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On my amp, I have a big round knob, that I can turn forever in both directions, to control volume. It simply turns up whenever I turn clockwise and down when I turn counter clockwise. There is no limit as to how far I can keep turning it.

How do I create this in my own project? I have an arduino where I want a knob like this, to control a value in the arduino memory. Simply, turning up, add to value, turning down, subtract from value.

I assume I need some kind of potentiometer, but they do not turn forever, do they?

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    \$\begingroup\$ Actually there are potentiometers, usually with two wipers, that do turn forever. However, those are not approriate for what you are asking about. \$\endgroup\$ – Olin Lathrop Jan 4 '14 at 16:37
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What you need in a rotary encoder, they have two outputs that generate pulses which are then decoded to read the direction of rotation.

Rotary mechanical encoder Rotary mechanical encoder waveform

Refer to http://playground.arduino.cc/Main/RotaryEncoders

To control the volume you can use Arduino to drive a volume control chip like PGA2311.
There are even 4 channel versions like PGA4311

You can also control digital potentiometers like MCP41xxx/42xxx

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    \$\begingroup\$ @DiegoCNascimento I suggest not to link Arduino directly with the answer The OP asked specifically for Arduino so why including an Arduino link with a library for his needs is wrong? \$\endgroup\$ – alexan_e Jan 4 '14 at 19:17
  • \$\begingroup\$ ops, have not read that, sorry :) \$\endgroup\$ – Diego C Nascimento Jan 5 '14 at 0:12

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