Serial USB communication to Nano_v3 connected to external stable +5V

Hi all,

I'm trying to understand what is the best approach to communicate with the Arcuino Nano v3.0 I'm using to manage my audio preamp. I have to communicate with the Arduino in order to fine tune some basic parameters (the Nano controls the attenuation of the preamp via a rotary encoder) and to do that I need it connected in the final position. Obviously the USB communication is not needed once the preamp will be used, so the Arduino and the preamp logic section have their own stable +5V. Now if I have well understood, there should not be any problem in plugging the USB cable if the Arduino is connected to an external power source via Vin pin but that's not my scenario (I'm using the +5V pin). Is there any easy way to connect the Arduino to send/receive simple input/output via serial monitor without the need for special cable/interface?

The only easy solution I can see is to cut the +5V line on the PCB, use the USB connection to power the Arduino and communicate with it, do my stuff and when everything is fine tuned just reconnect (re-solder) the +5V... not so elegant...

Why not use a Promini? Plug on a USB/Serial adapter (FTDI Basic) when serial interfacing with PC is needed, leave it off the rest of the time. Having 1 less high speed oscillator in the area would help with signal noise too.

Not sure the Pro Mini has enough input/output for my project.

Anyway I've found a simple solution that works in my case. I just disconnect the power supply from the audio section (it's a separate PCB) and plug the USB cable to the Arduino. The Nano itself provide the +5V to the rest of the system: 2 MCP42100, 4 photocoupler and a rotary encoder - nothing special. Obviously I'm not going to test the input channel selector that is based on relais. I didn't want to drain too much current from the Arduino but that's not a problem because I don't need to test or tune that portion of the preamp - it works for sure. I was able to tune everything else.

Generally speaking, I don't think this is a good practise, but in my case I needed no more than 100-150mA (a part from the Arduino itself) so it worked for me.

Hi, there's a diode between the 5V from USB and the 5V pin on the Nano, so that should protect the pc from the external 5V supply. The diode drops around half a volt, so no current should flow from USB when your external 5V supply is connected. So it sounds safe enough to me, but there are more knowlegeable people on the forum.

FYI the Pro Micro has pretty much the same number of I/O as the Nano. I seem to remember that the i2c connectors are in an awkward place if you are using a breadboard.