Some thoughts and research.

         Arduino stuff at bottom of page.

10 Aug 2013 ,,, Updated (See below)



 Rotary encoders keep popping up with cockpit building and its only natural that they get integrated with the projects that we build.

Although rotary encoders are basically a simple item in themselves, all too often the playing we do with them turns to custard and the end result is a "work around". (With speed and reliability)


ROTARY ENCODERS ,,, What are they ????

Rotary encodes enable a means of, by rotating a knob, increasing or decreasing the value of something. The most common ones that domestic users will come across is the volume control on modern car radios. (If it’s not done with buttons !) 

There is no mechanical “stop” to them ,, you can physically turn them for ever and not bust them.

Although there is many exotic types out there (more exacting,, positioning etc) we will only dig deep into the common ones that will be used by the average cockpit builder.


FIRSTLY ,,,, They are NOT rotary switches !!!!! Countless times a “Newbie” pops up in forums somewhere and after a lot of confusion ,, it turns out he though “rotary encoder” is just a fancy name for a rotary switch ,,,  In reality ,, I suppose you can relate to this because they are both presented to the end user as a “knob on the panel” that you turn to adjust something.


A rotary switch (in simple terms) changes a common terminal from one terminal to another terminal. Keep turning and (depending on the switchs design) it will change the common to even more terminals.  ,,, so basically you can choose which terminal is selected by turning the knob.


A rotary encoder is a different beast altogether.  Inside a rotary encoder there are actually 2 rotary switchs ,,  one making contact before the other one and the same sequence on opening,  also during the turning, both can be ‘open’ or both can be ‘closed’.

Its best to describe it in the term you will commonly see in DATA SHEETS.




















Interpreting the above …..

The dotted lines are the indent position ,, the “Click”. (Watch out here ,, sometimes dotted lines are shown as “transition points”.)

CH A is the same as “Switch A” ,,,  CH B is the same as “Switch B”

So in the above diagram ,,, pin C is the common and the 2 switches (Channels) are pins A and B.

Both switches are open (off) in the start position.

We move from left to right by turning the knob clockwise.

As the encoder is turned clockwise, switch A makes contact first ,, then switch B makes contact.

Now ,, on the first click (not counting the start position) ,, they are both closed. (on).

Turn it further clockwise and switch A opens first,, followed by switch B ,,, now they are both open (off) again.

The software detects from the above sequence that the knob is being turned in the clockwise direction.

In the above example, you have just turned it through a complete cycle ,, where the switch contacts are back to what they were to start.




OK, lets see what you know about these things  ,,,,,


Spot the difference ,,,,














OK ,,  they are all the same.   The point I’m trying to make is ,,,



Some important considerations  ,,,,


Get ones that can also be “panel mounted” so, if you want to,, you can mount them directly to the panel. (They have a thread and a nut on the shaft housing)

Rotary encoder terminals are made to suit mounting on a PCB ,, that’s fine for high volume production but for your “one of” project you probably want to mount it on a panel and run wires to the input card.


Ensure you can get Knobs that suits your chosen rotary encoder.

There’s a huge variation in shaft lengths, styles and mounting systems (for the knob).

All to often, the biggest problem is getting knobs that “look right” in your project and to actually find a knob that suits the encoder you are looking at buying is a huge mission. A lot of times ,,, looking for a knob to suit those cheap encoders you bought ,, ends in disaster.

Its no use paying $2 for a rotary encoder and then finding out the only “suitable” knob is going to cost you another $10.

Getting knobs for DUAL concentric encoders is even a bigger nightmare so be very careful if that’s what you need.

The only reseller I’ve struck that relates to this problem is Leo ,,,  he sells dual encoders WITH the suitable knobs.

Not the cheapest in town, but at least it’s all together.  (Near the bottom of the page)


Built-in “Push switch” ,,, Get the one with the inbuilt switch (push the knob and it switch’s something) ,,  real handy for a lot of purposes. (Mhz to Khz change-over or change input from one radio to the other. etc etc)


Choose the correct number of indents per revolution for the designed purpose. (The indent is the “click”)

Again,, this depends on what you are doing with them.  General flight sim stuff seems to suit 15 to 20 indents per revolution.

At 30 indents it gets a bit touchy to set a figure and 12 indents is getting a bit slow to make progress with your turn.

Another figure to think of here is built-in software acceleration ,, where the faster you turn it ,, or ,,  the longer you turn it, then the software takes bigger jumps in the output. The notes from the software developer should indicate if acceleration is “built in”.


Choose the correct “cycle per indent ratio”.

This is a real nightmare for the unskilled buyer.  Even big manufactures get this wrong sometimes. (Just ask anyone that bought an early Saitek radio unit)  Lots of sellers on ebay don’t even talk about this and don’t supply this crucial information.

Have you ever used something that for each click it jumps forward 2 or 4 places ??? ,,, or ,,,  you need to turn it 2 clicks for it to go forward 1 place ??  Well this is the area we are talking about.

