical sensor and apply a zebra strip inside the ring. or use a hall effect sensor and simply make teeth protrude inside.

[daedalus]

would love to see someone build their own optical sensors, but its a non-trivial job, and a bit of a waste of time considering the resolution you actually need. I work with 20k cpr sensors that cost me around 120gdp, that's hard to beat time wise. Im guessing 1k cpr would be more then enough for your application. There are 14-15 effective bit magnetic sensors out there for around the 3-4 gbp mark.

[Richard Crowley]

Look at the "teardown" of those digital calipers.
You could make a couple of complementary PC boards that, between them, would offer you as much resolution as you want...
https://www.flickr.com/photos/50241702@N04/5227386500/in/photostream/

The bigger issue would be mechanical: Namely some kind of bearing for the ring with a huge inside-diameter.

[ovnr]

Generate a flat Gray code strip and stick it on the inside of your ring. All you really need is a repeating 2-bit pattern which will give you quadrature encoding without any fuss and all you need to read it is a couple (IR) LEDs and phototransistors. You could obviously come up with some convoluted setup to read absolute position, but meh.



Disclaimer: Was super tired when writing the above, so YMMV.

[ovnr]

The bigger issue would be mechanical: Namely some kind of bearing for the ring with a huge inside-diameter.

Not actually a big issue. Easiest solution is just to have a free ring loosely clamped between two flanges, and then either let it slide on whatever is below, or use some small bearings as "wheels" for less friction - but in an encoder that the end user is supposed to use, you kind of want some friction. Of course, if you're 3D-printing it on a cheap process (FDM), it's going to be a nightmare and you'll pretty much need to add some bearings as wheels.

I'd add a mockup of it here if I were less lazy.

[komet]

Yes, it's clear that I can 3D print something, but I was wondering if there was an existing item for sale.

[eneuro]

Aren't all rotary encoders ring shaped?

It depends what you need 
It is easy solder a few elements and you have custom encoder even with... cheap photodiode ready to count six times per rotation and mount inside alternator