Archive for September, 2012

Mini project: How a rotary encoder is built

Wednesday, September 12th, 2012

The power button/volume knob on our minivan became flaky.  Rather than getting a whole replacement stereo (and giving up steering wheel controls), I decided to replace the part.  And since the old one doesn’t work, I get to take it apart and see what makes it tick.

Here’s the patient:

After removing the casing, it pulls apart into two main pieces.

The white circle in the middle of the bottom half is a rubber dome switch, much like what you find in inexpensive keyboards.  The two sets of three contacts on the left and right are connected to the common (center) contact.  The two sets of contacts on the far side are connected to the two outputs.  The top half holds a conducting spoked disc which at times connects the common pin to the two quadrature output pins.  How does it work?  Wikipedia to the rescue!

So that’s the electrical side of things.  What about the detents?  That mechanism is visible once you take apart the top half.

The black wheel has a ring of notches around its top side, and a copper disc spring provides friction.  A kink in the disc fits into the notches, giving the tactile feedback.

And that’s about it.  Simple, effective, and (usually!) reliable.