Hi, I found this schematics of a simple controller.
I would like to control the switches with arduino. I tryed with some npn transistor but it doesn't work well. Do you have any help or suggestion?
What connects to T and R?
Sorry!!!
Its a TRS cable. The original hardware is this: TC Helicon | Product | SWITCH-6
Practically, when the switch is activated the effect can be changed (6 different parameters can be tuned with this). My plans are to change the SPST with some arduino controlled switched (like npn transistors?)
Tensions involved should be around 3V (measured with the real thing).
100K is way too high for that circuit, I would use 4k7 resistors and get a decent impedance for driving your A/D converter, that is not shown.
Due tolerances on the resistors you will have to measure the numbers you get from the A/D and then adjust the table of thresholds so your actual readings sit in the middle between two adjacent switches.
Since the rest of the circuit is missing and polarity is not known you could use small read relays to replace the switches.
You can use these tiny solid state relays
@aleprivi
Here is the circuit
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The package is very perfboard friendly as well.
Regular-old electro-mechanical relays are the "safest" or an "unknown" circuit/application. A relay is an electrically-operated and electrically-isolated switch.
The "catch" is that the Arduino can't directly drive a relay coil, so you need a driver circuit. But you can buy "relay boards" with a built-in driver.
...Transistors are often used as "switches", but usually for DC (current doesn't flow backwards through a transistor), and in certain-limited circuits/applications. Unlike a relay, it's not a "drop-in" replacement for a switch and they don't turn 100% on or 100% off.
Thanks for all the suggestion!!! I'll try in the following days and I'll let you know! 
Have a nice day!
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