There are 2  area’s of concern here ,,, Obviously the encoder ,, the other one is the software.  The intended software should say if its ¼ cycle,, ½ cycle or full cycle. (Or some other way to describe the indents per switching cycle)

Once you know this,, you buy the encoder to suit that cycle.


NOW ,, Armed with all this knowledge ,,, Spot the difference in these three ,,,





















Encoder “A” has only gone thru 1 click and the 2 internal switches are back to the same electric state as what it was when it started  ,, so it’s a “full cycle” encoder.

Encoder “B” has gone thru 2 clicks to get back to the switches starting state ,,  so it’s a “half cycle” encoder.

Encoder “C” has gone thru 4 clicks to get back to the switches starting state ,,  so it’s a “quarter cycle” encoder.

There are other’s out there but that’s the main (common) three types.

What’s missing in the above picture is  Indents or Cycles per revolution ,, a very needed figure but it’s generally given elsewhere in the datasheet.



So my best advise before buying encoders is ,,,,


Study the datasheet on any rotary encoder that catch’s your eye.

Note .. There seems to be no set standard between manufacturers to describe the info needed for the “cycle to indent ratio” ,, but now you know what to look for.  You need to put this “Cycle to indent ratio” together yourself  out of the different specifications in the data sheets ,,, but it’s generally presented in a picture simular to the above.

“Specials” or  “Real cheap” ,,, There are some odd-ball ones out there, so be careful with “specials”. Some have no indents at all. !!!






These are real easy to integrate into our projects as its wired is just as a basic switch.

Alps and Knitter make pulse switch (turn them one way and a switch pulses, turn them the other way and another switch pulses with each click)

Although,, on a quick look I couldn’t find any knitter ones ,,, maybe they’ve stopped making them.

The disadvantage with these, is they are hard to find and are more expensive.



Rotary encoders with the Arduino.


As you know,, I love the Arduino cards and use them with my cockpits.

I liked pulse switches so rotary encoders was a bit of a new adventure for me but I had strict criteria for their use within my set-up.

My requirements were ,,,

Had to be fast ,,, FAST !!!!

Minimal errors on reasonably “cheapie” rotary encoders.

Ability to have heaps of them into one card.

Ability to handle ¼ , ½, and full cycle per indent.

Coding had to be simple to integrate into larger programs.

And the most important one ,,, It still had to perform well on an “already busy card”  !!!!


Well there seemed to be no contest here  ,,  I fiddled for ages with different ways to do this ,, some were great but fell down on one thing or another.   Towards the end of my research, I stumbled across Keith Neufeld’s blog and what a surprise. This Guys is a very talented man, not only in coding but also in electronics.


I can recommend with no hesitation Keith’s Quadrature library for the Arduino. (Only for IDE’s 0022 and 0023)

His website ….

His talk on this library ,,

His actual download file only worked with the early Arduino IDE’s but attached in my download (below) is a corrected version.


In the download the demo is set up for using with a Uno card (max pin = 14) but to use it with a Mega card ,, all that needs doing is to open out the 'quadrature.h' file and edit the line '#define DIGITAL_PINS (14)'.  Change the 14 to what ever pin no. is going to be your highest.


Thank you Keith !!


Unfortunately Keiths Quadrature library failed to work in the newer Arduino IDE version 1.

Bill Holland had the first fix and then I had another ,,,, so now it works with the newer versions of the IDE as well as the older ones.



Keiths library does half and full cycle encoders ,, there is however a another type on the block that is getting more

and more prevalent and that is a QUARTER CYCLE encoder. (One state change per click)

If your encoders are quarter cycle type then you don’t need to use a library at all.

I have enclosed a demo in the download called "quarter_cycle_encoder_demo" that has 2 different types of code showing how to do it.

DON'T  USE A LIBRARY if you have quarter cycle encoders.


You can download the new one here ,,,,




James's method as detailed in ,,,, 

Other links that use the common rotary switch are about the net in different formats ,, 1 pole 12 pos,,  2 pole 12 pos etc etc.

Somewhere on my travels I struck a guy using a potentiometer to "dial up" the required figure. (input to analogue pin)

Somewhere also I seen a circuit of  fixed resistors around a 12 position switch to again give different analogue values.

In some of the above switch modes ,, the "stop" was removed from the switch so it could endlessly rotate.


Writing Code for Rotary encoders ,,,,

OK,, now you know the in’s and out’s of rotary encoders, the normal thing is to take it all in and make the code to suit your project and the type of rotary encoder you have ,,,  that’s fine until you share it with some-one else.  What you need to do is include in your project notes, explanations on changing the rotary encoder input cycle to suit whatever they have. (Assuming their rotary encoders are different)


Keep at it Guys ,,, Jim


Dated 18 July 2011

Updated  3 Dec 2011

Updated 22 Aug 2012 added IDE v1 fix on Keith’s library

Updated 10 Aug 2013 general update (core library is the same)

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