Sunday 17 March 2024

Rotary Encoders

These are wonderful little things.

I got to thinking a bit about exactly how they worked, so I hooked one up to a power-supply and an oscilloscope.  There is 3V being delivered to the centre pin and one oscilloscope channel connected to each of the the two outer pins.  At rest, the centre pin isn't connected to either of the outer pins.  Turning the encoder clockwise... can see that the blue trace goes to 3V a little before the yellow trace (the blue channel is the left-hand "A" pin on the encoder). 

Turning the encoder counter-clockwise and the opposite happens:-

Here the yellow trace (the "B" pin) goes to 3V before the blue one.  The key thing is that as the encoder passes over every detent, there is a point at which both pins are connected to the centre pin, but the direction of rotation determines whether the "A" pin gets there first or the "B" pin.

What sent me down this particular rabbit-hole was a very interesting pair of YouTube videos about using using just digital logic (i.e. no microcontrollers), here and here.  I had never previously thought about using these without a microcontroller and thinking about them in this way does reveal some interesting insights.


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