I have this circuit:
It is a simple project - a focus system for a taken lens and anamorphic adaptor. I have three pots. The first one I use as a manipulator. The second is used to get the position of the continuous rotation servo (JX DC6015 Mod) - motor#1. The third pot is used to control the position of the other continuous rotation servo (JX DC6015 Mod) - motor#2. There are also some buttons and diodes for calibration purposes. Motor#1 rotates the taken lens and motor#2 rotates the adaptor.
Well, let's look at the circuit. I also have there a 1N4007 diode and a 47uF cap. I use this scheme because the circuit even didn't turned on when I tried to power it from a battery. Now at least it turns on. But let's leave it aside for a moment.
The main problem is that everything works perfect when I use an external AC-DC adaptor (or battery) with Nano's USB connected to my laptop. As soon as I unplug the USB cable the system goes crazy. In case it is connected to the AC-DC adaptor it just starts rotating servos several turns CW and then backwards. In case it is powered just with a battery the servos just rotates CCW or CW very fast and never stop.
It looks for me like I have a GND loop problem. I tried to decouple my pots by adding 3 caps for each pot connecting a signal pin with GND, but things went even worse. It didn't work even with USB connected to Arduino. Servos rotated CW and CCW changing the direction very fast.
I tried two other Arduino controllers, but it was all the same.
I've tested all the GNDs. They are all connected together.
Then I decided to try to isolate VCC and GND from servo putting NME0505SC between power source and Arduino. But it didn't help as well.
What could be done to make it work just from one power source?