Oh Terry, Great and Noble Forrest King!
I have seen your website and found the PC817 opto-isolator, but it looks to me, in my extreme ignorance, that ac current won't get through it when turned on?
I am feeling really ignorant, when I see all the replies suggesting various ways to build my circuit.
I am also realizing that I haven't been very clear in my questions.
My goal is reproduce as much as possible an electric guitar's mechaincal man-machine interface, which comprises pots & switches, by an Arduiono based programmable interface. Take a very simple circuit such as http://www.dimarzio.com/sites/default/files/diagrams/Eesqwiring.pdf. I would like to command the switching and the pots programmatically from the Arduino. In my naivety, I thought to use some MOSFETs to replace the switch, and to use 2 LDR-LED combinations to replace the pots. Then, I would use PWM on the LEDs to set the LDR resistance values.
I do not want to impact the signal coming from the pickup coil.
I have no idea what I am doing here, but I am prepared to hack at it! I have already got an old sacrificial guitar and an Arduino and lots of tools since I am an RC hobbyist. But, I am still trying to see what electronic components could be used to make all this work...
All your advice, and that over everyone on this forum is GREATLY APPRECIATED!
Thanks so much!