Feb 252021

I like minimal solutions to problems. I was playing with a CircuitPython-enabled QT Py on a breadboard with and a rotary encoder and I ended up making a USB knob, like many others have done before. But I realized: waitaminute, I can literally just plug the encoder directly onto the QT Py…

Thus was born the QTPy-knob. It’s one of the simplest USB knobs I’ve come across and it’s because the happenstance that a rotary encoder can usefully be plugged directly into a QT Py board and that CircuitPython is so powerful now it’s just a few lines to go from rotary encoder pulses to sending arbitrary USB keyboard or mouse commands.

I really liked the Griffin PowerMate from over a decade ago, so I decided to design a 3d-printable enclosure that echos the design of the PowerMate, but that works with the restrictions of the QTPy+encoder stackup and a Neopixel ring. The result is pretty good I think.

All design files are in the qtpy-knob github repo.

The code is must easier now than the media-dial project I started cribbing from. This is due to the fact that CircuitPython now has native support for rotary encoders and this knob is trying to do much less than media-dial project.

The electrical wiring is virtually non-existent, only needing three wires if you elect to add a Neopixel ring.

The assembly is described in the video above a bit, but you can also see what’s up in this CAD animation (thanks to the Adafruit_CAD_Parts repo for making the enclosure design easier)

The result with opaque plastic and clear LED diffuser ends up looking pretty cool.

  2 Responses to “QTPy-knob: Simple USB knob w/ CircuitPython”

  1. Hi Gordie!
    From looking at the CircuitPython source for IncrementalEncoder, it looks like on SAMD21 (as used in the QT Py) it’s just using the pin-change interrupts, so you can use any two pins.

  2. Do you know whether the rotaryio.IncrementalEncoder library use the hardware quadrature counter in the processor, or does it use interrupts?

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