Story
At first, I know the existence of a digital pot. 'Problem' is you need an IC to achieve this and also want to keep the circuits completly separated (and simple). Also want to use it with an Arduino audio project without external IC (got already a MCU that can do the 'settings' job). So I came up with the idea to use a led and a photoresistor to seperate circuits and avoid fixed 'digital' steps, just like a normal pot resistor, keep it smooth and natural but without the analog fail/aging problems.
Made two (quick!) drafts, one with simple hardware like an opacity wheel and one with use of an Arduino:
I figure out the basics of this idea using light to control volume is not a new one, found this:
http://www.tortugaaudio.com/evolution-of-ldr-volume-control/
And the Lightspeed Passive Attenuator:
http://diyaudioprojects.com/Solid/DIY-Lightspeed-Passive-Attenuator/
Try to find the component they are using, a led with a photoresistor but could not find it. I started to experiment with existing components, like this one (made an experimental board of an old canon printer, a board with two optical sensors, cut in half and added to audio sockets) to figure out it will work anyway:
The experiment succeed really well, very intresting to see it is actually working. The volume reduces when you put an object between the optical sensors or limit voltage input does the same. Except …… one side of the amplitude is chopped off. Because there are only four pins, the receiver could be an opto-resistor, however it seems to be a diode? Just listen to this soundtrack what happend:
Wave file: https://drive.google.com/file/d/0B2l-eQoHefcVdXRlUkM1bXJYek0/view?usp=sharing
Wave file (left+right combined): https://drive.google.com/file/d/0B2l-eQoHefcVVzZ3aWVrZ0hMVjg/view?usp=sharing
Question
Althought I can make the component myself by replacing the opto-resistor-diode with a photosensor I wonder if these components are available somewhere. Because available space is an issue, it would be handy it is a compact component (with four legs) sized like a opto-coupler for example. Any ideas? Any partnumbers?
Best Answer
Admirable attempt, and good debugging effort. Your Cannon opto-couplers are different animals than those described in the link you provided. They do use an infra-red diode to provide the light, but the photo-sensitive element is not a resistive one, but is a silicon photo-transistor. The current it provides only flows one-way - hence the clipped waveforms that yield highly distorted audio.
The photo-sensitive element in your link is a cadmium-sulphide or cadmium-selenide photoresistor. It is a resistor that allows current in both directions in a reasonably linear way. Search terms like photocell optocoupler might help.
Some currently available part #'s: NSL-33-007 from LUNA optoelectronics is a dual-channel photocell optocoupler.
NSL-32H-103 from LUNA optoelectronics is a single-channel version.